CLAY MINERALS BULLETIN

Transcription

1 CLAY MINERALS BULLETIN JULY, 196 Vol. 4, No. 23 CHANGES EFFECTED IN LAYER SILICATES BY HEATING BELOW 55~ * By C. M. WARSHAW, P. E. ROSENBERG and R. RoY. The Pennsylvania State University, University Park, Pa., U.S.A. [Received 5th January, 196] ABSTRACT The changes in X-ray diffraction patterns of layer silicates which result from dry heat-treatments below 55~ have been studied and the results are tabulated in order of increasing temperature at which these changes are observed; the structural implications are briefly considered. In order to evaluate the effects of composition, particle size and disorder, synthetic clay minerals have been used in addition to natural clays. This summary of X-ray data may be of value in the identification of clay minerals in mixtures. INTRODUCTION It has been well known for many years that the different types of clay minerals exhibit characteristic behaviour on heating. Earlier work consisted for the most part of correlating the amounts of water lost on ignition with the temperature, of determining the thermal energy associated with the dehydration, and of identification of the products formed at high temperatures. Much of this work is briefly summarized by Grim (1953) and in the symposium edited by Mackenzie 957). All the reactions described in this paper refer to "dry" heating in air; even although a phase persists under these conditions up to a certain temperature this cannot be used to indicate its thermodynamic stability under the conditions. Thus, there is no correspondence whatsoever between the temperature cited herein and those obtained for the upper stability limits of clays under hydrothermal conditions. In genera[, the clays can be heated a few hundred degrees above the true stability temperature of the phase in air. In the last decade much of the emphasis in the thermal investigation of layer silicates has been on determining the structural changes during the loss of hydroxyl water--see, for example, Bradley *Contribution No , College of Mineral Industries, The Pennsylvania State University. 113

2 114 c.m. WARSHAW, P. E. ROSENBERG AND R. ROY and Grim (1951) on montmorillonite, Brindley and Ali (195) on chlorites, Roy (1949) and Sundius and Bystrom (1953) on micas, Walker (1956) on vermiculite. Recently, Brindley and Nakahira (1959) have examined the structural changes in kaolinite and have summarized all the significant previous research on this subject. The changes in structure on heating are, of course, accompanied by changes in the X-ray diffraction patterns which are characteristic for the different mineral families, and which are used as a means of identification of some of the clay minerals. However, Nelson and Roy (1954) showed that certain variables other than temperature must be considered if one is to use these heating experiments for identification. Thus, in heating normal chlorites and 7 A minerals, chemical composition, particle size, degree of crystallinity, and length of heat treatment all affect the final product. At present, however, the data which have been accumulated on the thermal behaviour of different clay minerals are sufficient for difficulties not to be encountered when the chemical composition and degree of crystallinity are taken into account. Moreover, if samples are heated for longer times at lower temperatures than those suggested in the past, particle size need not be considered. The principal difficulty in attempting to utilize the information in the literature for identification by heat treatment is that very few authors have reported the duration of their he~ting experiments. Although positive identification of a clay mineral can rarely be made solely on the basis of its thermal behaviour, the changes observed in X-ray diffraction patterns as a result of heat treatment are frequently of considerable use as an auxiliary identification method or in confirming an identification. For this reason, heat treatments of clay minerals have been investigated as part of the identification scheme presented in a recent review (Warshaw and Roy, 196). As a result of these experiments, it has been possible to summarize the changes which occur on heating all the various layer silicates with respect to temperature of heating. The previous compilations of heating data, on the other hand, deal separately with each family of clay minerals. The summary in this paper should be of considerable use to those investigators who are dealing with assemblages of clay minerals. The nomenclature employed in this paper is the same as used in the above mentioned review article by Warshaw and Roy (196) and agrees, except in minor detail, with that used by Mackenzie (1959) in his classification of the clay minerals.

3 EFFECT OF HEAT ON LAYER SILICATES l 15 EXPERIMENTAL Specimens Investigated. The behaviour of most of the natural layer silicates upon heating is well established. Since many of these vary considerably in chemical composition, particle size, and crystallinity, it was decided to study synthetic clay minerals (in which these variables can be controlled) and their natural analogues to determine the influence of these factors. The synthetic specimens are all of very fine particle size regardless of their degree of crystauinity; thus, a synthetic well-ordered kaolinite is much finer than a natural one and may be comparable in particle size with natural disordered kaolinite (kaolinited). Over the past decade a large number of papers from this laboratory have described the pressure-temperature conditions for synthesis, the range of compositions and the properties of synthetic clay minerals (see Warshaw and Roy, 196). The methods of synthesis are described in some detail by Koizumi and Roy (1959) and by Warshaw (196). For the synthetic clays used in the present investigation, the starting materials were gels of the desired composition. These were reacted in sealed gold tubes at the appropriate temperature and at pressures of one to three thousand atmospheres. Before use the products were checked for phases present and degree of crystauinity. The synthetic minerals in Table 1 and the natural minerals in Table 2 were used in this study. Various combinations of these natural and synthetic clay minerals were also studied by X-ray diffraction following various heat-treatments. Binary, ternary, and a few quaternary mixtures were prepared by mixing together in a mortar for a short period of time (about one minute) small weighed portions of the separate phases. Of the many mixtures possible, only a limited number were made, the combinations being~ for the most part, those which are commonly found in nature. Combina- TABLE 1--Synthetic Minerals employed. Mineral Kaolinite Septeclinochlore Chrysotile Beidellite Saponite Muscovite Clinochlore Composition AI 4Si41 o( O H) s MgsA1Si3A11(OH) 8 Mg6SilOlo(OH) Nao. aa A u Si a. 67 AI o. 33 ~ 1(UIT1) 2 Nao-33MgzSia. 67A1.33 1(OH)2 KA12Si3A1Olo(OH)2 MgsAl Si3A 11 o(oh) s

4 116 C. M. WARSHAW, P. E. ROSENBERG AND R. ROY TABLE 2--Natural Minerals employed. Mineral Kaolinite Kaolinited Chlorite (septechlorite?, Fe-rich) Montmorillonite (or beidellite?) Montmorillonite (or beidellite?, natural organic complex) Montmorillonite (or beidellite?, natural organic complex) Illite Morttmorillonite Dioctahedral vermiculite Vermiculite Clinochlore Chlorite (very small 14/~ peak) Corrensite Mixture of montmorillonite (or beidellite?), mica (illite), kaolinite (disordered?) Dioctahedral chlorite Corrensite (dioct.?) Occurrence Flint clay Unknown Green pellets from Tertiary sediment Recent marine sediment Recent fresh water sediment Recent sediment Uaderclay, Fithian, Ill. Bentonil e Soil from Shenandoah Valley, Virginia Westtown, Pennsylvania Westchester, Pennsylvania amclay fraction of sandstone b---green pellets, Cretaceous sediment Residue from acid-treatment of limestone Tertiary sediment Clay fraction of sandstone Clay fraction of sandstone tions were selected with the following questions in mind: (a) Is there good resolution of the basal reflections of more than one mineral with the strongest diffraction peak near 7/k, and is there more, or less, resolution after heat treatment below their decomposition temperatures? (b) Are spacing changes observed with unresolved diffraction maxima when one component contributing to these maxima is decomposed before another? (c) Do the intensity changes which accompany changes in spacing upon heating render it impossible to detect certain phases in mixtures? (d) Do possible reactions between clays of different composition (e.g. between high-a1 and high-mg clay minerals) tend to promote changes at lower temperatures? Heating and Examination Procedure. The clay samples were allowed to sediment from water on to glass slides, dried at room temperature, and examined by X-rays using a Norelco Diffractometer. The slides in a special slide holder were heated at the desired temperature for a period of hours and then transferred to an oven at 11~ Each slide was removed from the oven in turn and again examined by X-rays. Rehydration while the slide was being examined was prevented by covering the opening in the scatter shield

5 EFFECT OF HEAT ON LAYER SILICATES 117 with cellophane tape and keeping a dish of magnesium perchlorate in the scatter shield. With the stainless steel holder it was possible to heat up to sixteen slides in the furnace at once while keeping all of them supported on a flat surface to prevent warping. Various temperatures between 27~ and 5~ were employed. One set of slide preparations was heated at a series of temperatures (e.g., periods of about 12 hours each at 4~ 45~ and 5~ while other slides of the same samples were heated directly at 5~ without prior heat-treatment. The lowest temperature of 27~ was selected because it is welt below temperatures at which most clay minerals begin to dehydroxylate and it is high enough for interlayer water to be essentially removed from smectites and for vermiculite to undergo profound changes. Only reactions up to 5~ have been considered, since these are usually sufficient to distinguish one type of clay mineral from another. Moreover, ordinary glass slides can be used up to this temperature. Some high-temperature reactions are also useful in the identification of "pure" specimens, but it is not yet known to what extent high-temperature (1~ heat-treatments can be used in the identification of clay minerals in mixtures, since interaction may occur. A heating period of 15 hours is in practice more convenient than periods of one to two hours, since this is approximately the length of time that samples can be heated unattended overnight. RESULTS The changes which occur in the diffraction patterns as a result of heating at various temperatures for periods of hours are summarized in Table 3 where the minerals are grouped in order of increasing temperature of the first observed change. Only the changes which occur in the low-angle portions of the patterns (below 2 ~ 2 with CuK~t) are necessary in identification procedures; thus, for simplicity, only these changes are noted, although others occur. It was found that prior heating at one of the lower temperatures makes little difference in the results obtained for any higher temperature. DISCUSSION Single Phases. The expanded 2:1 layer silicates or minerals containing some expandable layers (expandable clay minerals) exhibit changes, involving only interlayer water, at the lowest temperatures cited in Table 3. Smectites with essentially only monovalent and

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8 12 C.M. WARSHAW, P. E. ROSENBERG AND R. ROY divalent interlayer cations and containing no complexed organic material collapse readily with a decrease in peak intensity (cf. Milne and Warshaw, 1956, Fig. 2). In illites and related minerals and in non-complexed montmorillonite in young sediments the collapse is accompanied by a sharpening of the 1 reflection (cf. Milne and Warshaw, 1956, Figs. 5 and 7, respectively). The natural montmorillonite-organic complex exhibits only a partial collapse at 27~ a temperature greater than 3~ (Milne and Shott, 1958, Fig. 4) being necessary to effect the type of change shown by the montmorillonite itself. When trivalent interlayer ions (+organic material?) are present only a partial collapse can ever be obtained before dehydroxylation begins. An example of this behaviour is shown by dioctahedral vermiculite (Hathaway, 1955, Fig. 4), where the gradual collapse of the basal spacing is accompanied by a broadening of the 1 reflection. When vermiculite is heated at low temperatures (e.g., 12~ a collapse similar to that observed with montmorillonite occurs. The very strong 14.3/~ reflection is replaced by a less intense reflection at 11-6 A. When heating is carried out at the temperatures included in Table 3, however, the changes no longer involve simply the loss of interlayer water. Walker (1956) has proposed that a regular mixedlayer phase is formed with a basal spacing of 2-6/~ ( A), the strongest reflection being at 1-2 A (2); the changes which occur at increasingly higher temperatures reflect the gradual breakdown of this phase. Since the term vermiculite is used to cover a wide variety of phases both with regard to composition and degree of mixed-layering, or heteropolytypism (Weaver, 1958), the behaviour of various specimens on heating may vary considerably, necessitating some differences in interpretation of the changes. Thus, the sample of vermiculite examined here developed a peak at 8-7 A in addition to the one at 1-2 A, and a peak at 9.6 A replaced the 1-2A peak after heating at 5~ Walker (1956) has mentioned only a spacing of 9. A in addition to the 2.6 A phase. One feature of the dehydration of vermiculite which has not received sufficient emphasis is the actual appearance of the diffractometer traces of the heat-treated material; those obtained here are reproduced in Fig. 1. Corrensite is a regular mixed-layer mineral (heteropolytype) containing approximately equal numbers of chlorite and expanded 2:1 layers. Because of the presence of the latter, corrensite exhibits changes at low heating temperatures, which have been dis-

9 EFFECT OF HEAT ON LAYER SILICATES 121 cussed by Earley et al. (1956)and by Bradley and Weaver (1956). The change in the X-ray pattern for this mineral heated for periods of hours at temperatures below 5~ is similar to that obtained by Bradley and Weaver, whose sample contained smaller amounts of impurities than others which have been described. The pattern they obtained after heating at 55~ for only two hours corresponds to the patterns obtained here after heating for longer times at lower temperatures. The change observed after heating at d 3o ~ co" ~o" FIG. 1--X-ray diffractometer traces of vermiculite from Westtown, Pennsylvania, before and after heat treatment: a--before heat treatment, b---after treatment at 12~ c--after heat treatment at 445~ d--after heat treatment at 5~ The same slide preparation was used for all heat treatments (12-15 hours). Traces were obtained with a Norelco Diffractometer using CuKa radiation, scanning speed 2 ~ 2 per minute, scale factor 64. 5~ for 15 hours reflects the chlorite nature of the mineral. The interpretation of the basal spacings at low angles may be summarized as in Table 4. The spacings in Table 4 do not agree precisely with those obtained by Earley et al. (1956) and by Bradley and Weaver (1956), but provide an illustration of the degree of precision which can reasonably be expected in different laboratories working with natural materials which are mixtures of similar phases. In Table 3 dioctahedral chlorite has been grouped with the expanded 2:1 layer silicates or minerals containing expandable layers.

10 122 c. M. WARSHAW, P. E. ROSENBERG AND R. ROY The exact nature of this mineral has not yet been established, but the fact that its 1 reflection shows some collapse suggests that it may contain some vermiculite-type layers. Since most of the 7 A minerals exhibit changes in their X-ray patterns as a result of long periods of heating in the temperature range 4-5~ they can be grouped together. The changes observed are due to the loss of at least some of the hydroxyl water. The minerals examined are listed in order of increasing temperature of the beginning of this water loss. TABLE 4--Basal reflections of corrensite before and after heat treatment. 1 d (A) I Components Room temperature w s w 4 7'25 m 6 4"85 m Average basal spacing 29"2/~ 14.3A A chlorite saponite Dehydrated below 5~ 2 12'3 m b m b 14.3/~ -k 1-A m chlorite saponite Average basal spacing 24'3/~ Heated at 5~ s b 13-8/~ + 1-/~ chlorite saponite The iron-rich sedimentary chlorite which occurs as green pellets (somewhat similar to glauconite in appearance) exhibits some change at considerably lower temperatures than do the other minerals of this group; this is probably related to the oxidation of the iron as well as to dehydration. Sedimentary chlorites, even those which are or which contain normal chlorite, begin to decompose at a lower temperature than natural well-crystallized kaolinite. In general, it can be said that sedimentary chlorite and kaolinite show decreases in the intensity

11 EFFECT OF HEAT ON LAYER SILICATES 123 of the 7/~ and 3.5/~ basal reflections in the same temperature range. Thus, loss of these reflections as a result of heat-treatment cannot be used to distinguish these minerals in sediments. However, heat-treatment is useful if the 13-14A region is examined. The 14 A peak of normal chlorites increases in intensity as a result of heat-treatment, for about 15 hours at temperatures above 5~ * It has been reported by Nelson and Roy (1954) that all the septechlorites they examined developed a peak in this region as a result of high-temperature heat treatment; such behaviour is also exhibited by dickite (Hill, 1955) but not by kaolinite. The septechlorites examined in this study (synthetic, iron-free) did not develop the 14 A peak on heat-treatment (which was probably not high enough in temperature) but previous experience with sediments suggests that most sedimentary chlorites would show this peak after heat-treatment at 5~ The spacing of the 14 A peak of the sedimentary chlorite investigated decreased from 14.2/~ to 13.8/~ with the increase in intensity. Characteristically, the 14A peaks developed by septechlorites are actually broad peaks centered at about 13.5 A. Chrysotile, which has the highest thermal stability within the 7 A group, shows only slight changes at 5~ and actually has higher thermal stability than normal Mg-chlorites, which dehydroxylate in two stages. Only two minerals were investigated in the group with high thermal stability. All the 2:1 layer silicates and normal chlorites (which contain 2:1 layers) belong to this group (the smectites can be included once the interlayer water is removed.) Some minor changes in the spacings and intensities of the basal reflections may be observed when some of these minerals, especially the dioctahedral ones, are heated for a prolonged period at 5~ but these can be correlated with the loss of a considerable portion of the hydroxyl water to yield dehydroxylated forms (Bradley and Grim, 1951) which retain the layer structure. Thus the basal reflections instead of being destroyed, as with kaolinite, undergo very little change. The talc layers of the normal Mg-rich chlorites are not affected by heat-treatment at 5~ but the brucite layers are dehydroxylated at this or a slightly higher temperature, producing the change in X-ray pattern which is characteristic of chlorite, namely the increase in intensity of the 14A reflection. *This temperature is high enough for sedimentary chlorites, although a slightly nigher temperature may be needed for Fe-free chlorites.

12 124 C. M. WARSHAW, P. E. ROSENBERG AND R. ROY Mixtures. Results for the mixtures studied showed that the characteristic thermal behaviour of the clay minerals discussed is observed whether they occur singly or in mixture. That is, there is little interaction between clays in hours at the temperatures employed. Micas, illites, and most smectites present no difficulties in identification as components of a mixture, but some difficulties are encountered with mixtures of 7 A minerals, of chlorites and vermiculite, and of mixtures of all of these minerals. The 3.5 A reflection of a mixture of kaolinite and sedimentary chlorite (with a negligible 14A peak) may be resolved into two peaks, but frequently only one rather broad peak is observed at 25. ~ 2 (CuKa radiation). Increase in size or development of a 13-14A peak upon heat treatment at 475-5~ indicates that sedimentary chlorite is present, but the question remains as to whether kaolinite is also present. It is occasionally possible to detect a shift in the 25. ~ peak to 25.2 ~ (chlorite) or 24.9 ~ (kaolinite) after heat treatment at 4~ revealing that one component has lost intensity relative to the other (see Bradley, 1954). If this does not occur it may be necessary to use other methods, e.g., acid treatment or high-temperature heat-treatment, (see Warshaw and Roy, 196) to detect kaolinite in mixture with sedimentary chlorite. Mixtures of chlorite (usually the normal variety) with vermiculite and/or corrensite may also present some difficulties. Unless large amounts of the latter two minerals are present the broad peaks or bands which develop upon heat-treatment may not be detectable. However, a decrease in intensity of the 14 A reflection relative to that of the 7 A reflection as a result of heating at 375~ indicates the presence of vermiculite and/or corrensite in addition to chlorite. Large amounts of vermiculite or corrensite in a mixture can be distinguished by their characteristic broad peaks or bands (see Table 3). It should be noted, however, that the presence of corrensite may be confirmed by the low-angle peak (29 A) observed at room temperature. If sufficient corrensite is present to be detectable by heat-treatment, then there is also enough to yield the low-angle peak when the mixtme is examined by X-rays before heattreatment. The greatest difficulty is encountered with mixtures containing normal chlorite and septechlorite and/or kaolinite. The kaolinite may be detected by its greater resistance to acid, or by the development of mullite when the mixture is heated for a short time (1-2 hours) at 1-11~ Septechlorites may be separated from nor

13 EFFECT OF HEAT ON LAYER SILICATES 125 real chlorites by the method described by Brindley, Oughton and Youell (1951), i.e., the mixture is heated at a temperature high enough to dehydroxylate the brucite layer of the normal chlorite, but not to decompose the septechlorite, and the decomposed normal chlorite dissolved out with acid. This method has not, however, been adequately tested to confirm its universal applicability to chlorite mixtures. Acknowledgments.--The authors are grateful to the following for donations of natural clays: Professor G. W. Brindley of the Pennsylvania Slate University, Dr I. H. Milne and Dr J. W. Farley of the Gulf Research and Development Company, Dr C. E. Weaver and Dr J. F. Burst of the Shell Development Company, and Dr J. C. Hathaway and Dr L. G. Schultz of the U.S. Geological Survey. This work was supported by Grant C-294 from the Petroleum Research Fund administered by the American Chemical Society, and grateful acknowledgment is hereby made by the authors. REFERENCES BRAOt_~Y, W. F., Clays and Clay Minerals (A. Swineford and N. Plummer, editors). Nat. Acad. Sci.--Nat. Res. Counc., Washington. Publ. 327, p BRAOLEY, W. F., and GRIM, R. E., Amer. Min., 36, 182. BRADLEY, W. F., and WEAVER, C. E., Amer. Min., 41,497. BRINDLEY, G. W., and Au, S. Z., 195. Acta cryst., 3, 25. BmND1.EY, G. W., and NAKAHIRA, M., J. Amer. ceram. Soc., 42, 311, 314, 319. BRINDLEY, G. W., OUGHTON, B. M., and YOOELI, R. F., Acta cryst., 4, 552. EARLEY, J. W., BRINOt.EY, G. W., MCVEAGH, W. J., and VAND~N HEUVEL, R. C., Amer. Min., 41,258. GRIM, R. E., Clay Mineralogy. McGraw-Hill, New York. HATaAWAY, J. C., Clays and Clay Minerals (W. O. Milligan, editor). Nat. Aead. Sci.--Nat. Res. Counc., Washington. Publ. 395, p. 74. HILL, R. D., Acta cryst., 8, 12. KOIZtIML M., and RoY, R., Amer. Min., 44, 788. MACKENZIE, R. C. (editor), The Differential Thermal Investigation of Clays. Mineralogical Society, London. MACKENZIE, R. C., Slav Min. Bull., 4, 62. MILNE, I. H., and SrIOTT, W. L., Clays and Cla~ Minerals (A. Swineford, editor). Nat. Acad. Sci.--Nat. Res. Counc., Washington. Publ. 566, p MILNE, I. H., and WARSHAW, C. M., Clays and Clay Minerals (A. Swineford, editor). Nat. Acad. Sci.--Nat. Res. Counc., Washington. Publ. 456, p. 22. NELSON, B. W., and RoY, R., Clays and Clay Minerals (A. Swineford and N. Plummer, editors). Nat. Acad. Sci.--Nat. Res. Counc., Washington. Publ. 327, p. 335.

14 126 c.m. WARSHAW, P. E. ROSENBERG AND R. ROY RoY, R., J. Amer. ceram. Soc., 32, 22. SUNDIUS, N., and BYSTROM, A. i., Trans. Brit. Ceram. Soc., 52, 632. WALKER, (~. F., Clays and Clay Minerals (A. Swineford, editor). Nat. Acad. Sci.~Nat. Res. Counc., Washington. Publ. 456, p. 11. WAR.SHAW, C. i., 196. Clays and Clay Minerals, Proceedings of the Seventh National Clay Conference (Ada Swineford, editor). Pergamon Press, London, p. 33. WARSHAW, C. M., and RoY R., 196. In press. WEAVER, C. E., Amer. Min., 43, 839.