Clay Minerals (1967) 7, 215.

C H A N G E S IN P H L O G O P I T E S D U R I N G T H E I R

A R T I F I C I A L A L T E R A T I O N

A. C. D. N E W M A N

Rothamsted Experimental Station, Harpenden, Hefts

(Read at a joint meeting of the Clay Minerals Group and the Groupe Bel~e des Argiles at Brussels, 2 June 1967)

ABSTRACT: When phlogopites containing only a little iron are artificially altered with sodium tetraphenytboron solutions, the vermiculite-like products contain fewer interlayer cations than the original micas. Structural formulae of the products are calculated from the chemical analyses, together with infrared examination of the angular vibration band of water at 1650 cm -1 to distinguish between water of hydration and hydroxyl groups. The formulae obtained are consistent with the weight-loss curves and show that the products contain, more than 4(OH,F) per formula unit. The interpretation of these formulae is that the silicate layers lose net negative charge by protonation of structural oxygen anions to form new hydroxyl groups.

Although the micas of igneous rocks are considered to be relatively stable minerals (Pettijohn, 1949), they are much altered by weathering in soil (Denison, Fry & Gile, 1929). In the first stage of weathering, potassium between the silicate layers is replaced by hydrated cations. Because the silicate layers lose some of their net negative charge during the weathering processes, the potassium lost is often not electrically balanced by the replacing cations. For many years it was thought that negative charge was lost. because Fe z+ in the silicate layers was oxidized to Fe s+, but Bradley & Serratosa (1960) suggested that Fe z+ was oxidized in a reaction that did not change the net charge; evidence is now accumulating that their sug- gestion is correct. Raman & Jackson (1966) and Newman & Brown (1966) altered micas artificially with sodium tetraphenylboron solutions and found that the loss of layer charge was not balanced by the oxidation of ferrous iron; these authors independently proposed that the layer charge was decreased when protons were incorporated into the structure, a suggestion first made by Gruner (1934), or when structural hydroxyl ions were released to solution. Indirect evidence supported these suggestions (for instance, the p H of solutions in contact with micas always increases) but direct evidence from the determination of structural hydrogen in altered micas was equivocal. This is because hitherto it was difficult to differentiate by thermal methods between water of hydration and structural hydroxyls, especially

216 A. C. D. Newman

in vermiculites. However, infrared examination of the expanding minerals has shown that the angular vibration of water can be observed in addition to the OH stretching of water and structural hydroxyl groups (Fripiat, Chaussidon & Touillaux, 1960), so that, in principle, it should be possible to combine thermal and infrared analysis to determine structural hydroxyl.

If, as suggested above, the silicate layers lose charge by a mechanism that is independent of Fe 2+ oxidation, one critical experiment would be to exchange potassium from micas in which too little Fe +z is oxidized to alter the charge and find if the layer charge alters. Scott & Smith (1966) stated that the layer charge of muscovite, illite and phlogopite does not change with K-depletion, but this con- clusion was based on interlayer cation contents, expressed as meq/g; no allowance was made for any change in formula weight during K-depletion.

The first aim of the work reported here was to make a careful comparison of the layer charge of phlogopites containing little iron before and after artificial alteration. After the loss of charge previously reported by Newman & Brown (1966) was substantiated, attempts were made to establish the water and hydroxyl contents of the altered micas by chemical analysis, thermogravimetry, and infrared analysis, and find whether the hydroxyl content changed during their alteration.

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

Materials Two phlogopites were used, one obtained from Messrs Hart, Maylard & Co.,

London and the other from Ward's Natural Science Establishment, Inc., New York. The micas were ground and samples of the 100-300 mesh fraction were divided into two parts; one part was used for chemical analysis and the other for the alteration experiment.

Chemical analysis The rapid analysis scheme of Shapiro & Brannock (1956) was followed with

a few modifications. Magnesium was determined by the method of Meyrowitz (1964) using a purified Titan yellow reagent (King & Pruden, 1967). Fluorine was separated by pyrohydrolysis at 660 ~ C, using V20~ as the accelerator, and deter- mined in the distillate by titration with thorium nitrate using alizarin red S indicator (Bennett & Hawley, 1965). Exchange capacities were estimated by determining the calcium released by N NaC1 from Ca-saturated materials; the calcium was titrated with EDTA using murexide as the indicator.

A Iteration experiments The micas were altered by shaking with a solution containing NaC1 and tetra-

phenylboron in a thermostatic water bath at 60 ~ C; sodium enters the mica and replaces potassium, which is precipitated as potassium tetraphenylboron. The detailed method has already been described (Newman & Brown, 1966) but the efficiency of the extraction was improved by using a frothing technique to separate the bulk of the potassium tetraphenylboron (Scott & Reed, 1962) before washing with

Alteration of phlogopites 217 aqueous acetone--CaC12 solut ion. A f t e r four extract ions , 95% of the K in the micas had been rep laced and the exper iment was s topped . The a l te red micas were sa tu ra ted with Ca, washed with wa te r and e thanol , a i r -d r i ed and ana lysed in para l l e l with the una l te red micas (Table 1). Ca lc ium re leased b y N NaC1 was a lso de t e rmined (Table 2) and the NaCl -ex t r ac t ed ma te r i a l s were washed with wa te r and e thanol , and a i r -dr ied .

TABLE 1. Chemical analyses of phlogopites and their alteration products

P1 AP1 P3 AP3

SiO~ 40-87 37-39 40.57 38.58 AI~08 12"42 11 "20 16"39 15"08 Fe~O~ 0"31 0.33 0"96 0"94 FeO 2-71 2-59 1-67 1 "54 TiOz 0-46 0-43 0.59 0.57 MgO 26-64 23.71 24-78 22"67 CaO 0"27 5"39 0"27 5-24 MnO Trace Trace Trace Trace K~O 10.10 0.47 10.29 0.51 Na~O 0.25 0-03 0-30 0.01 Wt loss below 110 ~ 1.23 2.90 0-29 2-49 Wt loss 110-1000 ~ 3.42 t3.98 4-07 12.37 F 6.3 5.8 2.7 2.5

P1, Phlogopite, Ontario; P3, phlogopite, source unknown; AP1, altered phlogopite 1, Ca-saturated; AP3 altered phlogopite 3, Ca-saturated.

TAstE 2. Interlayer ion.contents of phlogopites and their alteration products, meq/g ignited weight

K Na Ca Total

P1 2.25 0-085 - - 2.34 Ca-AP1 0.120 0.012 2.20 2.33 Ca exchanged by Na 2.25

P3 2.28 0-098 - - 2.38 Ca-AP3 0" 127 0.003 2"08 2"21 Ca exchanged by Na 2.05

X-ray diffraction examination Orien ted specimens of the sod ium-sa tu ra t ed a i r -d r i ed a l te red micas were

examined as descr ibed before (Newman & Brown, 1966) and bo th gave a series of r a t iona l basa l spacings based on 12"2 A; the two samples d id no t differ s ignif icant ly e i ther in intensi ty or spacings of the first five orders of reflection.

218 A. C. D. Newman

15

!4

13

12

11

g

c

(a)

J :tJ !I

3ii 2 ~ ,1 0 200 400 6&3

J

(b)

y /

J f

P3 J P1 j

8 1000 200 400 600 800 I 0 Temperature (~

FtG. 1. Weight loss curves of phlogopites and their sodium-saturated alteration products. Solid lines show weight loss in vacuum under dynamic heating at 2" I o C/rain; dashed lines show weight loss after 16 hr at room temperature and after periods

(5 hr for Na-AP1 and 4 hr for Na-AP3) at 1000 ~ C. (a) Na-AP1, (b) Na-AP3.

Thermogravimetric analysis The change in weight of the micas and their sodium-saturated alteration products

was determined as a function of temperature, using a silica spiral thermobalance similar to that described by Greene-Kelly & Weir (1956). The balance has now been modified for vacuum operation and all weight changes were recorded at a pressure of about 10 -1 torr; no difficulties were experienced with spring oscillation, presumably because convection currents were largely eliminated. The spring was calibrated at several temperatures up to 1000 ~ C so that a correction could be made for the change in torsional modulus of quartz with temperature. The calibra- tion was checked by measuring the loss of weight of A.R, calcium carbonate, which X-ray examination showed to be a pure calcite. For a sample weight of 83 mg with a heating rate of 2-1 ~ C/min , the measured loss was 44"03 %, com- pared with a theoretical loss of 43"97%. All samples were examined under the same conditions; the weight loss curves obtained for the micas and their sodium- saturated alteration products (Na-AP1 and Na-AP3) are shown in Fig. 1. The water content of Na-AP1 and Na-AP3 was determined by the method of Jeffery & Wilson (1960) in which the sample is pyrolysed in a recycling current of air and the water evolved is absorbed in a weighed tube containing magnesium perchlorate (Table 3).

Alteration of phlogopites TABLE 3. Water and fluorine contents of altered phlogopites as percentage air-dry

weight

219

Temperature range of H,O analysis H~O F

Na-API 20-350 9.4 350-t000 3-2

Total 12.6 20-700 11-0

700-1000 1-0 Total 12.0 6.3

Na-AP3 20-400 9- 3 401)-1000 3.8

Total 13.1 2.4

Inirared spectroscopic examination (at the Macaulay Institute, Aberdeen)

Highly transparent films of the altered micas, suitable for infrared study, were prepared by the method of Walker & Garrett (1967). This involved swelling the propylammonium-saturated micas in water, followed by conversion of the swollen mica to a stable aqueous dispersion by treatment with a laboratory homogenizer. The dispersion was concentrated, with continuous stirring, to 30-40 mg mica/ml. Evaporation of 1 ml of this dispersion on fiat polyethylene sheet gave a trans- parent film, about 2-5 cm in diameter, which was stripped from the polyethylene, and converted to the sodium-saturated form by soaking in N NaC1 solutions. Excess salt was washed out with aqueous ethanol. The films were mounted in a quartz vacuum cell, designed by Angell & Schaffer (1965), and the spectra recorded after heating in vacuum to successively higher temperatures. The water content of the films was estimated from the intensity of the 1650 cm -1 angle-deformation vibration of water, and the results expressed as a percentage of the band intensity obtained at room temperature in vacuum (Fig. 2).

D I S C U S S I O N

The structural formula or unit cell content of a mineral can be calculated without assumptions if unit cell constants, density and analyses of all constituent cations and anions are known. Very often, some of this information is lacking and structural formulae of layer silicates are usually calculated from chemical analyses by making assumptions based on the knowledge that they are isostructural with other minerals whose structure is already known. For micas and other 2:1 layer silicates, a Widely used assumption is that there are twenty-four anions, of ideal composition O2o(OH,F)~, in the structural unit. The number of each cation in the structural unit is found from the atomic proportions, which are multiplied by a factor that

220

100

A. C. D. Newman

~ 6c

~ ~c

2C

: + ~ +

200 400 600 - 800 "temperature C~

1 FIG. 2. Absorbance of 1650 era- band (angular vibration of H=O) in altered micas after heating at temperatures up to 750 ~ C, expressed as a percentage of the absorbance of

the unheated sample. +, Na-AP1 ; O, Na-AP3.

makes the sum of the positive charges equal the forty-four negative charges of the anion framework. Formulae of micas obtained this way are usually satisfactory in the sense that rational numbers of tetrahedral, octahedral and interlayer cations are calculated (Foster, 1960). If the anlyses include hydrogen, this should in theory be included in the calculation, but in practice the number of (OH +F) calculated from the analyses rarely equals four, possibly for reasons discussed by Foster (1964).

With the unaltered phlogopites, there was no alternative to calculating the formulae by equating the charge of the cations excluding H to 44; this was because the T G A curves of the micas (Fig. 1) showed that they were incompletely decom- posed at 1000 ~ C, the maximum attainable in the tube furnace used for the water determination.

By contrast, the altered micas dehydroxylated readily so that their total water content could be determined without difficulty, but to use the data for calculating structural formulae, it was essential to distinguish between hydration water and hydroxyl in the analysis and the analytical separation has for long been uncertain. Vermiculites retain water between the layers at temperatures much exceeding 110 ~ C (Walker, 1956) so that the conventional analysis for H~O below and above 110 ~ C

Alteration of phlogopites 221 (so-called Hz O- and H20 +) was not likely to differentiate between I-I20 and OH; indeed, Fripiat et al. (1960) found that dehydroxylation began before all molecular water was released. Inspection of the T G A curves of the altered materials suggested that hydration water would be removed by heating to about 400 ~ C and the tem- peratures used in the water determinations (Table 3) were chosen on this basis.

M o r e definite information about the release of hydration water at different temperatures was obtained from the infrared spectra of the altered micas, for which I am greatly indebted to Dr V. C. Farmer and Dr J. D. Russell of the Macaulay Institute, Aberdeen. The angular vibration of the water molecule gives an absorption band at about 1650 cm -1, the intensity of which decreases as the water is driven off by heating. Fig. 2 shows the influence of temperature on the absorbance of this band, expressed as a percentage of the absorbance at room temperature; it is interesting to note that the alteration products differ in their water retention. After heating to 400 ~ C, the absorption of the water band in Na-AP3 was only 2% of the initial intensity, whereas Na-AP1 still contained an appreciable amount of water that was only removed by heating to above 700 ~ C. Because it is unlikely that dehydroxylation occurs below 400 ~ C in trioctahedral minerals, the water determinations for Na-AP3 (Table 3) may be used with reasonable confidence that they correspond to molecular HzO (RT to 400~ C) and OH (400-1000 ~ C). This is not so, however, with Na-AP1, for the infrared evidence shows that water deter- mined over the temperature range 350-1000 ~ C will include strongly-bound molecular water as well as structural hydroxyl[.

Two attempts were made to overcome this analytical difficulty neither of which was entirely satisfactory. In the first, water remaining at the analysis temperature was estimated from the intensity of the infrared absorption at 350 ~ C relative to the absorption at room temperature, and the analytical data were corrected for this residual water. In the other method, the water content of Na-AP1 was redeter- mined over the ranges RT-700 ~ C (molecular HzO) and 700-1000 ~ C (hydroxyl). However, this is likely to underestimate the hydroxyl content because above 400- 500 ~ C, Fe 2+ is oxidized by the reaction (Addison & Sharp, 1962)

4 Fe z+ + 4 O H - + O2 ~ 4 Fe ~+ + 4 0 2 - + HzO (I) which releases some structural hydroxyl as water.

Calculation of structural formulae from analyses including [-120, OH and F One of the aims of this investigation was to find what mechanism was responsible

for any loss of net negative charge when micas were altered, so that it was important not to use restrictive assumptions in calculating the structural formulae. For instance, the assumption that the anion content is O20(OH,F)4 causes apparent changes in the Si occupancy of the tetrahedral sites and hence in the AI content of the octa- hedral layer; in phlogopites these are the main alterations that seem to compensate for loss of interlayer cations. Newman & Brown (1966) argued that these complex rearrangements were unlikely, and showed that a simpler interpretation was obtained when the number of Si per eight tetrahedral sites was the same as in the unaltered mica; in the phlogopites, the loss of interlayer cations was then balanced mainly

222 A. C. D. N e w m a n d

by a decrease in anionic charge. They suggested that the anion charge was altered in one of the following reactions:

(A) structural hydroxyl ions were released; or (B) protons were sorbed at some hydroxyl sites.

An argument in favour of these reactions is that they suggest a reason why the substitution of fluorine for hydroxyl influences the ease of mica alteration (Raussell Colom et al., 1965). Raman & Jackson (1966) also found that micas lose charge by proton sorption when they are altered artificially but suggested that new hydroxyls were formed from oxygen ions in the tetrahedral layer (reaction C).

The products from these reactions may be given the following general structural formulae :

(A) N c , o + ~ O 2 o O H ~ F ~ . (y + 4 - - r)H20 Number of structural ( O + F ) : 20 +r ; charge 40+r .

(B) N t 4 0 + ~ O 2 0 O H ~ , _ ~ H 2 0 ~ F , . yH20 Number of structural (O + F) : 24; charge 40 + r.

(C) N~4o+~O~6+~OH<~OH~4_~F~. yH20 Number of structural (O + F) : 24; charge 40 + r.

In these formulae, Nc,o+,) represents the number of cations normalized to a total charge of (40+r) per formula unit, and r is a number that in formulae (A) and (B) cannot exceed 4 and in formula (C) cannot exceed ( 8 - x); when r = 4, the formulae revert to the conventional O20(OH,F)4 anion structure. These formulae all have the same hydrogen content (2y + 8 - r - - x ) , and the same anion or cation charge (40+r); they differ only in the distribution of hydrogen between structural hydroxyl, structural water and hydration water. The constants in the formulae are found from the hydration water, hydroxyl and fluorine contents, determined on the hydrated mineral, using equations derived from the formulae (A), (B) or (C). For (A) and (B), the equations are:

1 8 ( y + 4 - - r) Hydration water as H20 : hi -- M (1)

(r -- x)9 Hydroxyl as H20 : h2 -- M (2)

19x Fluorine: / = M__ (3)

M is the total formula weight, so that M = M'~,o+~ + 320 + 17r + 2x + 18y (4)

and M'~4o+~ is the sum of the weights of the cations in the formula unit. For (C), equations (3) and (4) are the same; the other equations are:

18y Hydration water as H~_O: h~ -- M (5)

(8 -- r -- x)9 Hydroxyl as H20 : h2 -- M (6)

Alteration of phlogopites TABLE 4. Structural formulae of phlogopites and their alteration products

223

Na-APlt

PI* (i) (ii) (iii) P3* Na-AP3t

Si 5-85 5-69 5.90 5-85 5.66 5.67 A1 2.10 2.01 2.08 2.07 2"34 2.33

27 7.95 7.70 7-98 7.92 8.00 8.00

AI . . . . 0.36 0.28 Fe 3§ 0.03 0-04 0.04 0.04 0.10 0.10 Fe 2+ 0-32 0.33 0.34 0-34 0.20 0-19 Ti 4§ 0.05 0-05 0.05 0.05 0-06 0.06 Mg 5-68 5.38 5.57 5.53 5.16 4.97 Ca 0.04 0.04 0-04 0-04 0.04 0"04

27 6-12 5-84 6.04 6-00 5.92 5-64

K 1.84 0.09 0-09 0.09 1.83 0.10 Na 0.07 1.67 1-75 1.74 0.08 1.57

27 1-91 1.76 1.84 1.83 1-91 1.67

O 20.00 18.36 19.90 19.59 20-00 19-15 OH 1.19 2-68 1-06 1.39 2.90 3.73 F 2.81 2-96 3.04 3-02 1.I0 1.12 H~O - - 4.93 5.62 5-41 - - 4.57

* Based on forty-four negative charges and twenty-four anions. t Based on F, OH and H~O analyses and twenty-four anions. For Na-AP1, formula (i) was based

on H~O determined at 350 ~ C and OH from 350-1000 ~ C, using infrared to estimate H,O retained at 350 ~ C; formula (ii) was based on H~O determined at 700 ~ C and OH from 700--1000 ~ C; and formula ('hi) was calculated as formula (ii) but allowing for OH released below 700 ~ C equivalent to pe a+ .

Table 4 gives the structural formulae of the micas and their a l terat ion products; the mica formulae were calculated to forty-four negative charges. The formulae of the altered micas can be calculated directly by solving equat ions (1)-(4) or (3)-(6) for x, y, r and M'(4o+r~, bu t in practice it is quicker to ob ta in the solutions by successive approximat ion. M is not very sensitive to r, so that if r is first set equal to 4 (equivalent to forty-four negative charges), x and y can be calculated from equat ions

(3), and (1) and (2) or (5) and (6). M44 is calculated by the usual procedure and M found from the first values of x, y and r. This enables better values of x, y and r to be obtained, and thence M. The refinemen~t of x, y and r is stopped when the new value of M does no t alter x, y and r; usuaUy this takes only two rounds of calculation.

Using the equat ions based on formulae (A) and (B), the impor tan t result was obta ined that r exceeded 4. As pointed out above, a value of r greater than 4 is meaningless in formulae (A) and (B), for it indicates that the n u m b e r of (O + O H + F ) exceeds twenty-four per structural formula and for structural reasons

2 2 4 A. C. D. Newman

this is extremely unlikely. The excess of hydroxyl ions eliminates reaction (A) immediately and also eliminates (B) unless it can be supposed that the association of a proton to a structural hydroxyl group forms a grouping with properties very different from water. The present evidence, therefore, shows that protonation of structural oxygen atoms, reaction (C), is the important process that decreases the anionic charge. The structural formulae of Na-AP1 and Na-AP3 in Table 4 were calculated on the basis of formula (C) and equations (3)--(6).

Three structural formulae were calculated for Na-AP1 using different estimates of the molecular H20 and structural OH contents. For formula (i), the intensity of the absorption band of H20 at 1650 cm -1 was used to estimate H20 retained by the sample at 350 ~ C relative to H_~O retained in vacuo at room temperature, and the analyses for H20 (RT-350 ~ C) and OH (350~ ~ C) were corrected for this residual water before calculating the structural formula. The infrared examination showed that negligible molecular H20 was retained above 700 ~ C and this temperature was chosen for the analyses for H20 (P,T-700 ~ C) and OH (700~ ~ C) used in calculating formula (ii). Formula (iii) was based on the same analyses as formula (ii) but corrected to allow for release of OH below 700 ~ C when Fe ~§ was thermally oxidized.

Of these three formulae, formula (i) was unsatisfactory in several respects; the fairly substantial deficit of tetrahedral cations and increase in OH content seemed unreasonable. Formulae (ii) and (iii) are both satisfactory from these aspects, but there seems little doubt that Fe 2+ is oxidized below 700 ~ C, so that formula (iii) is probably preferable to formula (ii). The failure of the infrared correction for formula (i) may be attributed to the very different physical state of the samples: water analysis was made on small single crystals, whereas the sample for infrared analysis had been largely dispersed into separate silicate sheets and then deposited as a highly oriented film.

Comparison of the structural formulae of each mica with that of its alteration product shows that:

(1) The interlayer cations are less in both alteration products in spite of appear- ing unchanged for P1 when expressed as meq/g (Table 2).

(2) The silica occupancy of the tetrahedral sites is unchanged. This is independent evidence that the structural formula calculation used by Newman & Brown (1966) was correct, and is supported by the silica levels in the extract solutions (Table 5). The total silica removed from the micas in four extracts would have changed the silica content by only 0"1%, with negligible effect on the structural formula.

Thermogravimetric analyses Having determined the structural formulae, it was possible to attempt an inter-

pretation of the T G A curves. The total water collected in the water determinations was tess than the recorded weight loss, which suggested that fluorine was being lost by the samples at high temperatures. Fluorine was determined in the residues from the thermobalance experiments and it was found that about one-sixth of

Alteration of phlogopites 225

the fluorine in Na-API and one-quarter in Na-AP3 had been lost; further experi- ments showed that most of this fluorine was evolved above 800 ~ C. The compound of fluorine that is volatilized is not known for certain, though evidence suggests I-IF in a hydrous atmosphere and NaF in anhydrous conditions (Eitel, 1954, p. 1390). Although loss of F as SiF~ may also be possible, the weight change was calculated by assuming, rather arbitrarily, that fluorine was lost as NaF; this tends to underestimate the weight loss from dehydroxylation.

The other side reaction, the thermal oxidation of ferrous iron (reaction I), causes only a small weight change (H lost equivalent to Fe 2+) but the full water equivalent is taken up in the absorption tube. The colour of the altered micas changed from silver-grey to brown between 500 ~ and 780 ~ C so that it was assumed that Fe 2+ was oxidized over this temperature range.

Tables 6 and 7 compare the TGA weight losses calculated in moles with the theoretical weight losses expected from the structural formulae. Under vacuum, the initially expanded (12.2 A basal spacing) micas collapsed, releasing 2-2.4 H.~O per Na; Van Olphen (1965) found that Na-Llano vermiculite released 2.06 H20 per Na.

TABLE 5. Silica and alumina in tetraphenyl boron solutions in contact with phlogopites

SiO, (ppm) SiO, (mg/g mica) A1,Os (ppm)

P1 1st extract 23 0-29 < 1 2ud extract 26 0.33 < 1

P3 I st extract 27 0.42 < 1 2nd extract 20 0-31 < 1

TABLE 6. Water and hydroxyls in Na-AP1 estimated from thermogravimetric analysis. Formula weight (iii) = 915

Temperature Weight loss Weight loss as range (~ per formula moles per formula

Evacuated, 20 20- 200

200- 500 500- 780

63"6 3"53 H~O (2"03 H20/Na) 2.7 0.15 H~O 13-7 0.76 H~O I 1.4 0.62 H,O 0.34 OH

(as H ~ Fe *+) 780-1000 29.7 0.96 OH 0.50 NaF

(as H~O) Totals 5.06 H~O 1-30 OH Total H 11.42

Structural formula (Hi) 5.41 H~O 1"39 OH Total H 12"21

G

226 A. C. D. Newman

TABLn 7. Water and hydroxyls in Na-AP3 estimated from thermogravimetric analysis. Formula weight = 885

Temperature Weight loss Weight loss as range (~ per formula moles per formula

Evacuated, 20 66.2 3.68 H~O (2.35 H20/Na) 20-200 2.4 0-13 H 20

200-550 6-6 0.37 H~O 550-780 10.4 0"19 OH

�9 (as H---- Fe ~-) 1.13 OH (as H~O)

780-1000 39-6 3-10 OH 0-28 NaF (as H~O)

Totals 4"18 H 20 4"42 OH Total H 12"78

Structural formula 4.57 H~O 3-73 OH Total H 12-87

Some water is trapped between the layers and under dynamic heating is released from Na-AP3 by 480 ~ C (a weak inflection) and from Na-AP1 in two stages com- plete at 800 ~ C (two strong inflections); these assignments are essentially based on the absorption of the 1650 cm -1 band under static heating conditions (Fig. 2).

The TGA data agree reasonably well with the structural formulae predictions, though there are some discrepancies. Water and total hydrogen in Na-AP1 are smaller when estimated from TGA, but in Na-AP3, TGA predicts more hydroxyls than the structural formula, although the total hydrogen agrees well. These dis- erepancics serve to underline the inherent dit~culties in obtaining accurate estimates of water and hydroxyls in hydrated alumino-silicates.

One of the interesting aspects of this work is the difference in behaviour between P1 and P3. P3 is more readily able to decrease its net negative charge than P1, and other work has shown that P3 also releases potassium much more readily. Na-AP1 retains more water to higher temperatures than Na-AP3, and TGA and infrared data both suggest that there are two different sites for water in the collapsed structure, t.~emically, the micas differ mainly in their aluminium and fluorine contents, but the relevance of these differencs to the formation of new hydroxyls is not clear. Possibly the ideal tetrahedral ratio of S i A l is 3 : 1 and any A1 in excess of this weakens the structure and allows protons to attack tetra- hedral oxygens. It is hoped that answers to these unexplained features may be found in detailed structural analyses at present in progress.

A C K N O W L E D G M E N T S

The author thanks G. Pruden for the analysis in Table I and Mrs S. Shepherd for technical assistance.

Alteration of phlogopites 227

R E F E R E N C E S

ADDISON W.E. & StLM~dP J.H. (1962) Clay Miner. Bull. 5, 73. ANGELL C.L & SCn.~ER P.C. (1965) 1. phys. Chem. 69, 3463. BEN~Tr H. & t-Ixw~y W.G. 1965) Methods of Silicate Analysis, Chapter 42, Academic

Press, London. BRADLEY W.F. & SE~TOS,~ J.M. (1960) Clays Clay Miner. 7, 260. DENSION I.A., FRY W.H. & Gu.~ P,L. (1929) Tech. Bull U.S. Dept. Agric. No. 128. ErrEL W. (1954) The Physical Chemistry of the Silicates, Chapter E.1, p. 1390, The University

of Chicago Press, Chicago, Illinois. FOST~ M.D. (1960) Prof. Pap. U.& geoL Surv. No. 354-B. Fosa~z M.D. (1964) Prof. Pap. US. geol. Surv. No. 474-F. FRIPIAT J.J., CI~USSIDON J. & TOUILLAUX R. (1960)/. phys. Chem. 64, 1234. Gas IL & WEre A.H. (1956) Clay Miner. Bull. 3, 68. GRtZr~R J.W. (1934) Am. Miner. 19, 557. JEFFERY P.G. & WmSON A.D. (1960) Analyst, 85, 749. K ~ o H.G.C. & PRUDEN G. (t967) Analyst, 92, 83. ME~OWITZ R. (1964) Am. Miner. 49, 769. NEWMAN A.C.D. & BROWN G. (1966) Clay Miner. 6, 297. V,~ OLPrmN H. (1965) I. colloid Sci. 20, 822. PEa"ruot-n~ F.J. (1949) Sedimentary Rocks, p. 380 et seq. Harper & Brothers, New York. RAMAN K.V. & JACKSON M.L. (1966) Clays Clay Miner. 14, 53. RAUSELL-CoLoM J., SWEATM.Mq T.R., WELLS C.B. & NORRISH K. (1965) Experimental Pedology.

Proc. 11th School Agric. Sci. Nottingham (E. G. HaUsworth and D. V. Crawford, editors), p. 40. Butterworths, London.

ScoTT A.D. & REED M.G. (1962) Proc. Soil Sci. Soc. Am. 26, 41. SCOTT A.D. & SMn~ S.J. (1966) Clays Clay Miner. 14, 69. SrlAPIZO L. & BRANNOCK W. (1956) Bull. U.S. geol. Surv. No. 103642. WALleR G.F. (1956) Clays Clay Miner. 4, 101. WALKER G.F. & GARRETr W.G. (1967) Science, N.Y. 156, 385.

Note added in proof

Very recently, W. T. Granqu i s t and J. V. Kennedy (1967, Clays Clay Miner . 15, 103) have shown tha t wate r is more s t rongly a d s o r b e d on c lay minera l s in which hydroxy l is rep laced by fluorine, an effect s imi la r to tha t found with the a l t e red phlogopi tes descr ibed here ( compare Fig. 2 in this p a p e r with Granqu i s t & Kennedy ' s Fig. 4).