Clay Minerals (1969) 8, 1.

A S C H E M E O F S E M I - M I C R O A N A L Y S I S F O R T H E M A J O R E L E M E N T S I N C L A Y M I N E R A L S , B A S E D O N M O D I F I C A T I O N S TO C O N V E N T I O N A L M E T H O D S

O F S I L I C A T E A N A L Y S I S

G. P R U D E N AND H. G. C. K I N G

Rothamsted Experimental Station, Harpenden, Herts

(Received 16 December 1968)

ABSTRACT: A scheme is presented for the semi-micro analysis for the major elements in clay minerals, based mainly on improvements to conventional colorimetric methods. Silica and alumina are determined after fusing a 50-rag sample with sodium hydroxide. Titanium, total iron, calcium, magnesium, phosphorus and manganese are determined colorimetrically on a hydrofluoric-sulphuric acid digest of a 50-mg sample; sodium and potassium are determined flame photometrically on the same digest. Ferrous iron is determined separately. Additional methods for the determination of total water, total sulphur and fluorine are indicated.

I N T R O D U C T I O N

Existing methods and colorimetric reagents used for analysing silicate minerals have been investigated and an improved scheme of semi-micro analysis for clay minerals, based on modifications to conventional methods, is proposed.

The sample material is brought into solution by methods similar to those of Shapiro & Brarmock (1962), i.e. by fusing 50 mg with sodium hydroxide before determining A120~ and SiO2, and by digesting a further 50 mg with a mixture of HF and H2SO~ before determining the remaining major elements. These treatments have been found satisfactory, although the authors are aware of other schemes of silicate analysis in which the sample is brought into solution with one reagent (Langmyhr & Graft, 1965; Suhr & Ingamells, 1966). The proposed scheme is con- cerned with improvements to the subsequent methods of colorimetric analysis rather than the preparation for analysis. A recently developed method for deter- mining Ca colorimetrically is described and the sensitivity of the determinations of A1208 and Mg by using specially purified reagents has been improved. The sensitivity of other methods has also been improved by substituting more effective reagents for those commonly used. The wavelengths of maximum optical density of coloured complexes have all been accurately determined. This is important because earlier quoted wavelengths were often fixed by the limitations of the

2 G. Pruden and H. G. C. King

spectrophotometer, and are not spectral maxima; for example, in the determinations of silica and titanium by Shapiro & Brannock (1962). An upper limit of determina- tion of each element is given, the value of which always lies on the linear portion of its calibration graph.

Because as little as 100 mg of sample is needed for determining the ten major elements the scheme is suitable for work involving the analysis of small samples, for example, small purified samples of individual minerals. The time taken to prepare a test solution is shorter than that of earlier schemes.

Iron (II) is determined on a separate 25-mg sample, which should contain rio organic matter. The weights of sample required for determining total water, total sulphur and fluorine must be estimated in preliminary experiments.

E X P E R I M E N T A L

Analytical grade reagents are used throughout.

Preparation of solution A A 50-mg portion of the sample is fused at dull red heat with sodium hydroxide

in a nickel crucible. After cooling, the melt is leached with water and the solution acidified with hydrochloric acid.

(a) Reagents. Sodium hydroxide so!ution--30 % w/v solution; store in a polythene bottle. Hydrochloric acid solution--1 : 1 v/v solution (6 N).

(b) Procedure. Weigh accurately about 50. mg of the sample, ground to pass 100 mesh, and 50 mg of a suitable standard sample, e.g.U.S. National Bureau of Stan- dards Sample No. 99 (feldspar), into separate nickel crucibles. Add 5 ml of 30% sodium hydroxide solution to each crucible and also to an empty crucible for a reagent blank. Evaporate the solutions to dryness on a hot plate. Heat the crucibles over a bunsen burner, gently at first, until the contents are fused, and then increase the heat to a dull redness. Keep the crucibles at this temperature for about 5 min, and then allow to cool. Add 50 ml of water to each crucible, cover and allow to stand overnight or until the melt disintegrates completely. Transfer the contents of the crucibles to 1-1 graduated flasks containing about 400 ml of water and 20 ml of 6 N hydrochloric acid. Use a polythene funnel with a stem of such a length that the strong alkaline solution does not touch the sides of the flask. Wash the contents of each crucible with water into the 1-1 flasks. A clear solution is obtained in about 15 min. Dilute each solution to volume and mix. Store in a polythene bottle if determinations cannot be completed the same day.

Determination o[ silica In the determination of silica the yellow silicomolybdate complex is reduced

to molybdenum blue, and the optical density of the solution measured at 810 nm. The method is similar to that described by Shapiro & Brannock (1962), but the conditions of acidity at the colour development stage have been changed to the

Semi-micro analysis of clay minerals 3 procedure used by the Overseas Geological Surveys, Mineral Resources Division (private communication, published by permission). Because of the limited range of their spectrophotometer Shapiro & Brannock measure the optical density at 640 nm, a wavelength on the steepest part of the spectrum. Measuring at 810 rim, the spectral maximum, doubles the sensitivity of the method. The adoption of more acidic conditions when developing the colour gives greater stability to the reduced silicomolybdate solution and suppresses interference due to phosphorus.

(a) Reagents. Ammonium molybdate solution--dissolve 7"5 g of crushed crystal- line ammonium molybdate in 90 ml of water. Add 10 ml of 50% v/v salphuric acid, mix and store in a polythene bottle. Sulphuric acid--25% v/v solution. Reducing solution--dissolve 1"4 g of sodium sulphite and 18 g of sodium meta- bisulphite in about 150 ml of water. Add 0"3 g of 1-amino-2-naphthol-4-sulphonic acid and dilute to 200 ml. Store in a polythene bottle, preferably in a refrigerator. This solution is stable for at least two weeks.

(b) Procedure. Pipette 10 ml of solution A, 10 ml of the reference standard solution and 10 ml of blank solution into separate 100 ml graduated flasks, and dilute each to about 50 ml with water. Add 3 ml of ammonium molybdate solution to each flask, swirl and allow to stand for 10 min. Add 25 ml of 25% sulphuric acid, followed immediately by 2 ml of reducing solution. Dilute to volume, mix and allow to stand for 30 min. Measure the optical density of each solution using the blank solution as reference on a spectrophotometer at 810 nm using 1 cm ceils. Calculate the percentage of silica in the sample from a calibration graph, or by using the following formula:

(~ sio2 in standard) x (weight of standard) optical density of sample % SiO2 in sample = x

optical density of standard weight of sample

Determination of alumtna Alumina is determined by measuring the optical density of a solution in which

the aluminium has been converted to a calcittm-Alizarin Red S complex (Parker & Goddard, 1950). Interference from iron is eliminated by the use of potassium ferri- cyanide and thioglycollic acid as complexing agents. Purified Alizarin Red S (King & Pruden, 1968) is used. The wavelength of maximum absorption of the complex is 490 rim, at which the interference due to titanium is negligible.

(a) Reagents. Calcium chloride solution--transfer 7 g of calcium carbonate to a 250 ml beaker, add i00 ml of water and 15 ml of concentrated hydrochloric acid, heat to boiling and boil for a few minutes. Cool the solution and dilute to 500 ml. Potassium ferricyanide solution--dissolve 0"375 g of potassium ferricyanide in 50 ml of water immediately before use. Thioglycollic acid solution--dilute 2 ml of the concentrated acid to 50 ml immediately before use. Hydroxyammonium chloride solution--freshly prepared t0% solution in water. Buffer solution pH 4.8---dissolve 100 g of sodium acetate trihydrate in about 400 ml of water, add 30 ml of glacial acetic acid and dilate to 500 ml. Alizarin Red S--dissolve 0.25 g of the purified dye (King & Pruden, 1968) in 500 ml of water.

4 G. Pruden and H. G. C. King

(b) Procedure. Pipette 15 ml of solution A, 15 m1 of the reference standard solution and 15 ml of blank solution into separate 100 ml graduated flasks. Add to each flask first 2 ml of calcium chloride solution, then 1 ml of hydroxyammonium chloride solution, 1 ml of potassium ferricyanide solution and finally 2 ml of thioglycollic acid solution, mixing after each addition. Allow the mixture to stand for 5 min and then pipette 10 ml of buffer solution into each flask and mix. After 10 rain pipette 10 ml of Alizarin Red S solution into each flask, dilute to volume and mix. Allow the coloured solution to stand for 45-75 min, and then measure the optical density of each solution, using the blank solution as reference, at 490 nm in 1 cm cells. Calculate the percentage of alumina in the sample from a calibration graph, or use the following formula:

(~ Al203 in standard) • (weight of standard) optical density of sample A1203 in sample = •

optical density of standard weight of sample

Preparation o] solution B The samples are digested on a sand bath with a mixture of hydrofluoric and

sulphuric acids. Total iron, magnesium, calcium, titanium, phosphorus, manganese, sodium and potassium are determined on the diluted digest. Minerals insoluble in hydrofluoric and sulphuric acids (e.g. tourmaline, futile, spine/, corundum) should be removed from the solution by centrifuging, dried, weighed and analysed separately after fusion with sodium hydroxide or lithium metaborate.

(a) Reagents. Sulphuric acid--18 N and 9 N. Hydrofluoric acid--40%. (b) Procedure. Weigh accurately about 50 mg of the sample into a 30 ml platinum

crucible, and moisten with a few drops of water. Add about 1 ml of hydrofluoric acid and digest for a few minutes on a sand bath. Cool the digest, add 2 ml of 18 N sulphuric acid and warm on the sand bath until sulphur trioxide fumes appear. Cool, add 1 ml of water and again evaporate until sulphur trioxide fumes appear. Allow the digest to cool, add 5 ml of water and warm to ensure complete dissolution, then finally cool and transfer quantitatively to a 100 ml graduated flask. Add 9 ml of 9 N sulphuric acid to the flask, mix and dilute to 100 ml. The normality of this solution with respect to sulphuric acid is approximately 1-2.

Determination of titanium Titanium is determined by measuring the optical density of the yeIlow complex

produced with tiron (disodium 1,2-dihydroxybenzene 3,5-disulphonate) (Yoe & Armstrong, 1947). Iron (III) also reacts with tiron to form a purple solution, which is decolorized by complexing with thioglycollic acid (Rigg & Wagenbauer, 1961). The optical density of the complex, buffered at pH 3.8, is measured at 377 nm. Because of the limitations of their spectrophotometer, Shapiro & Brannock (1962) measured the colour at 430 nm. Up to 5"3 % titanium dioxide can be determined using a 10 ml aliquot of solution B.

Semi-micro analysis o f clay minerals 5

(a) Reagents. Tiron--5% solution in water. Thioglycollic acid solution--20% v/v solution in water. Buffer solution, pH 3'8--add 390 ml of glacial acetic acid to 1 1 of molar sodium acetate solution. Standard titanium dioxide solution--heat 0"5 g of titanium dioxide with 10 g of ammonium sulphate and 25 ml of concentrated sulphuric acid until dissolved. Cool and dilute to 1 1. Dilute 20 ml of this solution to 1 1 with N sulphuric acid, to give a solution containing 10/~g of titanium dioxide per ml.

(b) Procedure. Pipette 10 ml of solution B into a 50 ml graduated flask, and add in turn 25 ml of buffer solution, 5 ml of tiron solution and 2 ml of the 20% thioglycollic acid solution. Mix, dilute to volume and allow to stand for at least 1 hr before measuring the optical density of the solution at 377 nm against a reagent blank. Calculate the percentage titanium dioxide in the sample from a calibration graph prepared from the standard solution.

Determination of total iron Iron is determined by measuring the optical density of the red colour developed

with 2.2'-dipyridyl (Hall, 1930), after reduction to the iron (II) state with glycin (p-hydroxyphenylglycine) (Parker & Griffin, 1939), in a solution buffered at pH 7-0 with ammonium acetate. Up to 12"4% total iron can be determined using a 5 ml aliquot of solution B. For the occasional sample in which the value exceeds 12.4%, a smaller aliquot can be taken. Pyrite (FeS2) is unattacked by hydrofluoric and sulphuric acids; if this mineral is present, total iron should be determined in a solution of the sample obtained after fusion with sodium carbonate.

(a) Reagents. Glycin~0"1% solution in 0.4% w/v sulphuric acid. 2-2"-dipyridyl solution--O.2% solution in 10% v/v acetic acid. Ammonia solution--s.g. 0"880. Ammonium acetate solution, pH 7-0--molar solution. Standard iron solution-- transfer 0"3511 g of ferrous ammonium sulphate to a 500 ml graduated flask. Add 16 ml of 50~163 v/v sulphuric acid and about 300 ml of water. Allow the salt to dissolve completely, then dilute to volume and mix. This solution contains 0"1 mg of iron per ml.

(b) Procedure. Pipette 5 ml of solution B into a 50 ml graduated flask, add 2 ml of glycin solution and mix. Add 2 ml of 2.2'-dipyridyl solution, followed by concentrated ammonia solution dropwise until the red colour of the iron (II)-dipyridyl complex is produced. Dilute to volume with ammonium acetate solution, mix, allow to stand for 1 hr to ensure complete reduction to iron (II), and measure the optical density of the solution at 523 nm against a reagent blank. Calculate the percentage of total iron in the sample from a calibration graph prepared from the standard solution.

Determination o[ calcium

Calcium is determined by forming the calcium complex of glyoxal bis (2-hydroxy aniI), (abbreviated to GBHA), at pH 12"6, and extracting the complex into chloro- form and /so.amyl alcohol (King & Pruden, 1969a). The optical density of the

6 G. Pruden and H. G. C. King

solution of the complex is measured at 531 nm against a reagent blank using 1 cm cells. Co-precipitation of calcium with magnesium and iron is prevented by adding mannitol. Up to 0-8% calcium can be determined using a 20 ml aliquot of solution B. When the calcium content exceeds 0"8%, a smaller aliquot of solution B can be taken and the corresponding amount of blank solution added to bring the volume to 20 ml.

(a) Reagents. Blank solution--l-2 N sulphuric acid. Standard calcium solut ion- dissolve 1 g of pure calcium carbonate in a small excess of hydrochloric acid and dilute to 1 1 with water, giving a solution containing 400 t~g of calcium per ml. Dilute an aliquot of this solution to give a working standard containing 10 ~g of calcium per ml. Sodium hydroxide solution--30% w/v in water. Glyoxal bis (2-hydroxy anil) reagent--0.5 % w/v solution in ethanol. The reagent can be obtained commercially, but it is preferable to use the reliably pure product made by Bayer's (1957) method, by condensing re-sublimed o-aminophenol and aqueous glyoxal at 80 ~ and recrystallizing the product from methanol. Extracting solvent--a mixture of equal volumes of chloroform and iso-amyl alcohol. Mannitol solution--10% w/v solution of mannitol in water.

(b) Procedure. Pipette an aliquot of solution B containing not more than 80 t~g of calcium, the upper limit of determination, into a 125 ml separating funnel, then add sufficient 1-2 N sulphuric acid to bring the total volume to 20 ml. Add 10 ml of mannitol solution, mix thoroughly and then allow to stand for 30 rain. Add 5 ml of sodium hydroxide solution, mix and then cool to room temperature. Add 1 ml of GBHA reagent, mix and immediately add 20 ml of the chloroform/iso-amyl alcohol mixture, then shake for about 30 s. Allow the phases to separate, discard the first small portion and collect 10-15 ml of the lower phase, which contains the calcium-GBHA complex. Clarify the solution by centrifuging briefly at 4000 rev/min and measure the optical density, without undue delay, at 531 nm, using 1 cm cells, against a reagent blank prepared in the same way with 20 ml of 1-2 N sulphuric acid. Calculate the percentage of calcium in the sample by reference to a calibration graph prepared from calcium standards in 1"2 N sulphuric acid.

Determination of magnesium Magnesium is determined by measuring the optical density of the complex formed

with purified Titan yellow at 545 nm (King & Pruden, 1967a; King, Pruden & Janes, 1967b). The method is similar to that described by Meyrowitz (1964), but the concentration of pure Titan yellow used is much lower than that recommended for the commercial dye. The pure reagent gives a small blank value and a linear calibration up to 150 tzg of magnesium. Up to 6% of magnesium can be determined using a 5 ml aliquot of solution B. For the occasional sample in which the magnesium content exceeds 6% a smaller aliquot can be taken, adding sufficient blank solution to give a total volume of 5 ml.

(a) Reagents. Blank solution--l.2 N sulphuric acid. Standard magnesium solu- tion--dissolve 500 mg of pure magnesium ribbon in 15 ml of 6 N hydrochloric acid.

Semi-micro analysis of clay minerals 7 Remove the excess acid by evaporation, dissolve, the residual magnesium chloride in water and dilute the solution to 500 ml. 10 ml of this solution, diluted to 1 1, give a working standard containing 10 /~g of magnesium per ml. Complexing solution~dissolve 16 g of potassium cyanide in 100 ml of water; add 100 ml of triethanoIamine, and mix the solution. Dilute to 250 ml with water and mix. Sodium hydroxide solution--30% w/v in water. Titan yellow reagent solution, 0-008%~dissolve 8 mg of pure Titan yellow in 100 ml of water. Mixed reagent solution--add 100 mg of poly(vinyl alcohol) to 150 ml of water in a 600 ml beaker. Heat gently on a hot plate, with constant stirring, until the temperature of the solution is 60 ~ C. Heat at 613-70 ~ C with constant stirring until the solution is clear. Add 300 ml of water, then 5 ml of 9 N sulphuric acid, 750 mg of hydrated aluminium nitrate AI(NO~)3.9H20 and finally 20 g of hydroxy-ammonium chloride, mixing after each addition. Cool the solution and dilute to 500 ml. Filter through a fine filter paper.

(b) Procedure. Transfer 5 ml of blank solution and aliquots of working standard magnesium solution, e.g. 20 to 140 #g of magnesium, to 100 ml graduated flasks. Add 5 ml of blank solution to the flasks containing the standard magnesium solution. Transfer aliquots of solution B, containing up to 140/~g of magnesium, to 100 ml graduated flasks, and add sufficient blank solution to each of the flasks to bring the volume to 5 ml. Add 5 ml of mixed reagent, 45 ml of water and 2 ml of com- plexing solution to each flask, mixing after each addition. Add 5 ml of Titan yellow reagent solution to the first of the flasks, and then immediately add 5 ml of 30% sodium hydroxide, swirling the contents of the flask. Dilute to volume and mix. Follow this sequence systematically through the whole series of flasks. Allow the solution to stand for 30 min before measuring the opticaI density at 545 nm against a reagent blank, using 4 cm ceils. CaIculate the percentage of magnesium in the sample by reference to a calibration graph prepared from the standard magnesium solution.

Determination o] phosporus

In the determination of phosphorus the yellow phosphomolybdate complex is reduced to molybdenum blue with ascorbic acid (Fogg & Wilkinson, 1958), and the optical density of the solution measured at 810 nm. Up to 1"4% phosphorus can be determined using a 10 ml aliquot of solution B.

(a) Reagents. Ammonium molybdate--sulphuric acid solution---dissolve 10 g of crystalline ammonium molybdate in about 70 ml of water and dilute to 100 ml. Add 150 ml of concentrated sulphuric acid to 150 ml of water, cool and then add the molybdate solution, p-Nitrophenol indicator---0"2% w/v in water. Ascorbic acid-- solid. Standard phosphorus solution--dissolve 1"098 g of potassium dihydrogen phosphate in 250 ml of water. This solution contains 1 mg of phosphorus per ml, and is diluted to give a working standard of 10 txg of phosphorus per ml.

(b) Procedure. Pipette 10 ml of solution B into a 100 ml beaker and add two drops of p-nitrophenol indicator. Neutralize the solution by adding N sodium

8 G. Pruden and H. G. C. King

hydroxide in slight excess, and then discharge the yellow colour by the dropwise addition of r~ sutphuric acid. Dilute to 40 ml, then add 4 ml of ammonium molybdate- sulphuric acid solution and 0.1 g of solid ascorbic acid. Heat the solution to boiling and boil for 1 min. Cool, transfer the solution quantitatively to a 50 ml graduated flask, then dilute to volume and mix. Measure the optical density of the solution against a reagent blank at 810 rim. Calculate the percentage of phosphorus in the sample by reference to a calibration graph prepared from the standard phosphorus solution.

Determination of manganese

Manganese is determined by oxidation to permanganate with potassium periodate (Willard & Greathouse, 1917). The optical density of the permanganate solution is measured at 525 nm. Up to 8 % of manganese can be determined using a 25 ml aliquot of solution B.

(a) Reagents. Phosphoric acid s.g. 1-7. Potassium periodate--solid. Standard manganese solution--dissolve 0.2878 g of potassium permanganate in about 250 ml of water in a 1-t volumetric flask. Add 20 ml of concentrated sulphuric acid and decolorise the permanganate by the careful addition of sodium metabisulphite solution. Oxidise the excess sulphurous acid by the addition of concentrated nitric acid, cool and dilute to volume. The solution contains 0"1 mg of manganese per ml.

(b) Procedure. Pipette 25 ml of solution B into a 50 ml conical flask, add 5 ml of phosphoric acid, mix and heat the solution almost to boiling on a hot plate, then add 0"3 g of potassium periodate. Maintain the temperature (90-100 ~ C) for 30 min for full colour development. Cool the solution and transfer quantitatively to a 50 ml graduated flask, dilute to volume and mix. Measure the optical density at 525 nm against a reagent blank. Calculate the percentage of manganese in the sample by reference to a calibration graph prepared from the standard manganese soIution.

Determination of sodium and potassium

Sodium and potassium are determined directly in solution B with the flame photometer. The sensitivity of the instrument should be such that full scale deflection can be obtained on 5 ppm of sodium and 10 ppm of potassium. Aliquots of solution B may have to be diluted so that the concentration of the element to be determined lies within these limits. The upper limits of determination using undiluted solution B are 1% of sodium and 2% of potassium in the sample.

(a) Sodium. Dissolve 2-5418 g of sodium chloride in distilled water and dilute to 1 1 in a graduated flask. Store in a polythene bottle. This solution contains 1000 ppm of sodium.

(b) Potassium. Dissolve 1-9069 g of potassium chloride in distilled water and dilute to 1 1 in a graduated flask. Store in a polythene bottle. This solution con- tains 1000 ppm of potassium. Prepare diluted standards containing 0-5 ppm of sodium and 0-10 ppm of potassium just before using.

Semi-micro analys& of clay minerals 9 (c) Procedure. Determine sodium and potassium directly in solution B, adjusting

the flame photometer to give full scale deflections with standard solutions containing 5 ppm of sodium and 10 ppm of potassium. Refer to a calibration graph to convert scale reading to ppm of alkali metal, and hence calculate the percentages of sodium and potassium in the sample.

Determination o] iron (I/)

Iron (II) is determined as the 2"2"-dipyridyl complex, as in the determination of total iron. The sample is digested with a mixture of hydrofluoric and sulphuric acids, care being taken to avoid oxidation of the iron (II). Up to 12-4% of iron (II) can be determined using a 20 ml aliquot of the acid digest solution. Pyrite, if present, is unattacked by the acid mixture, leading to an erroneously small value for iron (II). Also, i f carbonaceous matter is present in the sample, it will reduce some of the iron (III) and give an excessively large value for iron (II) (Pruden & Bloomfield, 1969).

(a) Reagents. Hydrofluoric acid 40%. Sulphuric acid--50% v/v (18 N). Boric acid (H~BO3)--saturated solution in water. 2"2'-Dipyridyl solution--0-2% in 10% v/v acetic acid. Ammonia solution, s.g. 0-880. Ammonium acetate solution, pH 7 . 0 ~ molar solution.

(b) Procedure. Weigh accurately 25 mg of the sample into a 30 ml platinum crucible with a well-fitting lid, and add a little water to moisten the sample. Add 10 ml of 50% v]v sulphuric acid and 5 ml of hydrofluoric acid, cover the crucible and rapidly heat the contents to boiling over a bunsen burner. Boil gently for I0 min, then remove the crucible with tongs and completely immerse it in a solution of 100 ml of saturated boric acid and 10 ml of 50% v/v sulphuric acid. With the aid of a glass rod remove the crucible and rinse it into the beaker. Transfer the contents of the beaker quantitatively to 200-ml graduated flask, dilute to volume and mix. Pipette 20 ml of this solution into a 50-ml graduated flask and add 2 ml of 2.2"-dipyridyl reagent, followed by ammonia solution added dropwise until the formation of the red iron (II)-dipyridyl complex is complete. Dilute to volume with ammonium acetate solution, mix, and measure the optical density against a reagent blank at 523 nm. Calculate the percentage of iron (II) in the sample by reference to a calibration graph prepared from a standard iron (II) solution, as in the deter- mination of total iron.

Determination o] water

(a) Moisture (HeO-). The sample is heated in a weighing bottle for 5 hr at 110 ~ C, and the moisture content calculated from the loss of weight.

(b) Total water. Total water is determined using a simplified form of the closed circulation system described by Jeffrey & Wilson (1960).* The apparatus consists of a silica tube containing the weighed sample in a platinum boat, which is heated

* The packing furnace containing heated lead chromate for the removal of oxides of suiphur may frequently be omitted.

10 G. Pruden and H. G. C. King

to 1000 ~ C in a tube furnace. A current of a i r f rom a smal l electr ic p u m p passes through a bubb l e r conta in ing phosphor ic acid, t h rough an abso rp t ion tube con- ta in ing soda t ime, th rough ano the r tube conta in ing magnes ium perchlora te , then over the sample and f inal ly th rough a weighed absorp t ion tube conta in ing magnes ium perchlora te before re turning to the pump.

(c) Combined water. The combined water in the sample is de te rmined by sub- t rac t ing the percentage of mois ture f rom the percentage of to ta l water.

Determination oJ total sulphur Sulphur is de te rmined using the tube furnace p rocedure (Bloomfield, 1962), in

which the sample is igni ted with v a n a d i u m pen tox ide in a cur rent of ni t rogen. Ox ida t i on of vola t i le p roduc ts of decompos i t ion is comple ted b y pass ing them over hot cupr ic oxide. H o t meta l l ic copper converts su lphur t r iox ide into the d ioxide , which is abso rbed in a solut ion of po t a s s ium te t rach loromercura te , and de te rmined co lor imet r ica l ly wi th pa ra rosan i l ine base and fo rmaldehyde (King & Prnden, 1969b).

Determination o] fluorine Fluor ine is de te rmined by the me thod descr ibed by N e w m a n (1968). F luo r ine

is l ibera ted f rom the sample b y pyrohydro lys i s a t 700-800 ~ C in a gas-heated fused- si l ica tube. T h e hydrogen f luoride evolved is abso rbed in a lka l i and de te rmined by t i t ra t ion with t ho r ium n i t ra te o r abso rp t iomet r i ca l ly wi th ce r ium a l izar in complexone .

TAnL~ 1. Analysis of G-1 and W-l, and comparison of results with preferred values

G-1 W-1

Preferred Preferred Constituent Mean value Differ- Mean value Differ-

% ence s % ~ ence

SiOz 72-42 72.52 -0 .10 0-10 52.53 52.58 -0 .05 0.10 A1203 14.16 14.08 +0.08 0.08 14.78 14-94 -0-16 0.08 TiOz 0-28 0-26 +0.02 0.01 1.08 1-08 0 0.04 Total Fe as

Fe203 1.88 - - - - 0'07 11"13 - - - - 0"11 FeO 0"91 0'94 -0"03 - - 8-62 8-71 -0-09 - - Fe203 0"87 0"85 +0"02 - - 1"54 1-38 +0"16 - - CaO 1"35 1-36 -0"01 0"03 11.00 10-92 +0"08 0'14 MgO 0"39 0"35 +0"04 0"03 6-67 6-52 +0"15 0"07 MnO 0"02 0-03 -0.01 0"005 0"18 0-17 +0"01 0-01 P205 0"09 0"09 0 0"008 0"16 0"14 +0"02 0"01 K20 5"47 5"52 --0"05 0"06 0"65 0"63 +0"02 0"01 Na20 3-34 3.29 + 0-05 0"02 2-09 2-15 -- 0-06 0-03 H20- 0-05 0-02 +0.03 - - 0-I2 0-08 +0-04 - - H20 + 0.42 0.25 +0.17 - - 0.55 0-45 +0.10 - -

Semi-micro analysis of clay minerals 11

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12 G. P r u d e n a n d H . G. C. K i n g

A C C U R A C Y A N D P R E C I S I O N O F A N A L Y T I C A L R E S U L T S

Standard deviations have been calculated for the analyses of the silicate rock standards G-1 (granite), W-1 (diabase), G-2 (granite), GSP-1 (granodiorite), AGV-1 (andesite), BCR-1 (basalt), PCC-1 (peridotite) and DTS-1 (dunite) (Tables 1 and 2). The accuracy of the results can be assessed by comparison with the preferred values for G-1 and W-1 (Ingamells & Suhr, 1963). Sufficient data for the other rock samples are not available. Standard deviations were calculated as

/S (x -- 2) ~

where n = 6 for G-1 and W-I, and n = 4 for other rock samples; x and ~ are individual and mean values respectively.

C O N C L U S I O N

In this paper no attempt is made to review or examine critically the existing schemes for analysing the major elements of silicate minerals. A relatively simple scheme is presented which is not unduly time-consuming. The good precision and accuracy of the results depends on modifications, some large, some small, to well- known methods. With the exception of K and Na, each element is determined individually and colorimetrically. No element need be isolated in order to determine it, nor is any element determined by difference. For the first time calcium in clays and other silicates can be determined specifically and colorimetrically.

By improving the quality of certain commercial reagents the sensitivity of the A1 determination with Alizarin Red S has been increased, doubling the length of linearity of the calibration graph. Titan yellow, which had fallen into disrepute because of its variable composition, can now be made reproducibly pure for deter- mining Mg. The sensitivity of other determinations, e.g. SiOz and TiO2, has been improved by measuring the colour of the complexes at the wavelength of maximum optical density. It is hoped that the scheme will be especially useful to analysts when only small samples (100 mg minimum) are available. The scheme is equally satisfactory, however, for samples in gram quantities.

REFERENCES

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Semi-micro analysis of clay minerals 13

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