Part 1. Role of the Exchangeable Cation in the Hydration States of Smectites.

Transcription

1 THERMAL REACTIONS OF SMECTITES By H. G. MIDGLEY 1 and K. A. GROSS z (1) Building Research Station, Watford. (2) Defence Standards Laboratory, Victoria, Australia. [Read 14th April, 1956.] ABSTRACT The thermal dehydration of a series of smectites has been investigated by X-ray methods. It has been found that the rate of collapse of the layers is governed by the type of exchangeable ion present. The high temperature changes of a new saponite have been investigated. The saponite dehydrate s to talc and then to enstatite, the 5"2/~ fibre axis remaining constant throughout. Part 1. Role of the Exchangeable Cation in the Hydration States of Smectites. INTRODUCTION A series of smectites, with different ions in the exchangeable position, has been examined by X-ray diffraction methods at temperatures up to 320~ The work was suggested by a study which was being made of the thermal reactions of a fibrous saponite from Church Cove, Lizard, Cornwall, found by one of us, (H. G. M.). This sample will be described for the first time in this paper. It is well known that when smectites are heated in the range 0-200~ they lose water reversibly, accompanied by a change in the basal spacing from 14/~ to 9.6/~ (Greene-Kelly (1953)). However, the intermediate condition was not examined. Experiments on smectites containing magnesium in the exchangeable position have shown that, on heating in the range up to 125~ an intermediate phase is produced which has a basal spacing of about 11.5 A; with further heating, values of about 10.5 A at about 200~ and 9-9 A at 320~ are obtained. When sodium is in the exchangeable position there is no discrete intermediate stage, the basal spacing decreases to 9.5,~ by at least 95~ and remains at this value with further heating. With calcium as the exchangeable ion there is a nearly continuous reduction of basal spacing until it reaches 9"5 A at about 200~ The difference in the amount of water absorbed appears to bear a relation to the cation present in the exchangeable position and this is discussed by Mackenzie (1950) and Hendricks et al. (1940). It seems likely that the stages in the dehydration of smectites represent differently bonded water. 79

2 80 H.G. MIDGLEY AND K. A. GROSS EXPERIMENTAL PROCEDURE A small furnace was made which could be mounted on the "Unicam" single-crystal goniometer. The specimen in the form of a fibre, flake, or powder in a capillary tube could be placed at the centre of the furnace and the diffraction pattern was recorded on a flatplate cassette. With CuKa radiation and a specimen-to-film distance of 6.0 cm the first and second orders of the basal spacings could be recorded. During heating the sample was exposed to the normal laboratory atmosphere, no attempt being made to control the humidity. The temperatures were recorded with a Pt-PtRh thermocouple placed close to the specimen. The accuracy of this measurement was about ~ 5~ Material. The following samples were used in the experiments. 1, saponite, Church Cove; 2, calcium-saturated saponite; 3, sodium-saturated saponite; 4, magnesium-saturated bentonite, Wyoming; 5, calcium-saturated bentonite, Wyoming; 6, sodiumsaturated bentonite, Wyoming; 7, magnesium-saturated bentonite, Cheta. The saponite has not been described before, so a description is given below. The other samples were prepared to have the requisite exchangeable cation, by shaking with the appropriate metal salt solution. The Cheta bentonite was a sample kindly provided by J. A. White of the Illinois Geological Survey. Saponite. The material occurs as a vein invading Kennack gneiss at Church Cove, Lizard, Cornwall. It is fibrous, green to pale green, soft H 2, soapy to the touch, and has refractive indices 1-520, ~0-005, 7:z 11 ~ positive elongation, the fibres giving an interference figure, biaxim positive, 2V small. A chemical analysis (by L. J. Lamer) gave SiO , FeO 0.43, Fe20 ~ 1.57, AlzO , CaO 0.80, MgO MnO 0.03, Loss at 1000~ 21.81, Loss at 110~ This chemical analysis is equivalent to a formula :-- [Mgs.s2Alo.xFe2+0.1] 6 [Si6.76All.04Fe3+0.2] 8 O20(OH)410H20 + Ca0.1 Mg0.48 Chemical tests have shown that magnesium and a trace of calcium are the exchangeable ions. This analysis is typical of a magnesium smectite. Powder X-ray diffraction patterns of the mineral and of a sample saturated with glycerol were taken on a 10 cm diameter camera with CuK~ radiation. The spacings are given in Table 1. The occurrence of a long spacing at 14 which, when the sample was saturated

3 THERMAL REACTIONS OF SMECTITES 8 1 TABLE 1--X-ray Powder Data of Saponites Church Cove Church Cove~,- Milford (Cahoon 1954) Transvaal (Schmidt 1953) Lizard{ (Midgley 195~) d I , " " "30 2 2" " " " "28 2 l " "89 2 d I 17" " " "55 6 2"97 5 2"70 1 2"60 7* * 2.2l " " " "74 4 l " " ' "27 1 d " , '26 2 "06 1 "72 1 " / d [ 0 14"65 vs m s m s 4 2"627 m ) 2487 s w w 1,689 w s w w d "00 2"605 2"48 2"26 2 "00 / l " !8 4 1 "268 2 Measurements of d in ~, l~ -intensity, * =diffuse, t =glycerol stabilised, s--strong, m--moderate, w~weak, v~very, with glycerol, expanded to 17 A, confirms the identity of the mineral as a smectite, The sample of saponite from the Lizard is strongly fibrous and a fibre X-ray photograph taken in an X-ray goniometer shows strong preferred orientation (Fig, 1 a). The photograph shows the repeat distance along the fibre axis to be 5-2 A, and the cell dimensions to be 5,2, 9-2 and 14.7 A. Three samples of saponite have recently been described in the literature, by Schmidt (1953) from the Krugersdorp District, Transvaal, by Cahoon (1954) from Milford, Utah, and by Midgley (1951) from the Lizard. The diffraction patterns are given for comparison in Table 1. The chemical compositions (Table 2) do not differ appreciably except for the exchangeable ions; the samples from Church Cove and from the Transvaal have predominantly magnesium as the exchangeable ion, the mineral from Church Cove having more than that from the Transvaal. The analysis quoted for the sample from Milford did not specify the exchangeable ion, but calculations from

4 82 H.G. M1DGLEY AND K. A. GROSS the chemical analysis show that it is probably calcium. The refractive indices vary considerably but, as the minerals are strongly fibrous, the estimation of refractive index is liable to considerable error. The X-ray patterns of the various saponites are similar except for the differences in the 00l spacings. In the two samples stabilised with glycerol the second order basal spacings are very much stronger than in the unstabilised samples, for example the specimen from Church Cove shows an increase in intensity from 1 to 6. The samples from Milford and Transvaal do not show the second order spacing, but in the unstabilised specimens it might be too faint to be recorded. Schmidt and Heystek (1953) in their discussion of the saponite from the Lizard say that it is impure and has the X-ray diffraction peaks of talc. It is probable that they mistook the second order 9.1 A spacing for the strong line of talc. Heating Experiments on Smeetites. The results of the experiments on heated samples of smectites in the high temperature X-ray camera are collected together in Figs. 1 and 2. These results show the relationship between the basal spacing and the temperature; they fall into three groups. (a) Magnesium-saturated, (b) Calciumsaturated, (c) Sodium-saturated. Those samples with magnesium ions in the exchangeable position show that, with dehydration by heating, the basal spacing decreases TABLE 2--Molecular Composition of Saponites exchangeable Church Cove Mg AI ? Fe J Si 3 '39 "] A ~4.00 Fe 3+ 0"10 J O 10 OH 2 nhzo 5 Ca 0.05 Mg 0.24 refractive indices ~, ~.005 Milford, Utah (Cahoon 1954) 2.85 " ~ , ~ 4.00 o.3oj l * Transvaal (Schmidt 1953) 2 '99 r~t3 o.oi j 3.63) A.on 0.37 f-~,,,, '1~ I.493 * Calculated by authors from analysis given in Cahoon (1954).

5 THERMAL REACTIONS OF SMECTITES 83 A B C FIG. 1--X-ray photographs of saponite heated to various temperatures. A--saponite, B--saponite heated to 550c'C (9.7)~ lattice), C--saponite heated to 950~ (enstatite) (fibre axes horizontal). \ CH.ROMEL-t~LUMEL. "~ TH ERMOCOUPLE$ Ooi II I: [p,,.,v1 2(! i - \ ~~ l~a:, r ~ \ ~) I MtN FtG. 2--Differential thermogram of saponite from Church Cove.

6 84 H. G. MIDGLEY AND K. A. GROSS from about 15 A at room temperature to a value of between 11 and 12 A at temperatures between 80 ~ and 150~ then to about 10-5 A at about 200~ and to about 10.0 A by 320~ (Fig. 2 a). The sodiumimpregnated specimens do not show the A intermediate stage, the basa! spacing is reduced to 9.5 A by at least 95~ and remains at that value with further heating (Fig. 2 c). In the specimens contain-. ing calcium, the basal spacing reduces in value fairly regularly from A between room temperature and 200~ Whilst there is no sharp arrest with values of A, there are, with saponite at least, several points of inflection that show some correspondence with the magnesium-saturated samples (Fig. 2 b). The lattice spacing is comparable with talc and it is likely that on dehydrating, the talc lattice is produced. With the magnesiumsaturated samples the value of 9.7 A for the basal spacing must also be related to the tale structure, the layer not being held so strongly as in talc. The samples when examined were very prone to rehydration. Exposure to the normal moist laboratory atmosphere caused the 9.7 A talc-like structure to rehydrate to give the montmorillonite structure. For example, if a sample which originally had a basal spacing of 13.5 A was heated to 140~ to give a basal spacing of 9.7 A, rehydration in moist air for 289 hours gave a value of 11-7 A and after 3 days 12-2 A. It is known that in the fully hydrated state, where the basal spacing is more than 14 A, more than one layer of water occurs. It is suggested from the above evidence that one layer of water is more strongly held than the other in those smectites that have a divalent ion in the exchangeable position. It is thought that when magnesium is the cation, some of the water is held much more strongly than the rest; experimental evidence for this is the formation of a lattice with a c spacing of 11-7 A which is stable for about 150~ This stage is probably equivalent to the talc lattice with a single water layer between. When sodium is the exchange cation this bound water is not held so strongly, and so is removed by heating and there is no intermediate stage. The intermediate behaviour of calcium cannot be explained readily but is, perhaps, due to the ionic radius of divaient calcium being rather larger than that of magnesium. However, it is also notable that, with one exception, the basal spacing (001 reflection) is always a sharp well-defined ring, indicating that the various intermediate values are discrete states rather than statistical means of the 14,

7 THERMAL REACTIONS OF SMECTITES 85 11"5 and 9"7 A values. This is not consistent with the concept of a strongly bound planar mono-molecular layer of water molecules between talc layers in the 11.5 A state, but does seem to agree with the results of MacKenzie's theoretical reasoning. The exception is the sodium-saturated compound which gives a broad ring when fully hydrated. Differential thermal analysis carried out on samples of smectites saturated with megnesium show a double peak at about 150~ The original sample of magnesium-saturated saponite showed such a double peak at 150~ but when saturated with Ca 2 the peak was single. A sample of bentonite when calcium saturated gave only a single peak on the d.t.a, but, when magnesium-saturated, gave a double peak at 150~ It should be noted, however, that a number of examples of d.t.a, curves given in the literature show double peaks and in many of these the exchangeable ion is stated to be calcium but, in all cases cited, the exchangeable ion is calculated from the bulk analysis and has not been determined independently. From the analyses given there is always sufficient magnesium present for this to be the exchangeable ion. Further work on a number of samples where the exchangeable ion is known, is needed to clear up this discrepancy. Part II. High-temperature Decomposition of Saponite INTRODUCTION The fibrous saponite from the Lizard, described earlier in this paper, when mounted with the fibre axis vertical gave a preferred orientation photograph (Fig. 1 a) very suitable for examining ehe thermal decomposition. By taking photographs of material either after air quenching from the required temperature or in a hightemperature X-ray camera the process of break-down of structure can be traced. EXPERIMENTAL A d.t.a, was carried out on a sample of saponite, in an apparatus having ceramic crucibles with chambers 7 I0 x 12 mm, and chromelalumel thermocouples. The temperature was measured in the sample and a heating rate of 10~ 0-2~ was used. For the saponite thermogram g of sample was used undiluted, the reference material being kaolinite fired to 1050~ The trace is reproduced unretouched as Fig. 2. Peaks occur at 142, 157, 266, 616, 680 and 837~

8 86 H. G. MIDGLEY AND K. A. GROSS Mg ++ 5A]'URATFb 14- IZ II ~'~ N\\. I \\~\ i L2" MORILLONITE VIONTMORILLONIT~...-~ (CHErA) 4. SAPONIT E 50 IO0 150, Co. "~" SATURATED I3 ;, Jz H I ~.~APONITE MOmMORU ~0 IO Z5C No. + 7-I SATURATED k II lo 9 i SAPON~TE MONTMORILL0 qlte 50 10o tso 2oo zso FIG. 3--Dehydration effects on basal spacings of various montmorillonites. (Co-ordinates in ~ A weight-loss curve was also carried out with the following results :-- T~ Wt. Loss (~) Wt. Loss Cumulative (~) " "94 6" !' I " '81 14"06

9 THERMAL REACTIONS OF SMECT1TES 87 Temperature peaks on the d.t.a, thermogram represent some change in the mineral, and so X-ray examinations between these temperatures were considered to be most profitable. Samples of saponite were heated at 180 ~ 300 ~ 500 ~ 550 ~ 600 ~ 650 ~ 750 ~ and 900~ in an electric muffle furnace, air quenched, then placed in a desiccator until required for the X-ray examination. A suitable fibre was selected by splitting the sample, and glued to a glass fibre with shellac. This was mounted in a 6 cm diameter camera, and the diffraction pattern photographed with CuKa radiation. The samples heated at 180 ~ and 300~ gave diffraction patterns identical with those obtained at room temperature. The picture consisted of a series of basal reflections showing most marked preferred-orientation maxima about the zero layer line, together with Debye-Scherrer rings, with maxima at the layer line positions, corresponding to a cell dimension of 5.2 A. These rings showed the typical diffuse outer edge of a two-dimensional lattice. The spacing value of the 00l basal reflection was 14.2/~. The diffraction pattern of the sample heated to 500~ showed a new set of basal spacings corresponding to a unit cell length of 9.7 A superimposed on the residual pattern of the 14' unit cell. A further series of heating experiments was carried out at closer intervals near to 500~ At 470~ samples showed no sign of the formation of the 9.7/~ dehydrated stage, even after 24 hours heating; those heated at 550~ showed no reaction after 1 hour, but after 5 hours the reaction was complete and only the 9.7/~ phase was found. At intermediate times composite patterns of the 14 and 9.7 A phases were obtained. In samples heated to 600, 650 and 750~ the 00/- spacing was further reduced to 9.5/~. At no stage up to 750~ does there appear to be any significant change in the two-dimensional hk reflections, either in position or intensity. Some care is needed in interpreting these results since the rate of rehydration in moist air can be very rapid. From the results of the experiments carried out in the high-temperature camera it is known that up to 320~ the variable 00/-spacing can be reduced to 9-9, and this spacing will, if the specimen is left in contact with the normal atmosphere, rehydrate and expand to 15 A. The sample heated to 550~ for more than 5 hours only showed a spacing of 9.7 A. It did not rehydrate rapidly; after 24 hours in water it did expand and give the 15 A lattice spacing. Also a specimen left in contact with the normal atmosphere for 1 year showed on examination the 15 lattice. Samples heated to 650~ however, give only a spacing of

10 88 H. G. MIDGLEY AND K. A. GROSS 9.5 A, and these samples will not rehydrate; for example, after soaking in water for 4 months the pattern was unchanged. A comparison of the powder data given by the heated samples is given in Table 3. TABLE 3--X-ray diffraction data of heated saponites. Saponite Saponite heated to 500~ and left in moist air Saponite heated to 600~ Talc (Brindley 1951) d I d!,4 1 d 1 14" "96 4" " s VS VW m m ms VW ~V 1.46 w 1.32 m 1.28 w W W W mw mw w w 14.2 vs 9.74 vs 4'92 w 4.63 s 3-97 mwb 3.55 w 3-22 s 2.63 m 2.53 m 2-33 w 1.92 m 1.73 mb 1.54 s 1.33 m 1.28 w 9.52 vs 4-72 vw 4"57 s 3.14 vs 2.60 ms 2-48 ms 2.27 wb 1.88 m 1-70 mwb 1.56 w 1.52 s 1.34 w 1.30 mw 9"4 8 4 ' "88 lb 3"37 3 3"11 I0 2 " " " " ' '632 I 1 " " "390 3b 1 " " '291 3 Measurements of d in A, s =strong, m =moderate, w=weak, b-- broad, v=very. From the preferred-orientation fibre X-ray photographs it is possible to deduce two unit cells: A, and x 9.5,~, both with a fibre axis of 5-2 A (Fig. 1 b.). The first unit cell is that of a smectite and the second probably that of talc, (5.26 x 9.1 x ~,) (Bragg 1937). There may also be an intermediate stage with a unit cell of A.

11 THERMAL REACTIONS OF" SMECTITES 89 Both Cahoon (1954) and Schmidt and Heystek (1953) carried out static heating experiments on sampjes of saponite: Cahoon does not give any precise details of his experiments but notes that on rehydration the 001 spacings will vary with water content. Schmidt and Heystek however noted a further effect: when they heated their sample of saponite from Travancore to 450~ and then allowed it to rehydrate, the diagram produced did not fully agree with the original sample in that three lines 4.87, 3.63 and 2.94/~ did not return. In the experiments reported in this paper, when the partly dehydrated stage is rehydrated either by contact with moist air or in water, the lines, 4.81, 3-63 and 2.94 A (or their equivalents) are always present and do not disappear. If the sample is further heated the 9.5 A (talc) lattice breaks down and it is possible, at temperatures between 750~ and 950~ to arrest the decomposition by air quenching; the X-ray diffraction pattern given by these consists only of faint broad bands at low angles, indicating that an ordered diffraction pattern is absent and the specimen exists in a disordered state. Finally, samples heated to above 950~ showed the formation of a completely new lattice. To investigate the formation of this, a fibre was mounted vertically in a single crystal goniometer and the diffraction pattern obtained. The sample was removed, heated to 950~ air quenched, and the fibre was mounted as before and the new diffraction pattern obtained (Fig. 1 c). The saponite pattern was the normal preferred-orientation one, while that given by the new lattice also Showed preferred orientation. In the latter case, maxima were observed about the zero layer and could be indexed as 0k0 and hko; maxima were visible to give the layer lines, this distance being equivalent to a lattice spacing of 5.2/~, (the fibre axis). It was possible to index the pattern approximately on a cell of ,8/~, in very close agreement with the unit cell of enstatite, MgSiO 3, A. The interesting fact from the X-ray photographs is that the diffraction pattern shows preferred orientation and the fibre axis is still 5.2,~. The enstatite lattice can be produced from the saponite or talc layer lattices by splitting the layers into chains and rearranging alternate chains. The habit of the original saponite, which is strongly fibrous, is reflected in this splitting of the layers, for as the original layer is trioctahedral there should be nothing to choose between three directions, mutually 120 ~ to each other; in fact however that direction which retains the 5.2/k fibre axis vertical is always preferred.

12 90 H. G. MIDGLEY AND K. A. GROSS The chemical reaction accompanying the decomposition of talc to enstatite yields an excess of SiO 2. This appears asamorphous silica, as shown by a broad band at about 4.2A on the photographs. Finally some attempt must be made to relate the peaks found in the thermogram of saponite to the lattice changes: Value of c spacing Peak T~ fl5a t decreasing to 35 I 11.5A 142 Reversible Range -~ 152!0-5 ~ A [ 616 9"7 ]k Enstatite Acknowledgment. This paper is published by permission of the Director of Building Research. The work was carried out, during the attachment of one of u s (K. A. G.) to the Building Research Station, as part of the programme of the Building Research Board. REFERENCES Bragg, W. L Atomic Structure of Minerals, New York. Brindley, G. W X-ray Identification and Crystal Structures of Clay Minerals, p Cahoon, H. P Amer. Min., 39, 222. Greene-Kelly, R Clay Min. B:dl., 2 (9), 52. Hendricks, S. B., Nelson, R. A. and Alexander L. T J. Phys. Chem., 62, Mackenzie, R. C Clay Min. Bull., 4, 145. Midgley, H. G Miner. Mag., 29, 526. Schmidt, E. R. and Heystek, H Miner. Mag., 30, 201.