A Study of a group of typical spinels

HILL IN I SUNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

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A STUDY OFTYPICAL

A GROUP OFSPINELS

CULLEN W. PARMELEEALFRED E. BADGERGEORGE A, BALLAM

SBULLETIN No. 248ENGINEERING EXPERIMENT STATION

PuaBLsgaD BY TH UtqVErrITY or ILU4Nox, URBAN

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UNIVERSITY OF ILLINOIS

ENGINEERING EXPERIMENT STATION

BULLETIN No. 248 JUNE, 1932

A STUDY OF A GROUP OF TYPICAL SPINELS

BY

CULLEN W. PARMELEEPROFESSOR OF CERAMIC ENGINEERING

ALFRED E. BADGERRESEARCH ASSISTANT IN CERAMIC ENGINEERING

GEORGE A. BALLAMFORMERLY RESEARCH ASSISTANT IN CERAMIC ENGINEERING

ENGINEERING EXPERIMENT STATIONPUBLISHED BY THE UNIVERSITY OF ILLINOIS, URBANA

5 -UNIVE32SI245000-6-32--2457 PRE:SS n

CONTENTS

I. INTRODUCTION . . . . .

1. Introductory . . . .2. Purpose of Investigation3. Plan of Investigation4. Occurrence of Spinels5. Use of Spinels . . . .6. Acknowledgments.

II. SYNTHESIS OF SPINELS

7. Previous Methods of Synthesis .8. General Method of Synthesis Used in r

gation . . . . . . .9. Synthesis of Zinc Aluminate .

10. Properties of Zinc Aluminate.11. Synthesis of Magnesium Aluminate.12. Properties of Magnesium Aluminate13. Synthesis of Ferrous Aluminate .14. Properties of Ferrous Aluminate.15. Synthesis of Manganous Aluminate.16. Properties of Manganous Aluminate17. Synthesis of Zinc Chromite18. Properties of Zinc Chromite .19. Synthesis of Magnesium Chromite20. Properties of Magnesium Chromite.21. Synthesis of Ferrous Chromite22. Properties of Ferrous Chromite23. Synthesis of Manganous Chromite24. Properties of Manganous Chromite25. Synthesis of Zinc Ferrite . . . .26. Properties of Zinc Ferrite. .27. Synthesis of Magnesium Ferrite.28. Properties of Magnesium Ferrite29. Synthesis of Ferrous Ferrite .30. Properties of Ferrous Ferrite.31. Synthesis of Manganous Ferrite.32. Properties of Manganous Ferrite

This Investi-. . . . 11

S . . 1213

. . . . 13

. . . . 1415

. . . . 15S. . . 15. .. . 16

17. . . . 17S. . . 17

S. . . 18. .. . 18S . . 19. .. . 19. .. . 20

20. . . . 2121

21

S. . 2222

. . . . 23. . . . 23

. .. . 24

. .. . 25

. .. . 25

PAGE

777789

10

: : : :

CONTENTS (CONCLUDED)

III. DETERMINATIONS OF SCRATCH HARDNESSES OF SYNTHETIC

SPINELS . . . . . . . . . . . . . 2533. Scratch Hardness Values of Synthetic Spinels. . 25

IV. DENSITIES OF SYNTHETIC SPINELS . . . . . . . 26

34. Method of Determining Densities . . . . . 2635. Density Values of Synthetic Spinels. . . . . 26

V. X-RAY ANALYSES OF SYNTHETIC SPINELS . . . . . 26

36. Importance of X-ray Diffraction Data . . . . 2637. Method of Obtaining X-ray Diffraction Data . . 2738. Computations of Lattice Dimensions of Spinels

from X-ray Diffraction Data . . . . . . 2739. Summary of Determinations of Lattice Dimensions

of Synthetic Spinels . . . . . . . . 2840. Comparison of Values of Lattice Dimensions Ob-

tained by Various Observers . . . . . . 2841. Computations of Theoretical Densities of Synthet-

ic Spinels from X-ray Diffraction Measurements 2942. Theoretical Densities of Synthetic Spinels . . 31

VI. HEAT CAPACITIES OF SYNTHETIC SPINELS . . . . . 3143. Method of Determining Heat Capacities . . . 3144. Experimental Data Concerning Heat Capacities of

Synthetic Spinels . . . . . . . . . 3745. Summary of Heat Capacity Determinations . 3746. Atomic Heat Capacities of Synthetic Spinels 38

VII. THERMAL EXPANSIONS OF SYNTHETIC SPINELS . . . 3847. Method of Determining Thermal Expansions . . 3848. Preparation of Specimens for Thermal Expansion

Tests. . . . . . . . . . . . . 4149. Thermal Expansion Measurements . . . . . 4150. Linear Thermal Expansion Coefficients of Synthetic

Spinels . . . . . . . . . . . . 41

APPENDIX . . . . . . . . . . . . . . 45Tables of Data for Computing Densities, Lattice Di-

mensions, Specific Heats, and Thermal Expansions ofSpinels

LIST OF FIGURES

NO. PAGE1. Thermal Expansion of Aluminates of Zinc, Magnesium, Iron, and

Manganese . ................... . 172. Thermal Expansion of Chromites of Zinc, Magnesium, Iron, and

Manganese . ..... ....... ..... 193. Thermal Expansion of Ferrites of Zinc, Magnesium, Iron, and

Manganese . ....... .... .... . 244. Lattice Dimensions of Synthetic Spinels . .. . . .. . . . 295. Densities of Synthetic Spinels as Computed from X-ray Data . . . 316. Apparatus for Determination of Heat Capacities of Spinels . . . . 327. Effect of Mass of Vitreous Silica on Rise of e.m.f. of Thermoelements

in Calorimeter . . . . . . . . . . . . . . . .. 358. Fizeau-Pulfrich Apparatus for Determining Linear Thermal Expansions

of Spinels . ................. . . 39

LIST OF TABLES

1. Densities of Powdered Synthetic Spinels . . . . . . . . .. 262. Lattice Dimensions of Synthetic Spinels .. . . . . . . . 283. Lattice Dimensions of Synthetic Spinels .. . . . . . . . 304. Computed Densities of Synthetic Spinels . . . . . . . . .. 305. Molar Volumes of Synthetic Spinels .. . . . . . .. . 316. Determination of Heat Capacity of Calorimeter . . . . . . .. 347. Mean Heat Capacities of Synthetic Spinels over Temperature Range of

50 to 1025 deg. C.. . . . . . . . . . . . . . . . 388. Atomic Heat Capacities of Synthetic Spinels over Temperature Range of

50 to 1025 deg. C.. . . . . . . . . . . . . . . . 399. Interferometric Data on Thermal Expansion of Platinum . . . .. 40

10. Thermal Expansion Values of Synthetic Spinels . . . . .. . . 4211. Coefficients of Mean Linear Thermal Expansion of Synthetic Spinels . . 4312. True Specific Gravities and Densities of Synthetic Spinels . . . .. 4513. Computation of Lattice Dimensions of Aluminates of Magnesium, Zinc,

Iron, and Manganese . . . . . . . . ... . . .. . 4614. Computation of Lattice Dimensions of Chromites of Magnesium, Zinc,

Iron, and Manganese . . . . . . . . . . . . . . 4715. Computation of Lattice Dimensions of Ferrites of Magnesium, Zinc, Iron,

and Manganese . . . . . . . . . . . . . . .. . 4816. Determination of Mean Heat Capacities of Synthetic Spinels over Tem-

perature Range of 50 to 1025 deg. C . . . . . . . . .. . 4917. Interferometric Data on Thermal Expansions of Synthetic Spinels. . . 52


I. INTRODUCTION

1. Introductory.-The term spinel denotes a compound which hasthe type formula R" R2'" 04, and whose structure* may be represented

as R" < - R"' = O Spinels possess the perfect symmetry of the0 - R" = 0O

isometric crystal system, and certain crystals, calcium chromite forexample, which have many characteristics in common with the truespinels, are eliminated from the spinel family since they are found tobe anisotropic. It is of common knowledge that many members ofthe spinel family intercrystallize to form one homogeneous crystal.

In this investigation the common divalent metals, zinc, magne-sium, iron, and manganese, were combined with the trivalent ele-ments, aluminum, chromium, and iron, to form groups of aluminates,chromites, and ferrites, which possess the spinel type of crystalstructure.

2. Purpose of Investigation.-Although various members of thespinel group have been synthesized during the past century, littleinformation is available relative to their physical and chemical prop-erties. Complete data are lacking even for common spinel, or magne-sium aluminate, which finds extensive use as a refractory. In thecases of ferrous chromite, or chrome-iron ore, and iron ferrite, ormagnetite, data are available for the natural minerals, which ofnecessity differ widely according to the compositions of these com-pounds. Gahnite, or zinc aluminate, has been described also, buthere again discrepancies exist, probably due to the varying compo-sitions of the minerals studied.

3. Plan of Investigation.-Accordingly, a study of a representativegroup of spinels was undertaken. This group included the aluminates,chromites, and ferrites, of zinc, magnesium, iron, and manganese,which embrace those compounds having refractory properties, andwhich are encountered frequently in the ceramic industry.

Since the investigation was primarily concerned with the prop-erties of spinels, the most convenient methods of syntheses whichproduced a fairly pure compound were adopted, and in each instance

*J. W. Mellor, Modern Inorganic Chemistry, p. 636, Longmans, Green and Co., London, 1916.

ILLINOIS ENGINEERING EXPERIMENT STATION

a method was evolved which could be repeated with the assurance ofreproducing the desired spinel. A product containing a maximum offive per cent of uncombined oxides or other impurities was consideredsatisfactory where the difficulties of synthesis precluded a higherdegree of purity. In each case the purity of the synthesized compoundwas verified by X-ray analysis, which generally indicated the presenceof considerably less than five per cent of impurity.

It has been the plan to report such properties of the syntheticspinels as the densities, lattice dimensions, thermal expansions, andspecific heats, with such other miscellaneous information as was ob-tained during the course of the investigation. The resulting datawere correlated in the light of those relations which exist among agroup of isomorphous compounds.

4. Occurrence of Spinels.-The spinels form a group of isomor-phous compounds which are found in nature as the followingminerals :*

Spinel .......................... MgA120 4Ceylonite (pleonaste) ................. (Mg.Fe) A120 4Chlorspinel.......................... M g (Al-Fe)20 4Picotite ................... .. .... .. (Mg-Fe)(Al.Fe-Cr) 20 4Hercynite ........................... FeA1AO 4Gahnite............... ........... ZnA120 4Disluite............................. (Zn-Fe-Mn)(A1-Fe)20 4Kreittonite ......................... (Zn-Fe-Mg) (A1-Fe)04M agnetite.......................... FeFe2O4Magnesioferrite ...................... MgFe20 4Franklinite......................... (Fe-Zn-Mn) (Fe-Mn)20 4

Jacobsite........................(Mn-Mg) (Fe.Mn)2 0 4Chromite..............................(Fe.Mg)(Cr.Fe) 20 4

The compositions of the naturally occurring minerals vary greatlydue to substitution by isomorphic oxides. Thus, in the case of

gahnite, or ZnAl2z 4, if ferrous iron and magnesia replace part of the

zinc, and if ferric iron replaces part of the alumina, the isomorphousmixture known as kreittonite, having the formula (Zn.Fe-Mg)(Al-Fe)20 4, is formed. The mineral franklinite, or (Fe.Zn.Mn)(Fe.Mn)20 4, represents a similar substitution.

The presence of gahnite (ZnAlO 4) in the walls of zinc retorts has

been observed by A. Scott,f who stated that "a microsection of the

(wall) material shows it to be partly crystalline and partly glassy.The original material of the matrix has been completely changed,

the alumina having reacted with oxidized zinc to form spinel, while

*W. S. Bayley, Descriptive Mineralogy, p. 196, Appleton, New York, 1917.tA. Scott, Trans. Ceram. Soc., England, 18, 512, 1918.


the silica appears partly as tridymite and partly as silicates of zincand iron."

Other spinels may be formed under the conditions existing inindustrial processes; thus, manganous aluminate is found in slags*although it is not found pure in nature; magnetite, or ferrous ferrite,is a furnace product and forms the iron scale of the blacksmith.tIn fact, such controlled conditions are more conducive to the forma-tion of pure spinels than those usually found in nature. The list ofnaturally occurring spinels previously given attests to this truth,since most of them which occur in nature are isomorphous mixtures ofvarious spinels. Even the common spinel, chromite, is not found pureexcept in meteorites.T Apparently the controlled conditions avail-able in the laboratory are usually necessary for the formation of purespinels.

5. Uses of Spinels.-In many metallurgical processes which re-quire refractories with exceptional qualities, advantage is taken ofthe resistance which spinels offer to the attack of acids, alkalies, andordinary fusion agents, as well as of the high melting points whichthey generally possess.

Bricks of chromite and magnesia spinel are in common use, andtheir application would be more widespread were it not for the com-paratively high cost of these refractories. The ordinary magnesiaspinel finds a restricted use in certain refractory porcelains. It hasbeen suggested also as a paint pigment. The transparent variety hassome value as a gem stone. Magnetite has been cast for use aselectrodes in the electrodeposition of copper. Heilmann¶ and Mayerand Havas§ proposed using zinc aluminate as an opacifier in enamels,and took out patents covering this usage. Quoting from a Bureau ofStandards report** with regard to this application of the compound:"When artificial spinels of these compositions (ZnAl20 4 and MgA120 4) are calcined at extremely high temperatures so as to be prac-tically insoluble in molten enamels they make satisfactory opacifyingagents. Most of the spinels put on the market have not been renderedsufficiently insoluble and therefore have partially dissolved in theenamel. This has reduced their opacifying power and at the sametime raised the fusion point of the enamels quite materially. Theyproduce pure white enamels of high gloss and are both cheap and

*J. Loczka, Z. Kryst. Mineral., 43, 571, 1907.tF. W. Clarke, The Data of Geochemistry, U. S. Geol. Survey, Bul. 770, Washington, Ed. 5,

p. 348, 1924.tL. W. Fisher, Am. Mineral., 14, 356, 1929.IE. Heilmann, Brit. Pat., 26, 498, 1912, taken from C. A. 8, 1652, 1914.§M. Mayer and B. Havas, U. S. Pat. 1104266, 1914, taken from C. A. 8, 3106, 1914.*H. F. Staley, Bur. Standards Technol. Paper No. 142, p. 58, 1919.


non-poisonous. The use of spinels as opacifiers has many desirablefeatures, but at present must be considered to be in the experimentalstage."

The great resistance which zinc aluminate and magnesium alum-inate offer to the attack of the fusion agents which are used in theirchemical analyses suggests that these spinels might be of value asglass furnace refractories. Furthermore, the slow solution of thesecompounds would not introduce any deleterious substances in theglass since zinc, aluminum, and magnesium are often incorporatedin glass batches.

In the series of spinels which were a part of this investigation,the orange color of the ferrites of zinc and magnesium are pleasingto the eye, and might be useful in enamels or glazes. The olive greencolor of zinc chromite is also agreeable, although it presents a grayishhue resembling suede when viewed under artificial light. Magnesiumchromite might be useful also. It is a very dark olive green in color.

6. Acknowledgment.-This investigation is a contribution of theEngineering Experiment Station of the University of Illinois, whichis under the administrative direction of DEAN M. S. KETCHUM.The work was done in the Department of Ceramic Engineering, ofwhich PROF. C. W. PARMELEE is the head. The X-ray diffractionpatterns of the synthetic spinels were obtained with the equipmentof the Chemistry Department, and the computations of their latticedimensions were made with the advice of PROF. G. L. CLARK. Aconsiderable part of the experimental work involved in the synthesesand the determinations of properties was carried out by ABDE ALLY,whose efficient aid is gratefully recognized.

II. SYNTHESES OF SPINELS

7. Previous Methods of Synthesis.*-There are many methodsavailable for the synthesis of individual spinels, and some of thesemay be used for the syntheses of all the members of a group of spinels.The "wet" methods of Berzelius,t Sander,t and others were tried anddiscarded on account of the difficulty experienced in filtering andwashing the gelatinous precipitates which were formed. The methodsof Daubree,¶ Deville and Caron,§ and other investigators involvedthe vaporization of metallic fluorides in the presence of lime or boric

*In this section extensive use was made of J. W. Mellor's "A Comprehensive Treatise on Inorganicand Theoretical Chemistry," Longmans, Green and Co., London, 1924.

tJ. J. Berzelius, "Lehrbuch der Chemie," 2, 310, Dresden, 1826.tT. Sander, Liebigs Ann. Chem. 9, 181, 1834.¶A. Daubree, Compt. rend. 39, 135, 1854.§H. Ste.-C. Deville and H. Caron, Compt. rend. 46, 764, 1858.


acid. These methods presented experimental difficulties for the prep-aration of any appreciable amounts of spinel, and were discarded.Some spinels may be prepared by direct union of the oxides whenheated to high temperatures. This method is applicable when thecomponent oxides are relatively stable, but fails in the cases of zinc,iron, and manganese compounds.

A general account of these syntheses may be found in Meunier'streatise* on the subject. More recently Rinnef has described thesyntheses of a large series of spinels.

8. General Method of Synthesis Used in This Investigation.-In 1848M. Ebelmen published a paperS on the syntheses of certain minerals.In this and a subsequent publication¶ he described the syntheses ofmany spinels by an original method, which consisted in the prolongedheating of the combining oxides in the presence of boric oxide. Theboric oxide acted as a flux and a solvent for the oxides which wereenabled to combine at a much lower temperature than would benecessary without the admixture of this flux. During the heat treat-ment the boric oxide was slowly volatilized and the synthetic spinelswhich were formed by the union of oxides remained in the form ofbeautiful octahedra, attaining a length of 5-6 mm. in some cases.

Ebelmen's method is clearly analogous to the formation of crystalsfrom an aqueous solution when the liquid is allowed to evaporateslowly. His experiments were carried out in the porcelain kilns atSevres, which provided the heat treatment of 24 to 30 hours whichwas necessary to volatilize most of the boric oxide. Even under theseconditions some boric oxide remained at the end of the test in theform of borates, and these compounds were removed by leaching withhydrochloric acid. Ebelmen prepared the aluminates of magnesium,manganese, iron, beryllium, cadmium, zinc, cobalt, barium, and cal-cium, besides many chromites and ferrites. Artificial minerals ofmore diverse nature have since been prepared by Mourelo§ accordingto Ebelmen's general procedure.

In the following investigations a mixture of a divalent and atrivalent oxide with boric acid was heated at temperatures rangingfrom 900 to 1300 deg. C. according to the compound sought, and theheating was continued for 24 to 48 hours in order to permit the boricoxide to escape. At these temperatures combination of the oxidesensued with the formation of a spinel. The reaction between the

*S. Meunier, "Les M6thodes de Synthese en Mingralogie," Baudry, Paris, 1891.tF. Rinne, Fortschritte Mineral. Kryst. Petrog. 12, 68, 1927, taken from C. A. 23, 1842, 1929.IM. Ebelmen, Ann. chim. phys. 3, 22, 211, 1848.¶M. Ebelmen, ibid., 3, 33, 34, 1851.§J. R. Mourelo, Rev. g6n. sci. 26, 394, 1915.


oxides was accompanied by a change in color and a considerableshrinkage. The mixture of oxides was usually pressed into briquetsto which a little dextrin solution was added for a binder, and thebriquets were placed on a layer of oxide corresponding to the basicoxide of the spinel in order to avoid contamination by the furnacerefractory.

In the syntheses in which the reaction took place with greatfacility, the oxides were introduced in molecular proportions. Ifexperience showed that the synthesis proceeded so slowly that apprec-iable amounts of uncombined oxides were likely to remain in theproduct, quite a large excess of the basic oxide was introduced inorder that no acidic oxide remain after the synthesizing period, sinceany uncombined acidic oxides, notably A120 3 and Cr20 3, could beremoved by acid leaching only with difficulty. On the conclusion ofthe heat treatment necessary for the synthesis the excess basic oxidewas dissolved in the acid used for the leaching treatment.

The leaching operation was made on the pulverized syntheticmaterial which had been passed through a 100-mesh sieve. The con-centration of the acid was varied from ten per cent upwards accordingto the material under treatment, and the leaching was continued on asteam bath for about 24 hours.

Details concerning the syntheses of the individual spinels areconsidered under the separate headings.

Zinc Aluminate (ZnAl20 4)

9. Synthesis of Zinc Aluminate.-J. J. Berzelius* found that alum-inum hydroxide abstracts zinc oxide from an aqueous solution ofammoniacal zinc oxide, and that zinc aluminate is precipitated fromthe same solution by adding a saturated solution of potassium alum-inate. T. Sandert reported a precipitate of zinc aluminate on mixingsolutions of potassium aluminate and ammonium zincate. J. J. Ebel-ment and J. R. Mourelo¶ synthesized gahnite by heating a mixtureof zinc oxide, alumina, and boric oxide at a bright red heat for 18hours in a porcelain muffle. The same product was obtained byS. Meunier§ on heating to bright redness a mixture of zinc oxide,alumina, cryolite, and aluminum chloride in a graphite crucible.A. Daubree** made gahnite by passing vapors of zinc and aluminumchlorides over red hot lime. H. Ste.-C. Deville and H. Carontfvaporized zinc and aluminum fluorides in the presence of boric oxide.

*J. J. Berzelius, loc. cit.tT. Sander, loc. cit.J. J. Ebelmen, loc. cit.J. R. Mourelo, loc. cit.

§S. Meunier, loc. cit.**A. Daubree, loc. cit.ttH. Ste.-C. Deville and H. Caron, loc. cit.


The method of synthesis which was adopted was carried throughas follows: Zinc and aluminum oxides were weighed out in themolecular proportions of 1.5 : 1, and half of the total weight of boricacid was added. The three substances were then ground intimatelyin a mortar. A small amount of dextrin solution was mixed in to actas a binder, and the mix was briquetted in a small press. The briquetswere dried over night at 110 deg. C. and then heated at 1115-1130deg. C. in a gas-fired muffle for about 24 hours. An oxidizing atmos-phere was maintained to minimize the reduction of ZnO and itsconsequent loss by volatilization. A chemical analysis of the briquetafter concluding the heat treatment showed that little uncombinedZnO remained.

The crude spinel was crushed and ground to pass through a 100-mesh screen, and the resulting powder was then digested with twenty-per-cent HC1 for 48 hours on a steam bath, in order to remove anyuncombined ZnO. After the purified spinel had been washed anddried, microscopic examination showed small inclusions of a birefring-ent material (probably aluminum borate) in the spinel crystals. Thematerial was therefore ground still further and digested for severaldays in hydrochloric acid as before, when a very pure product wasobtained as proven by X-ray analysis.

10. Properties of Zinc Aluminate.-The pure zinc aluminate wasthus obtained in the form of a white powder, of hardness 7.5 onMohs' scale. Its density at 20 deg. C. was found to be 4.54 gramsper cu. cm., which agrees well with the value of 4.55 given by Larsen*and Ebelmen's value of 4.58 as listed by Groth.t X-ray analysisshowed that it has a unit cell dimension of 8.062 A.U., correspondingto a theoretical density of 4.615 grams per cu. cm. Its mean heatcapacity from 50 to 1025 deg. C. is 0.92 joule per gram per deg. C.The coefficient of mean linear thermal expansion from 25 to 900deg. C. is 91 X 10- 7 cm. per cm. per deg. C. Figure 1 depicts thecourse of the thermal expansion curve as obtained from interfero-metric data,t and expansion coefficients over intervals below 900deg. C. may be found in Table 11.

Zinc aluminate resists the attack of fusion agents, such as sodiumcarbonate, potassium bisulphate, and sodium peroxide, but dissolvesin a mixture of sodium carbonate and borax. When subjected to thetest described in the following, it was found to dissolve somewhat inconcentrated H2SO 4, but to be insoluble in other acids and alkalies.

*E. S. Larsen, "The Microscopic Determination of the Nonopaque Minerals," U. S. Geol. SurveyBul. No. 679, Washington, p. 180, 1921.

tP. Groth, Chemische Krystallographie, Vol. 2, p. 753, Engelmann, Leipzig, 1908.JSee p. 42.


Zinc aluminate, ground to pass a 300-mesh screen, was washed inhydrochloric acid to remove impurities introduced in grinding andscreening. After the material had been calcined, samples of one grameach were placed in large Pyrex test tubes, and to each was added100 ml. of the following reagents:

(1) Hydrochloric acid, concentrated.(2) Nitric acid, concentrated.(3) Aqua regia, mixture of above acids.(4) Sulphuric acid, 60 deg. Be.(5) Sodium hydroxide, 30 per cent solution.

(6) Hydrofluoric acid, concentrated (test tube ceresine coated).

The tubes were allowed to stand for 30 days, being shaken occa-sionally. The sulphuric acid and sodium hydroxide solutions wereheated periodically. The other solutions were kept at constant vol-ume by periodic additions of the concentrated reagents. At theexpiration of this time the solutions were examined for zinc, controlsbeing examined simultaneously. In the case of the sulphuric acid azinc content of 0.0015 g. was found; all other titrations correspondedto the blanks.

Other properties of zinc aluminate which have been reported in-clude the index of refraction, which Larsen* lists as 1.80 .

Magnesium Aluminate (MgAl20 4)

11. Synthesis of Magnesium Aluminate.-Since both MgO andA1203 are stable at high temperatures, the general method used toform synthetic magnesia spinel is to heat a mixture of these oxides toa temperature near 1700 deg. C., when combination occurs. Theaddition of boric oxide permits the use of lower temperatures.The batch used in the synthesis of MgA1204 consisted of

MgO, 181 parts by weightAl20s, 306 parts by weightHaBOs, 244 parts by weight

which signifies a molecular ratio of MgO to A120 3 of 1.5 to 1. Thebatch was ground intimately in a porcelain mortar and then mois-tened with a ten-per-cent dextrin solution. Briquets were pressedfrom this mass and dried at 110 deg. C. They were then placed in acrucible, the bottom of which was covered with MgO to preventcontamination of the batch by the crucible material. The mixturewas heated in a gas furnace for 36 hours at temperatures of 1275 to1300 deg. C.

On the conclusion of the heat treatment the synthetic material wasground in an iron mortar to pass through a 100-mesh sieve. Any iron

*E. S. Larsen, loc. cit., p. 178.


introduced by this grinding was removed with a magnet, and thepowdered material was then leached with twenty-per-cent HC1 for20 hours on a steam bath. After the leaching treatment the purifiedspinel was washed and calcined to 350 deg. C. X-ray analysis showeda high degree of purity of this product.

12. Properties of Magnesium Aluminate.-Pure magnesia spinelwas obtained as a white powder resembling zinc aluminate in itsappearance. Its scratch hardness is about 7.5. Its density at 20 deg.C. was found by pycnometer measurements to be 3.53 grams percu. cm. which agrees with Larsen's* value of 3.6 ±. The side of its unitcell is 8.086 A.U., and from this value a theoretical density of 3.548grams per cu. cm. may be computed. Rinnet found a correspondingvalue of 3.5775. Its mean heat capacity from 50 to 1025 deg. C. is1.19 joules per gram per deg. C. Table 11 lists the thermal expansiondata, from which its coefficient of mean linear thermal expansion is

a9 = 81 X 10- 7 cm. per cm. per deg. C. Figure 1 illustrates thecourse of the thermal expansion curve.

Other properties of magnesium aluminate are its index of refrac-tion, which is given by Larsent as 1.723±. Its fusion point is givenby Winchell¶ as 2135 deg. C. This authority states that it is slowlysoluble in H 2S0 4, but insoluble in most other reagents. A recentdetermination§ of the mean thermal expansion of a commercial grademagnesia spinel brick was found to be 86 X 10- 7 cm. per cm. perdeg. C. between 20 and 1400 deg. C. Rieke and Blicke** give a meancoefficient of 67 X 10- 7 between 25 and 800 deg. C. on a 7-cm. sample.

Ferrous Aluminate (FeA120 4)

13. Synthesis of Ferrous Aluminate.-Considerable difficulty wasencountered in the synthesis of ferrous aluminate. Some preliminaryexperiments, which were made with a mixture of ferrous scale, alumi-num oxide, and boric oxide, were unsuccessful, due to the tendency ofmagnetite to form in preference to ferrous aluminate. The synthesisof ferrous aluminate was finally effected by placing a briquet of thesematerials on a layer of metallic iron and enclosing the whole in asagger to minimize oxidation. However, the conditions during thissynthesis were not definitely, known, and additional experiments weremade in an attempt to formulate a more satisfactory procedure.

*E. S. Larsen, loc. cit., p. 177.tF. Rinne, Neues Jahrb. Mineral. Geol. 58 A, 70, 1928.tE. S. Larsen, loc. cit., p. 177.¶N. H. and A. N. Winchell, Elements of Optical Mineralogy, Part 2, Ed. 2, p. 63, Wiley, New

York, 1927.Bur. Standards Technical News Bul. No. 152, p. 121, 1929.*R. Rieke and K. Blicke, Ber. deut. keram. Ges. 12, 181, 1931.


If a mixture of A1203 with powdered metallic iron were heated at1200 deg. C. in a current of steam, this treatment should produceFeO, which should in turn react with the A120 3 present to formFeAl20 4. This experiment did not result in the formation of thedesired spinel, but the lack of success might have been due to the factthat air was not excluded perfectly from the charge.

Another set of experiments was made, using a mixture of powderediron, magnetite, and aluminum oxide, in the proper proportions toform FeAl20 4. These materials were heated in an iron bomb in orderto exclude air, but the results of the test were not encouraging.

Successful results were finally obtained by the use of the followingbatch:

Fe203, 479.1 parts by weightAl, 54.2 parts by weightA120 3, 511.0 parts by weight

The finely powdered materials were placed in a covered crucible andheated for about 32 hours in a gas furnace in a strongly reducing at-mosphere. The temperature as measured by an optical pyrometervaried from 1320 to 1420 deg. C.

Under these conditions the following reaction would be expected:

2 Al + 3 Fe20 3 = 6 FeO + A120s,

and since ferrous oxide is a violent flux, it should unite with the Al 20 3present to form FeAl20 4. The proportions in the batch were sochosen that no excess oxides were present.

The material synthesized according to these directions was ob-tained as a crumbly black mass. It was ground in an iron mortar topass through a 120-mesh sieve, and was then digested with twenty-per-cent HCI on a steam bath. The washed and dried product wasthen analyzed by X-rays to prove its purity.

14. Properties of Ferrous Aluminate.-Pure ferrous aluminate wasprepared as a black powder having a hardness of about 7.5. At 20deg. C. its density is 4.22 grams per cu. cm., which compares with avalue of 3.9 given by Larsen.* Its theoretical density was computedas 4.392 on the basis of a lattice dimension of 8.119 A.U. Its meanheat capacity from 50 to 1025 deg. C. is 1.04 joules per gram per deg.C. Table 11 shows that the coefficient of mean linear thermal expan-sion from 25 to 850 deg. C. is 90 X 10- 7 cm. per cm. per deg. C.Figure 1 shows the course of the thermal expansion curve.



/00

S80 -- - - - - - -- - -- - -- - - - - - - -.

.A/c mleli

0 /00 00 300 400 500 600 700 800 900Temp7erafcre /in de. C..

FIG. 1. THERMAL EXPANSION OF ALUMINATES OF ZINC, MAGNESIUM,IRON, AND MANGANESE

The index of refraction of ferrous aluminate (hercynite) is givenas 1.80± by Larsen.*

Manganous Aluminate (MnAi20 4)

15. Synthesis of Manganous Aluminate.-The batch used for thesynthesis of manganous aluminate consisted of

MnCO 3, 227 parts by weightAl203, 153 parts by weight

which represents an excess of MnO over the theoretical combiningproportions. It was not necessary to use a flux of boric oxide in thesynthesis of this spinel.

The charge was heated in an electric furnace for 24 hours at1000 ± 15 deg. C. Microscopic examination then showed inhomo-geneities in the batch, so the substance was reground and subjectedto the same heat treatment. After this second heat treatment thecrude spinel was ground to a powder and leached with ten-per-centHC1 to remove the excess manganese compound. X-ray analysisdid not show diffraction patterns of any impurity in the remainingpowder.

16. Properties of Manganous Aluminate.-Pure manganous alu-minate was prepared as a chocolate-colored powder with a scratchhardness of about 7.5. Its density at 20 deg. C. was determined to


bPkF


be 4.01 grams per cu. cm., which compares with Winchell's value* of4.05 grams per cu. cm. and that of 4.12 as listed in the InternationalCritical Tables.t The theoretical density is 4.031 grams per cu. cm.,as computed from a unit cell dimension of 8.271 A.U. Its mean heatcapacity is 0.89 joule per gram per deg. C. over the temperature rangeof 50 to 1025 deg. C. Its expansion curve is shown in Fig. 1, andTable 11 states that the coefficient of mean linear thermal expansionfrom 25 to 900 deg. C. is 73 X 10- 7 cm. per cm. per deg. C.

Few properties of this spinel are known. Winchellt lists the indexof refraction as about 1.8.

Zinc Chromite (ZnCr20 4)

17. Synthesis of Zinc Chromite.-Zinc chromite was synthesizedby Ebelmen¶ in 1851 by heating zinc and chromic oxides togetherto a white heat.

The batch which was used in this investigation consisted of anintimately ground mixture of ZnO and Cr20 3 in the molecular pro-portions of 1.5 : 1, with the addition of half their combined weights ofH3 B0 3 . This mixture was pressed into briquets after the addition of asmall amount of ten-per-cent dextrin solution. The briquets werefired in a gas furnace for about 40 hours at 1000-1100 deg. C., and theywere then powdered and leached on a hot plate with twenty-per-centHC1 for about 24 hours. X-ray analysis showed a pure product.

18. Properties of Zinc Chromite.-An olive-green-colored powderwith an unctuous feel was obtained in the synthesis of zinc chromite.Its hardness is about 6.5 on Mohs' scale. At 20 deg. C. it has adensity of 5.30 grams per cu. cm., a value which agrees closely withEbelmen's old value of 5.309. A lattice dimension of 8.296 A.U.corresponds to a theoretical density of 5.393 grams per cu. cm. Themean heat capacity from 50 to 1025 deg. C. is 0.77 joule per gramper deg. C. The coefficient of mean linear thermal expansion is

920 = 101 X 10- 7 cm. per cm. per deg. C., as shown by Fig. 2 andTable 11.

This spinel was found to be infusible in sodium carbonate or in amixture of sodium carbonate and borax. When fused with potassiumbisulphate, a residue of Cr 203 remained which was dissolved byprolonged fusion with sodium peroxide.

*N. H. and A. N. Winchell, loc. cit., p. 63.tI. C. T. I, 137.

RN. H. and A. N. Winchell, loc. cit., p. 63.[M. Ebelmen, loc. cit.


/00 ---

8ognes/'um Chrom/ir , / .

Manglanous (C'/rom'Iwe

7 ^ ^ Ferrous C/,rom/f7/e40 --

0 /00 200 300 400 -00 600 700 800 900Te-'pera'/re //7 adeg'. C.

FIG. 2. THERMAL EXPANSION OF CHROMITES OF ZINC, MAGNESIUM,IRON, AND MANGANESE

Magnesium Chromite (MgCr2O 4)

19. Synthesis of Magnesium Chromite.-The batch used in thesynthesis of MgCr20 4 consisted of

MgO, 121.0 parts by weightCrOa, 456.0 parts by weightHsB0 3, 288.5 parts by weight

These amounts represent molecular proportions of MgO and Cr20O,which were mixed with half their combined weights of H3BO 3 bygrinding together in a mortar. A small amount of ten-per-centdextrin solution was added as a binder for the powdered material,and the mass was formed into briquets in a small press. The briquetswere then dried at 110 deg. C. and heated to 1275-1325 deg. C. for18 hours to effect the combination of the oxides.

The synthetic product was crushed and ground to pass through a100-mesh sieve, and then leached with twenty-per-cent HC1 on asteam bath for 18 hours. The purified spinel was then dried at350 deg. C., and X-rayed to prove its purity.

20. Properties of Magnesium Chromite.-Pure magnesium chro-mite powder has a deep olive green color with plainly visible crystal-line spangles. It has a scratch hardness of about 6.5 on Mohs' scale.Its actual density at 20 deg. C. is 4.41 grams per cu. cm. which agreeswith Groth's value* of 4.415. The theoretical density as obtainedfrom X-ray measurements is 4.429 grams per cu. cm., which corresponds

*P. Groth, loc. cit., p. 750.


to a unit cell dimension of 8.305 A.U. The average heat capacity overthe temperature interval of 50 to 1025 deg. C. is 0.92 joule per gramper deg. C. Its coefficient of mean linear thermal expansion isa90 = 93 X 10- 7 cm. per cm. per deg. C., as shown by Table 11 and

Fig. 2.Magnesium chromite was found to resist the attack of fluxing

agents, but fusion was completed in two hours using a mixture ofsodium carbonate and borax.

Ferrous Chromite (FeCr20 4)

21. Synthesis of Ferrous Chromite.-Great difficulty was encoun-tered in synthesizing this commonly occurring mineral. After theusual boric oxide method failed, it was hoped that a highly reducingatmosphere might aid in the formation of ferrous chromite. Thesehopes were in vain, and magnetite was usually formed instead ofchromite. Finally FeCr 20 4 was synthesized by heating a mixture ofFesO 3 and Cr203 in molecular proportions to a temperature rangingfrom 1600 to 1675 deg. C. for 3 hours in a surface combustion furnace.X-ray analysis showed about 95 per cent of FeCr20 4.

These troubles in synthesizing chromite should be expected fromthe experiences of other investigators. For instance, Meunier usedan ingenious method of synthesis by oxidizing an alloy of iron andchromium in which these metals were in molecular proportions.

22. Properties of Ferrous Chromite.-Pure ferrous chromite is abrownish-black substance of hardness about 6. Its density at 20deg. C. was found to be 4.93 grams per cu. cm. by the pycnometermethod. Larsen* gives a value of 4.5 for the density. Its latticedimension was found to be 8.344 A.U. from which the theoreticaldensity was computed as 5.085 grams per cu. cm. Its mean heat capac-ity over the temperature interval of 50 to 1025 deg. C. is 0.82 jouleper gram per deg. C. Table 11 shows that the coefficient of meanlinear thermal expansion is as0 = 85 X 10- 7 cm. per cm. per deg. C.This value agrees quite well with Austin's determinationt of a80 =82.5 X 10- 7, which applies to a sample of natural chromite. Thecourse of the thermal expansion curve of synthetic ferrous chromiteis depicted in Fig. 2.

The value of the index of refractionS for natural minerals variesfrom 2.07 to 2.16.

*E. S. Larsen, loc. cit., p. 181.tJ. B. Austin, J. Am. Ceram. Soc. 14, 807, 1931.TE. S. Larsen, loc. cit., p. 181.


Manganous Chromite (MnCr20 4)

23. Synthesis of Manganous Chromite.-The batch used for theproduction of manganous chromite contained no addition of boricoxide flux. It was as follows:

MnCO 3, 172.5 parts by weightCr 20 3 , 152.0 parts by weight

A 1.5 :1 molar ratio of MnO : CrsO 3 is represented in this batchwhich was briquetted in the customary fashion.

The charge was fired in an electric furnace for 24 hours at 1000 +15 deg. C. Microscopic examination showed that the batch was notaltogether homogeneous, so the briquets were reground, formed intobriquets again, and refired to the same extent. The briquets werethen ground to pass through a 100-mesh sieve and leached withten-per-cent HC1 to remove the excess manganese compound.X-ray examination then showed a pure product.

24. Properties of Manganous Chromite.-Pure manganous chro-mite was obtained as a chocolate brown powder of hardness about 6.5.Its density at 20 deg. C. is 4.80 grams per cu. cm. The InternationalCritical Tables* list a value of 4.87. The side of its unit cell is 8.436A.U., from which a theoretical density of 4.900 grams per cu. cm. maybe computed. Its mean heat capacity over the temperature rangefrom 50 to 1025 deg. C. is 0.74 joule per gram per deg. C. Figure 2shows the course of its thermal expansion curve, and Table 11 indi-cates a coefficient of mean linear thermal expansion equal to 89 X10- 7 cm. per cm. per deg. C. from 25 to 900 deg. C.

Zinc Ferrite (ZnFe20 4)

25. Synthesis of Zinc Ferrite.-Zinc ferrite was prepared bySwartz and Krauskopft by heating an intimate mixture of ZnO andFe203 at about 650 deg. C.

For the synthesis of zinc ferrite the following batch was weighedout:

ZnO, 244.1 parts by weightFe20, 479.0 parts by weightH3BOa, 362.0 parts by weight

which represents molecular amounts of zinc oxide and ferric oxidewith the addition of half of the total weight of boric acid. The sub-stances were ground together in a porcelain mortar. A small amountof ten-per-cent dextrin solution was mixed into this mass to act as a

*I. C. T. I, 133.tC. E. Swartz and F. K. Krauskopf, Mining Met. 9, 28, 1928.


binder, and briquets were then formed in a small press. These weredried at 110 deg. C. and heated for 18 hours at 1100 deg. C. Thefired product was ground so as to pass through a 100-mesh sieve andleached with ten-per-cent HC1. X-ray analysis was used in order toprove the purity of the final product.

26. Properties of Zinc Ferrite.-Pure zinc ferrite obtained as apowder is deep orange in color, and has a scratch hardness of about5.5. It is non-magnetic and thus differs from the other ferrites whichwere studied, since all the latter show strong ferromagnetism. Itsdensity at 20 deg. C. is 5.27 grams per cu. cm., and its theoretical densitywas computed as 5.322 grams per cu. cm. from a lattice dimension of8.423 A.U. Groth* lists variations in densities of various samples asranging from 5.132 to 5.33. The average heat capacity was found tobe 0.73 joule per gram per deg. C. from 50 to 1025 deg. C. Thecoefficient of mean linear thermal expansion is 99 X 10- 7 over thetemperature interval of 25 to 850 deg. C. as shown by Table 11 andFig. 3. This spinel is appreciably soluble in concentrated acids, and isreadily fusible in potassium bisulphate.

Magnesium Ferrite (MgFe20 4)

27. Synthesis of Magnesium Ferrite.-This spinel was made byH. Ste.-C. Devillet who heated magnesia and ferric oxide in a streamof HC1. It was also formed by Forestier and Chaudront by precipita-tion with NaOH from a neutral mixture of ferric and magnesiumchlorides.

In the present series of experiments some high temperature testswere made in attempts to form MgFe20 4 from a mixture of the oxides,but it seemed that magnetite formed in preference to magnesiumferrite. The following low temperature experiment was successful:

The batch consisted of powdered MgO and Fe20 3 in the molecularratio of 1.5 : 1. The blend of oxides was mixed with a small amountof dextrin solution and pressed into a briquet. The briquet was driedand then heated to 960 deg. C. for 22 hours in an electrically heatedfurnace. After this heat treatment the briquet, consisting of MgFe204with excess MgO, was ground to a powder which was passed througha 100-mesh sieve and leached with twenty-per-cent hydrochloric acidfor 18 hours on a steam bath, and then the purified spinel was washedand dried at 110 deg. C. X-ray analysis proved its purity.

*P. Groth, loc. cit., p. 754.tH. Ste.-C. Deville, loc. cit.1H. Forestier and G. Chaudron, Compt. rend. 181, 509, 1925.


28. Properties of Magnesium Ferrite.-Powdered magnesium fer-rite resembles zinc ferrite in color, being a deep orange. It differsmarkedly from zinc ferrite in its magnetic properties, being stronglyferromagnetic. Its hardness is about 5.5 on Mohs' scale. An actualdensity of 4.61 grams per cu. cm. may be compared with a theoreticalvalue of 4.506 computed from a lattice dimension of 8.366 A.U.Winchell* lists a specific gravity of 4.6. The average heat capacityis 0.86 joule per gram per deg. C. from 50 to 1025 deg. C. Figure 3shows the course of the thermal expansion curve, and Table 11 affordsa value of the coefficient of mean linear thermal expansion of 129 X10-7 cm. per cm. per deg. C. from 25 to 900 deg. C.

The index of refraction of magnesium ferrite is very high, asWinchellt lists a value of nLi = 2.35.

Ferrous Ferrite (FeFe20 4)

29. Synthesis of Ferrous Ferrite.-Magnetite, or ferrous ferrite,was synthesized by Ebelmen,t who prepared it by fusing an ironsilicate and lime. H. Ste.-C. Deville¶ prepared magnetite by heatingferrous oxide in a stream of HC1. M. Sidot§ obtained magnetite bythe calcination of ferric oxide alone, and his method was applied inthe following preparation of magnetite from ferric oxide.

When Fe20 3 is heated in air above about 1348 deg. C. it losesoxygen, giving products intermediate in composition and approachingmagnetite as the heating continues and the temperature rises.**The dissociation pressure becomes one atmosphere at 1430 deg. C.

Small briquets were pressed from pure Fe203 without the additionof any binding material. These were placed in a gas-fired furnace andheated for five hours at 1350 to 1475 deg. C. At the end of the heattreatment an iron pipe was thrust into the crucible which containedthe briquets, and nitrogen gas from a cylinder was blown on thebriquets in an attempt to cool them rapidly in a neutral atmosphere.Also the crucible was removed from the furnace, in order to aid in therapid cooling of the charge. When the briquets were at a dull redheat they were dumped out of the crucible, since the latter retained ahigh temperature for some time. The whole cooling operation did nottake over five minutes, and the cooling through the sensitive hightemperature range where oxidation occurs readily must have been

*N. H. and A. N. Winchell, loc. cit., p. 63.tN. H. and A. N. Winchell, ibid.

M. Ebelmen, Compt. rend. 33, 525, 1851.TH. Ste.-C. Deville, Compt. rend. 53, 199, 1861.§M. Sidot, Compt. rend. 69, 201, 1869.**0. C. Ralston, U. S. Bur. Mines Bul. No. 296, p. 8, Washington, 1929.


/40

K /'-/

3rFerri -Ferr u ferrof

6 80 / 'ni

40

0 L

ap / -i Ferrc /-

00 /00 200 300 4602 5620 600 702 80 2 92T0dperrwoon /t 1 deg. C.

FIG. 3. THERMAL EXPANSION OF FERRITES OF ZINC, MAGNESIUM,IRON, AND MANGANESE

very rapid. After the briquets cooled they were ground to a powderin an iron crusher and later leached with dilute hydrochloric acid.The purity of the magnetite was verified by X-ray analysis.

30. Properties of Ferrous Ferrite.-Pure ferrous ferrite was ob-tained in the form of a sparkling black powder. This compound isthe commonly known mineral magnetite, and it shows strong ferro-magnetism. It has a scratch hardness of about 5.5. Its density at20 deg. C. was found to be 5.18 grams per cu. cm. Winchell* lists avalue of 5.17 and Grotht gives it as 5.2-5.25. The side of its unit cellis 8.374 A.U., from which its theoretical density was found to be5.201 grams per cu. cm. The mean heat capacity was found to be 0.96joule per gram per deg. C. over the temperature range of 50 to 1025deg. C. It is of interest to note that FeO30 4 undergoes an inversionSat 593 deg. C. and the resulting heat effect is included in the averagevalue given. This temperature¶ is closely associated with the Curie

N. H. and A. N. Winchell, loec. cit., p. 64.tP. Groth, loc. cit., p. 753.0. C. Ralston, loc. cit., p. 68.

¶O. C. Ralston, ibid.


point, beyond which magnetite becomes non-magnetic. Figure 3shows the course of its thermal expansion curve, and Table 11 liststhe values of the coefficients of mean linear thermal expansion, fromwhich a ',0 = 151 X 10- 7 cm. per cm. per deg. C.

Manganous Ferrite (MnFe20 4)

31. Synthesis of Manganous Ferrite.-The spinel manganous fer-rite was readily synthesized by heating the following batch in anelectric furnace for 40 hours at about 1000 deg. C. The relativelylow temperature is advisable since magnetite tends to form at moreelevated temperatures. The batch consisted of

MnCO3, 459.6 parts by weightFe203, 479.1 parts by weightH3BOs, 346.0 parts by weight

The excess manganese in the resulting spinel, as well as any boratespresent, were removed by digesting with HC1. X-rays were used toverify the purity of the final product.

32. Properties of Manganous Ferrite.-Pure manganous ferritewas obtained as a black powder, strongly ferromagnetic, of hardnessabout 5.5. Its density at 20 deg. C. is 4.74 grams per cu. cm., whichagrees well with Winchell's value* of 4.75. The side of the elementaryunit cell is 8.457 A.U. from which a theoretical density of 5.029grams per cu. cm. was computed. Its mean heat capacity is 0.87 jouleper gram per deg. C. from 50 to 1025 deg. C. Figure 3 and Table 11show the thermal expansions at various temperatures, from which

2a = 83 X 10- 7 cm. per cm. per deg. C.

III. DETERMINATIONS OF SCRATCH HARDNESSES OF

SYNTHETIC SPINELS

33. Scratch Hardness Values of Synthetic Spinels.-By using min-erals of known hardness values, the scratch hardnesses of the series ofsynthetic spinels were determined approximately. The results indi-cated that the ferrites of Zn, Mg, Fe, and Mn all have a Mohs' scalehardness number of about 5.5; the chromites of these metals havevalues near 6.5, while the aluminates attain a hardness of about 7.5.These approximate values are in inverse order to the size of the latticedimensions, e.g., the aluminates have the smallest-and hence themost compressed-lattices, and this results in greater hardness asa rule.

*N. H. and A. N. Winchell, loc. cit., p. 64


TABLE 1

DENSITIES OF POWDERED SYNTHETIC SPINELS

Determined by pycnometer measurements at 20 deg. C.

Densitygrams per cu. cm.

DivalentElement

Aluminates Chromites Ferrites

Zn 4.54 5.30 5.27Mg 3.53 4.41 4.61Fe 4.22 4.93 5.18Mn 4.01 4.80 4.74

IV. DENSITIES OF SYNTHETIC SPINELS

34. Method of Determining Densities.-The synthetic spinels werepowdered to 100-120 mesh and their true specific gravities at 20 deg.C. obtained by means of pycnometer measurements according to astandard test method.* The densities were then computed from thesevalues by multiplying them by the density of water at this temper-ature.

Table 12 in the Appendix lists the true specific gravities and thedensities of the spinels. The latter values are reported only to thesecond decimal place, since the purity of the spinels probably doesnot warrant a greater precision.

35. Density Values of Synthetic Spinels.-Table 1 provides a sum-mary of the densities at 20 deg. C. of synthetic spinels as obtainedby pycnometer measurements.

V. X-RAY ANALYSES OF SYNTHETIC SPINELSf

36. Importance of X-ray Diffraction Data.-In the syntheses ofspinels, certain oxides were caused to combine to form the chemicalcompounds known as spinels. The extent to which the oxides com-bined, i.e., the purity of the spinels, could not well be judged by chem-ical analyses, since the leaching out of any uncombined oxides mightnot have been complete. The absence of impurities in the syntheticspinel could be checked somewhat by microscopic examination, sinceany birefringent material would indicate an impure product. How-ever, microscopic examination of spinels is beset with difficulties dueto the. opacity of these compounds. The most reliable tool in veri-

*Standard Test Method No. 52, J. Am. Ceram. Soc. 11, 453, 1928.tA more complete account of these measurements may be found in G. L. Clark, Abde Ally, and

A. E. Badger, Am. J. Sci. 22, 539, 1931.


fying the nature of any compounds which were present is found inX-ray analysis, and the purity of all the spinels which were used inthis investigation was checked by this means.

In addition to the aid rendered by X-rays in verifying the purityof the synthetic spinels, the value of this method of analysis is en-hanced by the fact that the X-ray diffraction data may be used toobtain the length of a side of the elementary unit in the spinel crystal,i.e., its lattice dimension. Such data may be used in computing thetheoretical densities of the materials. A consideration of thesesubjects is undertaken in the following account.

37. Method of Obtaining X-ray Diffraction Data.-The Hullpowder method was used to obtain the X-ray diffraction data in con-junction with a multiple unit apparatus manufactured by the GeneralElectric Company. A molybdenum target tube was utilized to pro-duce X-radiation having the wave length 0.712 A.U. The termd

- in the fundamental diffraction equation,*n

d X- = , (1)n 2 sin (

was obtained with the aid of a scale which is provided with thediffraction apparatus.

38. Computations of Lattice Dimensions of Spinels from X-rayDiffraction Data.-Tables 13, 14, and 15 in the Appendix show theresults of the measurements of the X-ray diffraction patterns, the

dvalues of - for each of the twelve spinels being placed in the first

ncolumn under each compound. The estimated relative intensities ofthe interference lines are placed in the second columns.

If the Miller indices of the reflecting crystal planes are denoted h,k, 1, it is well known that in the cubic system the ratio of the values ofsin2o for any two planes, as computed from Equation (1), is equal tothe ratio of the values of (h2 + k + 12)n 2 for these two planes, wheren is the order of reflection. Column 3 under each spinel lists the ratiosof sin 26 which have been chosen to match the expected values of(h2 + k2 + 12)n2.

A fundamental law of crystallography states that the quantity(h2 + k2 + 2)n2 must be an integer. These integral values are foundin column 2 of each table, and the indices of the planes to which theyapply are placed in the first columns.

*G. L. Clark, Applied X-rays, p. 133, McGraw-Hill, New York, 1927.


TABLE 2

LATTICE DIMENSIONS OF SYNTHETIC SPINELS

Lattice DimensionsAngstrom units

DivalentElement


Zn 8.062 ± 0.001 8.296 + 0.002 8.423 ± 0.001Mg 8.086 ± 0.003 8.305 ± 0.001 8.366 ± 0.001Fe 8.119 + 0.002 8.344 ± 0.003 8.374 ± 0.003Mn 8.271 ± 0.002 8.436 ± 0.001 8.457 ± 0.002

The deviations from whole numbers which are observed in column3 under each spinel are due to the inexactness of the measurements.In the computation of the size of the unit cell, ao, as found in thefourth column under each compound, the integral values in column 2of each table were substituted in the formula*

a, = - (h2 + k2 + 12)n 2.n

39. Summary of Determinations of Lattice Dimensions of SyntheticSpinels.-The mean values of the lattice dimensions, ao, with theprobable errors of the means which result from the use of Student'stfactors, are summarized in Table 2. The table shows that the latticedimensions of chromites are generally intermediate between the lowervalues of the corresponding aluminates and the higher ferrite values.The unit cell of zinc ferrite is an exception to this rule. This spinelexhibits another anomaly in that it is non-magnetic while the otherferrites show ferromagnetism. The table shows that with the singleexception of zinc ferrite, the lattice dimensions increase in the orderZn, Mg, Fe, and Mn. Figure 4 illustrates these relations.

40. Comparison of Values of Lattice Dimensions Obtained by Vari-ous Observers.-For purposes of comparison the results of other in-vestigators are collected in Table 3. No previous references to thelattice dimensions of synthetic ferrous aluminate or of ferrous chro-mite were found in the literature. The divergences in the variousvalues may be explained in part by the solution of contaminatingsubstances which may distort the lattice structures of the spinels.The temperature at which the spinel was synthesized may affect theunit cell dimensions also. For example, Posnjak's value of 8.03 A.U.

*R. W. G. Wyckoff, The Structure of Crystals, p. 97, Chem. Cat. Co., New York, 1924.t"Student," Biometrika 6, 1, 1908; 11, 416, 1917.


Chrom/'e5 '

I

' 86300

8./00

%.9000

7.900

\J4~

FIG. 4. LATTICE DIMENSIONS OF SYNTHETIC SPINELS

in the case of MgA12O 4 refers to spinel which was synthesized atabout 1700 deg. C., and it is conceivable that such material may havea lattice dimension somewhat different from the specimen studied inthis paper, which was formed at about 1325 deg. C.

41. Computations of Theoretical Densities of Synthetic Spinels fromX-ray Diffraction Measurements.-With the knowledge that eightspinel molecules are associated in the unit cell, the theoretical densi-ties may be computed from the formula

nMP= 3a0

where p is the density, n the number of molecules in the lattice,ao the length of the unit cube, and M the absolute mass of the mole-cule (equal to 1.649 X 10- 24 of the molecular weight). The computeddensities are summarized in Table 4. They represent the maximumtheoretical densities of the spinels and should be greater than valueswhich are obtained experimentally, since the latter determinationsinvolve any voids and inclusions which may be present in the crys-talline material. This difference varies from nearly zero to about0.29 unit. The actual density of magnesium ferrite was found to begreater than the theoretical maximum value by 0.10 unit. Thisanomalous relation is probably due to the formation of a smallamount of magnetite with the desired spinel. A comparison of thesedensity values may be obtained by an examination of Tables 1 and 4.

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TABLE 3

LATTICE DIMENSIONS OF SYNTHETIC SPINELSComparison of results of various investigators

Spinel InvestigatorSize of Unit Cell,Angstrom Units

8.099 ± 0.0038.0508.062 ± 0.0018.093 ± 0.004

8.03 ± 0.018.090 ± 0.0038.0508.086 ± 0.0038.059 ± 0.004

8.119 ± 0.002

8.263 ± 0.0078.271 ± 0.002

8.323 ± 0.0048.2808.296 ± 0.002

8.290 ± 0.0058.328.305 ± 0.001

8.344 ± 0.003

8.487 ± 0.0028.436 ± 0.001

8.403 ± 0.0048.3508.423 ± 0.001

8.342 ± 0.0058.3608.366 ± 0.001

8.4008.4178.374 ± 0.003

8.572 ± 0.0068.5158.457 + 0.002

*S. Holgersson, Lunds Univ. Arsskrift, N. F., Avd. 2, 23, 9; Kgl. Fysiogr. Sellk. Handl. N. F. 38,1-112; taken from C. A. 24, 804, 1930.

tL. Passerini, Gazz. chim. ital. 60, 389, 1930; taken from C. A. 24, 4439, 1930.tH. Hauptmann and J. Novak, Z. physik. Chem. B, 15, 372, 1932.1E. Posnjak, Am. J. Sci. 16, 528, 1928.§L. Passerini, Atti accad. Lincei VI, 9, 338, 1929; taken from C. A. 23, 4167, 1929.**S. Holgersson, Z. anorg. allgem. chem. 192, 123, 1930.ttA. A. Claasen, Proc. Phys. Soc. London 38, 482, 1926.

TABLE 4

COMPUTED DENSITIES OF SYNTHETIC SPINELS

Densitygrams per cu cm.

DivalentElement


Zn 4.615 5.393 5.322Mg 3.548 4.429 4.506Fe 4.392 5.085 5.201Mn 4.031 4.900 5.029

Zn AO104

Mg A1204

Fe AlsO 4

Mn AO120

Zn CrsO4

Mg CrsO0

Fe CrsO4

Mn CrnO4

Zn Fe2O4

Mg Fe204

Fe Fe2O4

Mn Fe2sO

Holgersson*PasserinitAuthorsHauptmann and Novakt

Posnjak¶Holgersson*PasserinitAuthorsHauptmann and Novakt

Authors

Holgersson*Authors

Holgersson*PasserinitAuthors

Passerini§Holgersson**Authors

Authors

Holgersson*Authors



ClaasenttHolgersson*Authors


j


Fia. 5. DENSITIES OF SYNTHETIC SPINELS AS COMPUTED FROM X-RAY DATA

TABLE 5

MOLAR VOLUMES OF SYNTHETIC SPINELS

Computed from X-ray measurements

Molar Volumes*cu. em.

DivalentElement


Zn 39.8 43.2 45.3Mg 40.1 43.3 44.4Fe 39.6 44.0 44.5Mn 42.9 45.5 45.8

*At room temperature.

42. Theoretical Densities of Synthetic Spinels.-Table 4 provides a

summary of the densities of synthetic spinels which were computedfrom measurements of the X-ray diffraction patterns, and Fig. 5shows the results graphically.

By means of the data in Table 4, the volumes which would beoccupied by one gram-molecular weight of each spinel may be com-puted. These molar volumes are based on the theoretical valuesobtained from X-ray diffraction measurements. An inspection of the

values given in Table 5 shows that the molar volumes differ only byfairly small amounts, and hence it would be expected that thesespinels would readily intercrystallize and form isomorphous mixtures.

VI. HEAT CAPACITIES OF SYNTHETIC SPINELS

43. Method of Determining Heat Capacities.-The mean heat ca-pacities were determined by a comparison method which was based


FIG. 6. APPARATUS FOR DETERMINATION OF*HEAT CAPACITIES OF SPINELS

on the use of vitreous silica as a primary standard. A diagram of theapparatus is shown in Fig. 6. A weighed charge of spinel was con-tained in a platinum crucible at S which was suspended from amechanism M. Rotation of the mechanism M through a small arccaused the crucible and contents to fall at the will of the experimenter.The temperature of the charge was measured by a thermocouple Twhich was placed about a centimeter above the top of the samplecontained in the crucible. The temperature of the thermocouple wasread by means of a Leeds & Northrup Type K potentiometer, and acorrection was applied to this indicated temperature to obtain theaverage temperature of the sample.

The furnace winding was of a platinum-rhodium alloy. This was


surrounded by powdered magnesia insulation B and powdered Sil-O-Cel Si. The furnace was supported on a slide which permitted theremoval of the furnace from its regular position above the calorimeter.The bottom of the furnace was closed by means of an easily removableplug and shield Sh, which was held in position by a strip spring Sp.A transite shield Tr aided in decreasing the radiation of heat to thecalorimeter.

When the sample S had reached the desired temperature the bot-tom shield was removed from the furnace and the mechanism Mrotated with the fingers. This rotation caused the sample to dropinto the calorimeter which was placed directly below.

The calorimeter consisted of a quart-size Dewar flask D in whicha known mass (2000 grams) of mercury was placed. The temper-ature of the mercury was measured by eight copper-constantanthermoelements C which were held in small glass tubes placed sym-metrically around the interior surface of the Dewar flask. The coldjunctions of these thermoelements were surrounded by an ice-watermixture, and the electromotive force developed by the series of eightcouples was measured by the potentiometer already mentioned. Anichrome screen Sc extended from the top of the Dewar flask to thebottom, where it dipped below the mercury surface and preventedthe hot crucible from coming in contact with the glass sides of theflask. The Dewar flask was held in a brass tube which was surround-ed by a water bath W contained in a stoneware crock. The temper-ature of the water was maintained constant to within about 1 deg. C.by a heating coil and thermoregulator H. A thermometer Th andstirrer St were provided for the water bath. Felt packing L aided inthe exclusion of outside effects on the temperature of the bath. Thedropping mechanism is essentially that which has been described byW. P. White.*

The efficacy of this method of determining specific heats dependson the maintenance of similar conditions when the calibrating sub-stance (vitreous silica) and the spinel samples were used. In all teststhe furnace temperature was maintained nearly constant and thetemperature of the calorimeter was adjusted to approximately thesame initial reading of the copper-constantan thermocouples.

The calibration consisted in dropping various masses of vitreoussilica in their platinum container from the same furnace temperatureinto the calorimeter. Each mass of vitreous silica produced a rise inthe temperature of the mercury in the Dewar flask which was pro-portional to the mass of silica used, and this rise was measured by the

*See W. P. White, The Modern Calorimeter, Chem. Cat. Co., New York, 1928.


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increase in the electromotive force developed by the series of eightcopper-constantan thermocouples. Table 6 shows these measure-ments.

The initial temperature of the sample was nearly the same in allcases. However, the final temperature of quenching varied with themass of vitreous silica which was used. This difference in temper-ature change was adjusted to a common basis of a temperature changeof 975 deg. C. by assuming a linear relation between the temperaturerise and the mass of the sample. The actual temperature differencesof the tests are shown in the next to the last line of Table 6. They areall near 975 deg. C. The last line indicates the rise in the electromo-tive force of the series of thermocouples which would result if thetemperature change were 975 deg. C.

The data thus obtained are shown in Fig. 7, which depicts therise in the electromotive force of the series of copper-constantanthermoelements in the calorimeter which would be occasioned by aknown mass of vitreous silica in its platinum container as it coolsthrough a temperature range of 975 deg. C. The points of this curvedo not represent a straight line theoretically, but one which is prob-


ably concave to the horizontal axis due to the increased radiationloss which occurs as the mass of silica increases. However, this con-cavity is slight and the straight line y = 1.37 x - 1.90, which wasobtained by the method of least squares, fits the data quite well.

The equation, y = 1.37 x - 1.90, in which y represents the massof silica contained in the platinum crucible and x is the rise in theelectromotive force of the series of thermocouples, then serves as ameasure of the heat capacity of the calorimeter. If a substance ofunknown specific heat is substituted for the vitreous silica, and theincrease in electromotive force of the thermocouples is noted when theplatinum crucible and contents cool through 975 deg. C., then theequation y = 1.37 x - 1.90 gives the amount of vitreous silica whichwould be necessary to bring about this temperature change under thesame conditions. The heat capacity of vitreous silica at various tem-peratures was obtained from the International Critical Tables,* fromwhich it may be deduced that the mean heat capacity of vitreoussilica from 50 to 1025 deg. C. is 1.096 joules per gram per deg. C.This value was used as a primary standard in the heat capacity deter-minations which are listed in the Appendix, Table 16. This tableshows how the heat capacity of one gram of each spinel is obtained interms of the mass of vitreous silica. When this value is multiplied by1.096, the mean heat capacity of the spinel over the range 50 to 1025deg. C. is obtained. In general the mean of three determinations wasrecorded.

The accuracy of the method depends on the degree to which theexperimental procedure can be duplicated. Thus the upper temper-ature of the sample before dropping should be very nearly the samein all cases, and the furnace was held constant at this point for about15 minutes to insure that the difference in temperature between thefurnace thermocouple and the sample would be the same. It wasfound that a thermocouple embedded in various positions in thesample read about 18 ± 5 deg. C. higher than the thermocouple indi-cation. This correction was applied to the thermocouple reading toobtain the average temperature of the sample.

The calibrating substance should be of the same thermal conduc-tivity as the spinels in order that the radiation losses entailed whilethe calorimeter was rising in temperature should be equal in all cases.Although the thermal conductivities of the spinels and of vitreoussilica undoubtedly differ, the error is judged to be small. Further,the physical condition of all substances should be the same, and forthis reason powdered vitreous silica was used instead of larger parti-

*International Critical Tables V, 105.


cles. No devitrification of the vitreous silica took place during thecalibration runs.

Concerning the heat capacity of the platinum crucible and thatof the mercury nothing need be known, since the masses of the cruci-ble and of the mercury were unchanged throughout the tests.

The heat losses of the calorimeter due to radiation, convection,conduction, and volatilization of the mercury do not matter providingthat they are the same in each test. Attempts were made to effectthis duplication in the experimental procedure.

The mode of procedure indicates that the calibration of thecopper-constantan thermocouples is unnecessary, the only criterionbeing their constancy of indication.

A consideration of these factors results in a value of 2 per cent as aconservative estimate of the precision of the results.

In order to compare the results obtained with the mercury calo-rimeter with the values obtained by other methods, the heat capacityof artificial corundum ("R. R. Alundum" from the Norton Company)was measured. It was found to be about 3 per cent less than a valuewhich was computed from the data of Cohn,* who used a differentialheating method.

44. Experimental Data Concerning Heat Capacities of SyntheticSpinels.-Table 16 in the Appendix shows the readings and finalvalues of the heat capacity determinations. The lowest horizontalline gives the mean values of these determinations.

45. Summary of Heat Capacity Determinations.-The mean heatcapacities of synthetic spinels over the temperature range 50 to 1025deg. C. are listed in Table 7, which shows in column 2 the values injoules per gram per deg. C. The corresponding heat capacities on agram-molecular basis are shown in column 3. Conversion to caloriesmay be effected by dividing the given values by 4.186.

Table 7 shows that the molar heat capacities of eight of the twelvespinels are very nearly equal, varying from 169 to 184 joules per molper deg. C. The molar heat capacities of MnAI20 4 and MnCr20 4 aresomewhat lower, being 154 and 165 joules per mol per deg. C., respec-tively. The ferrites of iron and manganese have considerably highervalues for the molar heat capacities-222 and 201 joules per mol perdeg. C., respectively. These ferrites show the phenomenon of ferro-magnetism, but the increase in heat capacity cannot be ascribedwholly to this effect, since the ferrite of magnesium possesses the

*W. Cohn, J. Am. Ceram. Soc., 7, 482, 1924.


TABLE 7

MEAN HEAT CAPACITIES OF SYNTHETIC SPINELS OVERTEMPERATURE RANGE OF 50 TO 1025 DEG. C.

Mean Heat Capacity, 50 to 1025 deg. C.

Kind of Spinel

joules per gram joules per molper deg. C. per deg. C.

Zn A1204 0.92 169Mg AhO4 1.19 170Fe A1204 1.04 178Mn A1iO 0.89 154Zn Cr204 0.77 180Mg Cr2O4 0.92 177Fe Cr2Ol 0.82 184Mn Cr2tO 0.74 165Zn Fe204 0.73 176Mg FesO4 0.86 172Fe FeO04 0.96 222Mn FeiO4 0.87 201

normal value and yet it is strongly ferromagnetic. Since very smallchanges in the atom profoundly modify its magnetizability the lackof obvious changes in the specific heat and other properties is to beexpected. The similarity in the heat capacities of most of the spinelsverifies to a limited extent the truth of Neumann's law, that com-pounds with similar molecular structures have the same molar heatcapacities.

Although an intimate relation must obtain between heat capacityand thermal expansion, no clear correlation is evident from thesemeasurements.

46. Atomic Heat Capacities of Synthetic Spinels.-At high temper-atures the atomic heat capacities of compounds-i.e., the molecularheat capacity divided by the number of atoms in the compound-approach a value of about 6.0 cal. per deg. C. The molecular heatsof Table 7 may be used in this manner to compute the atomic heatcapacities given in Table 8, which are average values for the temper-ature interval from 50 to 1025 deg. C. They should, therefore, besomewhat less than 6.0 in magnitude. This relation is verified inmost cases, but the value for manganese aluminate is quite low, being5.26 while those of the ferrites of iron and manganese are inordinatelyhigh.

VII. THERMAL EXPANSIONS OF SYNTHETIC SPINELS

47. Method of Determining Thermal Expansions.-The Fizeau-Pulfrich principle was utilized in the thermal expansion apparatus


TABLE 8

ATOMIC HEAT CAPACITIES OF SYNTHETIC SPINELS OVERTEMPERATURE RANGE OF 50 TO 1025 DEG. C.

Atomic Heat CapacityKind of Spinel (50-1025 deg. C.),

cal. per deg. C.

Zn AlO04 5.77Mg AlO04 5.80Fe A120 4 6.07Mn AlO04 5.26Zn Cr2sO 6.14Mg Cr2zO 6.04Fe Cr204 6.28Mn Cr204 5.63Zn Fe20 4 6.00Mg Fe 2 04 5.87Fe Fes0 4 7.58Mn FeOl4 6.86

FIG. 8. FIZEAU-PULFRICH APPARATUS FOR DETERMINING LINEARTHERMAL EXPANSIONS OF SPINELS

which is illustrated by the diagram in Fig. 8. The measurements ofthermal expansion were made on small pieces of spinel which were inthe form of tetrahedrons with altitudes of a few millimeters each.Three such pyramids were placed between optically flat vitreous silicaplates in an electrically heated furnace, and this system formed theinterference element in the Fizeau-Pulfrich method of determininglinear thermal expansions.


TABLE 9INTERFEROMETRIC DATA ON THERMAL

EXPANSION OF PLATINUM

L2 = 0.2996 cm.

InterferenceFringe Number

056

14152425343545465556666777788990

Temperature ofSample, deg. C.

2792

104190201293304393402495504592604693707792802895905

Expansion of Sampimicrons per cm.

05.446.49

14.7315.7424.8425.8534.8635.8545.8146.8055.7256.7166.6167.6077.4878.4689.3090.29

Figure 8 indicates how light from a helium discharge tube at A

passed through the collimating system at B. The totally reflecting

prism C then caused the light to pass down into the furnace through

the vitreous silica convection shields D. Interference took place be-

tween the surfaces of the vitreous silica plates separated by the sam-

ples at E, and the expansion of the sample caused a passage of the

interference fringes which were viewed through the optical system

F, G. A thermocouple H was used to measure the furnace tempera-

ture, from which the temperature of the sample was deduced by

means of a correction term. The references* may be consulted for

a detailed analysis of the method.In order to check the accuracy of the expansion tests which were

made by the interference method, the expansion of platinum was

determined. The data relating to this test are found in Table 9, from

which the mean coefficient of linear expansion of platinum between

27 and 604 deg. C. was found to be 98 X 10- 7 cm. per cm. per deg. C.

The mean coefficient from 0 to 600 deg. C. as computed from the

equationIt = I0 (1 + 8.786 X 10- 6t + 3.118 X 10-9t 2

- 5.27 X 10-12t3 + 4.095 X 10-15t4),

*C. G. Peters and C. H. Cragoe, Bur. Standards Sci. Paper No. 393, Washington, 1920.L. D. Fetterolf and C. W. Parmelee, J. Am. Ceram. Soc. 12, 193, 1929.


which was obtained from the International Critical Tables,* is 97 X10- 7 cm. per cm. per deg. C. The agreement between the experi-mental and the computed values indicates that the thermal expansioncoefficients which are given in Table 11 may be relied on to about1 per cent.

48. Preparation of Specimens for Thermal Expansion Tests.-As arule, small pieces of spinel were broken from the larger block of syn-thesized spinel in order to produce fragments suitable for the thermalexpansion tests, and these pieces were then ground to the desiredpyramidal form on a carborundum stone. In some cases the syntheticspinel was too friable for such treatment and another means of form-ing the test pieces was resorted to. This method utilized the spinel ina finely powdered form, which was mixed with five per cent of itsweight of equivalent parts of the constituent oxides plus five per centof boric oxide. Sufficient saccharine solution was added to facilitatemolding and the mixture was then pressed, dried, and fired to such anextent that the added oxides combined to form a spinel which actedas a cement for the original spinel grains. The boric oxide was vola-tilized by this heat treatment and did not vitiate the test specimens,which were ready for the grinding operation after this heat treatment.

49. Thermal Expansion Measurements.-The thermal expansionsof the spinels as determined by the interference method are given inthe Appendix, Table 17, from which the expansion values at desiredtemperature intervals may be deduced by interpolation. These inter-polated values, as set forth in Table 10, were obtained by the inter-polation formula of Lagrange in case the required temperature wassufficiently distant from the measured value to necessitate the useof this formula.

50. Linear Thermal Expansion Coefficients of Synthetic Spinels.Table 11 lists the values of the coefficients of mean linear thermalexpansion of synthetic spinels over various temperature intervalswhich extend to 900 deg. C. in most cases. Figures 1, 2, and 3illustrate the courses of the expansion curves of these spinels andprovide a graphic comparison of the expansions of various aluminates,of chromites, and of ferrites.

Considering the aluminates, it is apparent that up to about 450deg. C. the expansions decrease in the order Zn, Mg, Fe, and Mn.The reverse order holds for the lattice dimensions of these spinels.Since a smaller lattice dimension denotes a greater compression of the

*International Critical Tables II, 461.

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atoms in the molecule, it would be expected that such a substancewould have a greater expansion than one which possessed a largerand less compressed unit cell. At higher temperatures the expansionof ferrous aluminate increases more rapidly than that of its com-panions.

In the cases of the chromites and ferrites of the metals studied,these relations do not hold. The ferrites particularly show irregularexpansions which attain quite high values. Magnetite especiallyshows a sudden rise in expansion between 500 and 600 deg. C., andthis increase is undoubtedly intimately connected with the inversionwhich is known to occur near 593 deg. C. Of the ferrites, only that ofzinc is non-magnetic, but no effect of this property which woulddistinguish it from its fellows is apparent from the course of theexpansion curve.

APPENDIX

TABLES OF DATA FOR COMPUTING DENSITIES, LATTICE

DIMENSIONS, SPECIFIC HEATS, AND THERMAL

EXPANSIONS OF SPINELS

TABLE 12

TRUE SPECIFIC GRAVITIES AND DENSITIES OF SYNTHETIC SPINELS

Determined by Standard Test Method No. 52 of American Ceramic Society

Aluminates Chromites FerritesDivalentElement

Specific Density Specific Density Specific DensityGravity g. per cu. cm. Gravity g. per cu. cm. Gravity g. per cu. cm.

Zn 4.545 4.54 5.307 5.30 5.273 5.274.547 5.302 5.277

Mg 3.527 3.53 4.404 4.41 4.612 4.613.532 4.441 4.629

Fe 4.220 4.22 4.937 4.93 5.175 5.184.234 4.944 5.147

Mn 4.000 4.01 4.798 4.80 4.757 4.744.033 4.810 4.728


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Bulletin No. 198. Results of Tests on Sewage Treatment, by Harold E. Babbittand Harry E. Schlenz. 1929. Fifty-five cents.

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Bulletin No. 200. Investigation of Endurance of Bond Strength of Various Claysin Molding Sand, by Carl H. Casberg and William H. Spencer. 1929. Fifteen cents.

Circular No. 19. Equipment for Gas-Liquid Reactions, by Donald B. Keyes.1929. Ten cents.

Bulletin No. 201. Acid Resisting Cover Enamels for Sheet Iron, by Andrew I.Andrews. 1929. Twenty-five cents.

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Bulletin No. 203. Dependability of the Theory of Concrete Arches, by HardyCross. 1929. Twenty cents.

Bulletin No. 204. The Hydroxylation of Double Bonds, by Sherlock Swann, Jr.1930. Ten cents.

Bulletin No. 205. A Study of the Ikeda (Electrical Resistance) Short-Time Testfor Fatigue Strength of Metals, by Herbert F. Moore and Seichi Konzo. 1930.Twenty cents.

*Bulletin No. 206. Studies in the Electrodeposition of Metals, by Donald B.Keyes and Sherlock Swann, Jr. 1930. Ten cents.

*Bulletin No. 207. The Flow of Air through Circular Orifices with RoundedApproach, by Joseph A. Poison, Joseph G. Lowther, and Benjamin J. Wilson.1930. Thirty cents.

*Circular No. 20. An Electrical Method for the Determination of the Dew-Pointof Flue Gases, by Henry Fraser Johnstone. 1929. Fifteen cents.

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Bulletin No. 209. Heat Transfer in Ammonia Condensers. Part III, by AlonzoP. Kratz, Horace J. Macintire, and Richard E. Gould. 1930. Thirty-five cents.

Bulletin No. 210. Tension Tests of Rivets, by Wilbur M. Wilson and William A.Oliver. 1930. Twenty-five cents.

Bulletin No. 211. The Torsional Effect of Transverse Bending Loads on ChannelBeams, by Fred B. Seely, William J. Putnam, and William L. Schwalbe. 1930.Thirty-five cents.

Bulletin No. 212. Stresses Due to the Pressure of One Elastic Solid uponAnother, by Howard R. Thomas and Victor A. Hoersch. 1930. Thirty cents.

Bulletin No. 213. Combustion Tests with Illinois Coals, by Alonzo P. Kratzand Wilbur J. Woodruff. 1930. Thirty cents.

*Bulletin No. 214. The Effect of Furnace Gases on the Quality of Enamels forSheet Steel, by Andrew I. Andrews and Emanuel A. Hertzell. 1930. Twenty cents.

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Bulletin No. 226. Laboratory Tests of Reinforced Concrete Arches with Decks,by Wilbur M. Wilson. 1931. Fifty cents.

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Pickels. 1931. Seventy cents.*Bulletin No. 233. An Investigation of the Properties of Feldspars, by Cullen W.

Parmelee and Thomas N. McVay. 1931. Thirty cents.*Bulletin No. 234. Movement of Piers During the Construction of Multiple-Span

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Oil, by Carl H. Casberg and Carl E. Schubert. 1931. Fifteen cents.*Bulletin No. 236. The Electrolytic Reduction of Ketones, by Sherlock Swann,

Jr. 1931. Ten cents.*Bulletin No. 237. Tests of Plain and Reinforced Concrete Made with Haydite

Aggregates, by Frank E. Richart and Vernon P. Jensen. 1931. Forty-five cents.*Bulletin No. 238. The Catalytic Partial Oxidation of Ethyl Alcohol, by Donald

B. Keyes and Robert D. Snow. 1931. Twenty cents.*Bulletin No. 239. Tests of Joints in Wide Plates, by Wilbur M. Wilson, James

Mather, and Charles O. Harris. 1931. Forty cents.*Bulletin No. 240. The Flow of Air through Circular Orifices in Thin Plates, by

Joseph A. Polson and Joseph G. Lowther. 1932. Twenty-five cents.*Bulletin No. 241. Strength of Light I Beams, by Milo S. Ketchum and Jasper 0.

Draffin. 1932. Twenty-five cents.*Bulletin No. 242. Bearing Value of Pivots for Scales, by Wilbur M. Wilson,

Roy L. Moore, and Frank P. Thomas. 1932. Thirty cents.*Bulletin No. 243. The Creep of Lead and Lead Alloys Used for Cable Sheathing,

by Herbert F. Moore and Norville J. Alleman. 1932. Fifteen cents.*Bulletin No. 244. A Study of Stresses in Car Axles Under Service Conditions,

by Herbert F. Moore, Nereus H. Roy, and Bernard B. Betty. 1932. Forty cents.*Bulletin No. 245. Determination of Stress Concentration in Screw Threads by

the Photo-Elastic Method, by Stanley G. Hall. 1932. Ten cents.*Bulletin No. 246. Investigation of Warm-Air Furnaces and Heating Systems,

Part V, by Arthur C. Willard, Alonzo P. Kratz, and Seichi Konzo. 1932. Eightycents.

*Bulletin No. 247. An Experimental Investigation of the Friction of ScrewThreads, by Clarence W. Ham and David G. Ryan. 1932. (In press.)

*Bulletin No. 248. A Study of a Group of Typical Spinels, by Cullen W. Parmelee,Alfred E. Badger, and George A. Ballam. 1932. Thirty cents.

UNIVERSITY OF ILLINOISHARRY WOODBURN CHASE, Ph.D., LL.D., L.H.D., President

The University includes the following departments:The Graduate SchoolThe College of Liberal Arts and Sciences (Curricula: General with majors,

in the Humanities and the Sciences; Chemistry and Chemical Engi-neering; Pre-legal; Pre-medical; Pre-dental; Pre-journalism; AppliedOptics)

The College of Commerce and Business Administration (Curricula: Gen-eral Business, Banking and Finance, Insurance, Accountancy, Trans-portation, Industrial Administration, Foreign Commerce,' CommercialTeaching, Trade and Civic Secretarial Service, Public Utilities, Com-merce and Law)

The College of Engineering (Curricula: Ceramics; Ceramic, Civil, Electri-cal, Gas, General, Mechanical, Mining, and Railway Engineering; En-gineering Physics)

The College of Agriculture (Curricula: General Agriculture; Floriculture;Home Economics; Smith-Hughes-in conjunction with the College ofEducation)

The College of Education (Curricula: Two year, prescribing junior stand-ing for admission-General Education, Smith-Hughes Agriculture,Smith-Hughes Home Economics, Public School Music; Four year, ad-mitting from the high school-Industrial Education, Athletic Coaching,Physical Education. The University High School is the practice schoolof the College of Education)

The College of Law (three-year curriculum based on a college degree, orthree years of college work at the University of Illinois)

The College of Fine and Applied Arts (Curricula: Music, Architecture, Ar-chitectural Engineering, Landscape Architecture, and Painting)

The Library School (two-year curriculum for college graduates)The School of Journalism (two-year- curriculum -based on two years of

college work)The College of Medicine (in Chicago)

The College of Dentistry (in Chicago)The College of Pharmacy (in Chicago)The Summer Session (eight weeks)Experiment Stations and Scientific Bureaus: U. S. Agricultural Experiment

Station; Engineering Experiment Station; State Natural History Sur-vey; State Water Survey; State Geological Survey; Bureau of Educa-tional Research; Bureau of Business Research.

The Library Collections contain (July 1, 1931) 832,643 volumes' and 221,000pamphlets (in Urbana) and 45,241 volumes and 7,875 pamphlets ,(inChicago)

For catalogs and information address TmE RiwcSTRAR, Urbana, Illinois.

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A Study of a group of typical spinels