Interaction of kaolinite with calcite on heatingI. instrumental and procedural factors for one kaolinite in air and nitrogen

Clay Minerals (1988) 23, 191-203

I N T E R A C T I O N OF K A O L I N I T E W I T H C A L C I T E ON H E A T I N G : I I I . E F F E C T OF D I F F E R E N T K A O L I N I T E S

R. C. M A C K E N Z I E , L. H E L L E R - K A L L A I * , A. A. R A H M A N t AND H. M. M O I R

Departments of Chemistry and Soil Science, University of Aberdeen, Meston Walk, Old Aberdeen AB9 2UE, Scotland, and *Department of Geology, Institute of Earth Sciences, The Hebrew University,

Jerusalem 91904, Israel

(Received 17 January 1988; revised 16 March 1988)

ABSTRACT: The degree of distortion of the calcite endotherm on differential thermal analysis (DTA) curves for kaolinite-calcite mixtures depends on the kaolinite sample used as well as on the factors previously established. Although no two of the ten kaolinite samples employed, even if from the same general locality, produced identical effects under all experimental conditions, a general relationship between locality of origin and degree of distortion was noted. The crystalline species detected in the products of heating, after standing in air at room temperature, included portlandite, calcite, aragonite, vaterite and the high- temperature phases gehlenite, c(-Ca2SiO, and 12CaO. 7A1203. The actual species present again depended on the kaolinite sample. Although the results cannot be directly related to the volatiles evolved along with water on dehydroxylation of the different kaolinites, a tentative explanation is offered on the basis of the effects of the volatiles on the surfaces of the particles present.

Earlier studies (Mackenzie & Rahman, 1987; Mackenzie et al., 1988) have established that the configuration of the calcite endotherm on D T A curves for kaolinite-calcite mixtures is highly dependent on instrumental and procedural variables, whether the atmosphere be air, nitrogen or carbon dioxide. The complexities were not due to solid-state reactions and were tentatively at tr ibuted to desorption of CO2 sorbed on a fresh metakaol ini te surface and/or the effects of the complex volatiles released by kaolinite on heating (Keller, 1986; Heller-Kallai et al., 1986, 1987, 1988; Heller-Kallai & Mackenzie, 1987). As the composit ion of the volatiles varies considerably from one sample of kaolinite to another and the D T A studies were all performed with one kaolinite sample, the effects of different kaolinites on the calcite endotherm under a range of experimental condit ions were investigated in an a t tempt to elucidate the mechanism.

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

Materials

Ten kaolinite samples covering various formational types, localities of origin, degrees of order and particle sizes were employed: C-1. Well-ordered 'A1 BP' kaolinite from Cornwall, England (Mackenzie & Rahman,

1987) - as received, and the < 4 #m fraction. C-2. Well-ordered kaolinite (03) from Cornwall, used in the O E C D inter-laboratory study

(Van Olphen & Fripiat , 1979), as received.

1"Present address: Bangladesh Centre for Advanced Studies, 626 Rd. No. 20 (Old) 10-A (New), Dhanmondi, Dhaka, Bangladesh.

�9 1988 The Mineralogical Society

192 R . C . M a c k e n z i e et al.

C-3. Well-ordered Cornish kaolinite from the Macaulay Insti tute collection, < 2 #m fraction, Na-saturated.

Ga-1. Well-ordered kaolinite (KGa-1) from Georgia, USA, used in the Clay Minerals Society inter-laboratory study (Van Olphen & Fripiat , 1979), < 2 #m fraction, Na-

saturated. Ga-2. Poorly-ordered kaolinite (KGa-2) from Georgia, used in the same study, as received. P-K. Well-ordered kaolinite from Pugu, Tanzania, described by Robertson et al. (1954),

< 5/~m fraction. P-D. Poorly-ordered kaolinite from the same locality, rock sample, unseparated. Ione. Well-ordered kaolinite from Ione, California, USA, Na-saturated. WaG. Well-ordered kaolinite from "Warsaw geodes", Keokuk, Iowa, USA, described by

Hayes (1963) and Keller et al. (1966), 10-20 #m fraction. Chemical analyses of some samples are given in Table 1 ; from these, C-1 would also seem to

be Na-saturated.

TABLE 1. Chemical analyses of some of the kaolinite samples used.

C-l* C-2~- Ga-2:~ P-Kw

SiO2 46-29 46-18 44.05 48.14 TiO2 0-03 0-03 2.13 0.76 Al/O3 37.78 38.28 37.85 35-74 Fe203 0.42 0.67 1.17 1.45 MnO 0-02 0.01 0.00 n.d. MgO 0.18 0.14 0.04 0.11 CaO 0.08 0.26 0.04 0.18 Na20 0.23 0.09 0.01 0.04 K20 0.80 1.27 0.05 0.14 Loss on ignition 13.95 12.80 14-01 13.62 P205 0.18 0.13 0.15 0-05 CO2 0.00 0-07 n.d. n.d. S O 3 0.04 0.04 0.05 0.05 F n.d. 0.11 0.02 n.d. C1 n.d. 0.01 n.d. 0.00 Total 100.00 100.09 99.42 100.14

* Analyst, D. C. Bain. t Mean of 18 analyses (Van Olphen & Fripiat, 1979).

Mean of 2 analyses (Van Olphen & Fripiat, 1979). w Robertson et al. (1954).

The calcite used was Ana laR calcium carbonate, which consisted of rhombs with edge lengths of 3 12 /~m. Mixtures with a kaolinite :calcite ratio of 1.25:1, theoretically the opt imum for gehlenite formation, were prepared by dry mixing (Mackenzie & Rahman, 1987).

Ins trumentat ion

The thermoanalyt ical instruments used were a Stone Model DTA-200 with a SH-8BE specimen holder that permit ted gas flow through the sample during the determination, a Stanton Redcroft Model 672 D T A instrument with gas flow over the sample and a Stanton Redcroft STA-780 simultaneous T G - D T G - D T A instrument, also with gas flow over the

Kaolinite-calcite interactions on heating 193

sample. For conciseness, these are referred to as the Stone, 673 and STA instruments, respectively. The N2 supplied to the Stone instrument was either dried by bubbling through concentrated sulphuric acid, or moistened by bubbling through water at room temperature, whereas the gases supplied to the other two instruments were taken directly from the cylinders. Experimental conditions were maintained as constant as possible and replicability was excellent. When static air was used in the Stone instrument, the gas supply lines to the bases of the specimen wells were closed off to prevent convection. The reference material throughout was a-alumina.

X-ray diffraction (XRD) data were determined on a Philips 1730 diffractometer with Cu- K~ radiation.

R E S U L T S

Thermal analysis

Mixtures with the Cornish kaolinites C-1 and C-2 in static air in the Stone instrument yielded similar curves (Fig. 1 A,D), apart from temperature differences. In the STA instrument, however, with N2 flowing over the sample, the calcite endotherm was modified on the low-temperature limb for the mixture with C-l, but on the high-temperature limb for the mixture with C-2 (Fig. 2 A,B): the correspondence between the DTA and derivative thermogravimetry (DTG) curves confirms earlier observations (Mackenzie & Rahman, 1987) that the complexity of the calcite endotherm is caused by variation in the rate of CO: evolution. DTA curves for these two mixtures determined on the Stone instrument with dry or moist Nz flowing through the samples (Fig. 1 B,C,E,F) and on the 673 instrument in a CO2 atmosphere (Fig. 3 A,B) also showed marked differences. Mixtures with the fine fraction of C- 1 and with C-3 yielded, irrespective of atmosphere, distinctly different curves (Fig. 1 G-L; Fig. 3 C,D) and no specific feature can be attributed to particle size alone. The unusual sharpness of the calcite endothermic system on curves for the mixture with C-3 (Fig. 1 J-L; Fig. 3 D) is noteworthy. A low-temperature component of the calcite peak system for all mixtures containing Cornish kaolinites was enhanced by moisture (Fig. 1), presumably because of the catalytic effect of moisture on calcite dissociation (see Paulik et al., 1980).

For mixtures with Georgia kaolinites, the calcite endotherm appears to be relatively unaffected by the presence of kaolinite (Fig. 2 C,D; Fig. 3 E,F; Fig. 4), but close examination reveals significant variations. Even in Fig. 2 C,D, the lead-in to the calcite DTA peak differs for the two samples (confirmed by the DTG curve) and on curve E, Fig. 3, and curve A, Fig. 4, a series of very small endotherms occurs between the kaolinite and calcite peaks : these are absent on curve F, Fig. 3, and curve D, Fig. 4. Passage of dry N2 through the samples also brings out differences in behaviour (Fig. 4, B,E) and in a CO2 atmosphere, shoulders appear on different sides of the calcite peak (Fig. 3 E,F). The effect of moisture is less obvious than for the Cornish mixtures but is evidenced by the incorporation of the minor endotherm at 690~ into the calcite peak for the Ga-1 mixture (Fig. 4, B,C) and the increased asymmetry of that peak for the Ga-2 mixture (Fig. 4 F,G).

The significant quartz content of the Pugu sample P-D is obvious on all the DTA curves. In the Stone instrument in static air, P-K had little effect on the calcite peak, but P-D produced a slight shoulder on the low-temperature limb (Fig. 5 A,D). There are also some slight differences for curves determined with dry and moist N: flowing through the sample (Fig. 5 B,C,E,F), the most obvious being the differences in symmetry of the calcite peaks caused by

194 R . C . Mackenzie et al.

AT

5~B A

~ B

83O

~ E

793 820

Temperofure / ~

AT 883

5~0

78o 8,o F I 54O

797

Temperature/~

AT 565

895

~ H

790

Temperature l~

AT 862

54O

815

gO2

Temperature l~

FIG. 1. DTA curves from the Stone instrument at 10~ for 1.25 : 1 mixtures of C-1 (A-C), C-2 (D-F), C-1 < 4 pm (G-I) and C-3 (J-L) kaolinites with calcite: A,D,G,J--in static air; B,E,H,K--with dry N2 through the sample at ~ 15 ml/min; C,F,I,L--with moist N: through

the sample at ~ 15 ml/min.

Kaolinite-calcite interactions on heating 195

774 510

DTG

DTA

A

530

TA

76# 500

78/*

TEMPERATURE I oC

FIG. 2. DTG and DTA curves from the STA-780 instrument at i0~ with N2 flowing over the sample at 40 ml/min for 1.25 : 1 mixtures of calcite with : A--C-I ; B--C-2; C--Ga-1 ;

D---Ga-2.

the catalytic effect of moisture (curves C,F). In CO2, the calcite endotherm for the mixture with P-K (Fig. 3 G) shows a distinct shoulder on the high-temperature limb, whereas that for the P-D mixture displays a more general broadening (Fig. 3 H).

The Ione sample appears to have little effect on the calcite endotherm in the Stone instrument (Fig. 6 A-D), apart from the usual effect of moisture that was accentuated for a sample mixed in the wet state (Fig. 6 C,D). A slight doubling of the calcite peak occurred in CO2 in the 673 instrument (Fig. 3 I).

The only effects that could be observed for mixtures with the very coarse W a G sample (Fig. 3 J; Fig. 6 E - G ) were a very slight shoulder on the lead-in to the calcite peak on curve E, and a suggestion of the effect of moisture on the lead-in to the same peak on curve G.

196 R.C. Mackenzie et al.

AT

992

995

9 929 ~89

~ ~74

571 966

989

981

~./ "96/* 560

2 936

557 P56

937 971

9/.7 99/*

56/*

9~7 989

933

TEMPERATURE I ~ FIO. 3. DTA curves from the 673 instrument at 10~ with CO2 flowing over the sample at 50 ml/min for 1.25 : 1 mixtures of calcite with : A--C- 1 ; B--C-2; C--C- 1 < 4 #m; D--C-3;

E--Ga-1; F--Ga-2; G P-K; H--P-D; 1--1one; J--WaG.

X-ray diffraction

Samples from the above thermoanalytical study, as well as some from earlier experiments (Mackenzie & Rahman, 1987; Mackenzie et al., 1988)--some 40 samples in all--were examined by XRD to check whether the phases formed would throw any light on the mechanism of interaction.

Correlation of the high-temperature phases, gehlenite, a'-Ca:SiO4, possibly with some larnite, and 12CAO. 7A1~O3 (Table 2), with the DTA curves showed that gehlenite always occurred in samples heated to beyond the start ( ~ 970~ of the kaolinite exotherm. In this connection it must be remembered that, after switching off, the temperature would increase slightly before cooling, and, with a sharp exotherm like that of kaolinite, the effect of this might be considerable. As only a single exotherm was observed (e.g. Fig. 3 E), it appears that either (a) gehlenite crystallized at the same temperature as the kaolinite exotherm, or (b) the heat evolved during the exotherm caused the gehlenite to crystallize. On the other hand, ~'- CazSiO 4 occurred in several samples appreciably before the exotherm, forming at as low a temperature as 880~ in the mixture with C-3 : whether this (and perhaps even the sharpness of the calcite endothermic system) was in any way associated with the trace of NaC1 detected in C-3 is uncertain. In any event, the formation of c(-Ca2SiO4 seems to have been gradual, as no exotherm can be associated with it. No reason for the formation of 12CaO. 7A1203, which was detected in only two samples (Table 2), can currently be advanced.

As these samples had been stored for some months in closed containers before X-ray

Kaolinite-calcite interactions on heating 197

AT

850

790 807

780

Temperature / o[

870

E

5~3

530

753

Temperature l~

FIG. 4. DTA curves from the Stone instrument at 10~ for 1.25:1 mixtures of Gad (A-C) and Ga-2 (D-G) kaolinites with calcite: A,D--in static air; B,E--with dry N z through the sample at ~ 15 ml/min; C,F--with moist N2 through the sample at ~ 15 ml/min; G--with moist

N2 through the sample at ~45 ml/min.

examination, rehydration and recarbonation of the CaO had occurred. The variety of C a C O 3

phases formed (Table 2) was particularly interesting. Calcite, aragonite and vaterite were all represented, the phase composition depending mainly on the kaolinite sample used. Aragonite and vaterite appeared to be mutually exclusive, but both occurred along with calcite, which was the only phase detected in samples heated in CO2. Vaterite was detected in samples containing C-l, Ga-1 and P-D, which tended to be those most strongly affecting the low-temperature limb of the calcite peak in the Stone instrument (Fig 1 A-C; Fig. 4 A-C; Fig. 5 E-G). The fact that a sample from a mixture made up with C-1 calcined at 1000~ (Mackenzie & Rahman, 1987, Fig. 1 E) also yielded vaterite demonstrates that neither the volatiles evolved during kaolinite dehydroxylation nor freshly formed metakaolinite surfaces determined the phases formed. The absence of a C a C O 3 phase in the product from the mixture with C-3 cannot meantime be explained.

Volatiles evolved

Selected species evolved along with the dehydroxylation water from C-l, C-2, Ga-1 and Ione, determined as described by Heller-Kallai et al. (1988), are listed in Table 3; the pH of the dehydroxylation water is also given. If, as is believed, the high values for B, Si and possibly Na for the lone sample can be attributed to attack on borosilicate glass by the highly

198 R . C . Mackenz ie et al.

AT

A

B63

8O7

793

Temperature / ~

AT B87

532

8t+5

797

E

Temperature / ~

FIO. 5. DTA curves from the Stone instrument at 10~ for 1.25:1 mixtures of P-K (A-C) and P-D (D-F) kaolinites with calcite: A,D--in static air; B,E--with dry N2 through the sample

at ~ 15 ml/min; C,F--with moist N2 through the sample at ~ 15 ml/min.

alkaline dehydroxylation water, the relative var iabi l i ty is small and is not commensurate with the difference in the effects exercised on the calcite endotherm. However, it is known that H F and other gases can escape during t rapping (Heller-Kallai et al., 1988) and the wide pH range indicates that other species, such as NH~ (Keller, 1986), may have been present.

D I S C U S S I O N

The configuration of the calcite endotherm on D T A curves for 1.25:1 kaolinite-calcite mixtures clearly depends on the part icular kaolinite used. Although no two samples, even if from apparent ly the same locality, gave identical results under all experimental conditions, the degree of distortion of the calcite endotherm seems to depend on the locality of the kaolinite in the order:

Cornwall > Georgia > Pugu > I o n e > "Warsaw geode",

suggesting that some environmental factor is involved. Certainly, the effects of different methods of extraction and processing may be superposed, but, as Tsvetkov et al. (1964) also observed a distorted calcite endotherm for a mixture of a USSR kaolinite with calcite in the ratio 7:3, the phenomenon seems to be a general one.

All the Cornish samples contained mica but this impurity is most unlikely to affect the calcite peak, as a mixture of Ione (which contains no mica) with 7~o illite, when mixed with calcite in the usual 1.25 : 1 ratio, caused no greater distortion than Ione alone. However, the

Kaolinite-calcite interactions on heating 199

AT

900

. 810

837

803

13

AT 875

817

B25

Temperature/~ Temperature / ~

FIG. 6. DTA curves from the Stone instrument at 10~ for 1.25 : 1 mixtures of lone (A-D) and WaG (E-G) kaolinites with calcite: A,E--in static air; B,F--with dry N2 through the sample at ~ 15 ml/min; C,G--with moist N2 through the sample at ~ 15 ml/min; D--wet-

mixed, with moist N2 through the sample at ~ 15 ml/min.

heated material from the mixture with Ione+ illite yielded vaterite instead of calcite on standing, so contaminants may affect the CaCO3 phases formed. Whether the mutual exclusion of aragonite and vaterite (Table 2) is due to preferential nucleation of one, or inhibition of the other, or what the controlling factors are, cannot presently be determined.

Much of the material remaining after calcite dissociation but before crystallization of the high-temperature phases was, at least in some of the samples, non-crystalline. Thus, thermogravimetric (TG) examination of the product from the sample giving curve C, Fig. 9, in Mackenzie & Rahman (1987), which contained only portlandite and vaterite as crystalline phases, revealed that almost 70~ of the CaO must be in non-crystalline form. Presumably, this indicates migration of CaO into the disordered metakaolinite structure, as proposed for BaO by Guillem Villar & Guillem Monzonis (1984). The amount of non-crystalline CaO can vary appreciably, however, as in the product from the sample giving curve D, Fig. 4, which contained portlandite, calcite and aragonite as crystalline phases, with some 55~ of the CaO in the form of calcite and aragonite: although the hygroscopic moisture and portlandite could not be quantitatively distinguished, the amount of non-crystalline CaO in this sample must have been relatively small. It was also noted that, under the electron microprobe, the product from curve I, Fig. 1, showed regions (~ 200 #m across) with AI, Si and Ca as well as some segregated Ca, whereas the product from curve F, Fig. 4, showed only regions containing A1

200 R . C . Mackenzie et al.

TABLB 2. Phases observed in some samples giving the DTA curves in Figs. 1 and 3-6, after removal from the DTA instrument and exposure to the air for some months.

Kaolinite ~ "~ ~ '~ ~ ~ ~ ~ ~ sample Atmosphere Max. T(oC) o ~ ~ ;> ~ .~ ~ ~ ~ ,<

C-1 Static air 970 x x x x . . . . Dry N 2 through 925 x x - x - Moist N2 through 910 x x - x - CO2 through 1000+ - x x x - - -

C-1 < 4 #m Static air 970 x x x x . . . . Dry N2 through 930 x x - tr - - - Moist N2 through 920 x - x - x -

C-3 Moist N2 through 880 x . . . . x x - Ga-1 Static air 920 x x - x - x

Dry N: through 900 x tr - x x Moist N2 through 875 x x - x x CO2 over 1000+ tr x x x x x

Ga-2 Static air 925 x x x tr . . . . . x Moist N2 through 860 x x x - - - x Moist N2 through (fast) 820 tr x . . . . x

P-K Moist N 2 through 840 x x x x P-D Static air 925 x x - x x - x

Dry N2 through 900 x tr - tr - x - x Moist Nz through 885 x tr - x - x - x

Ione Dry N2 through 900 x x x x - x Moist N2 through 910 x x x - x

(wet-mixed) Moist N 2 through 860 x x x x - x ( + 7 ~ illite) N 2 over 1000+ x - x x - x - x

WaG Dry N2 through 920 x x x x Moist N: through 875 x x x x

KEY: x - Present; tr - trace; - absent.

a n d Si a n d s e p a r a t e Ca. T h e s e o b s e r v a t i o n s s u g g e s t tha t , if m i g r a t i o n o f C a O in to the

m e t a k a o l i n i t e s t r u c t u r e does occur , t he kao l in i t e u sed m a y be o f g r e a t e r i m p o r t a n c e t h a n the

a t m o s p h e r e in w h i c h the s a m p l e w a s hea t ed . F u r t h e r e v i d e n c e is, h o w e v e r , des i rab le .

F r o m the a b o v e resu l t s a n d ear l i e r o b s e r v a t i o n s ( M a c k e n z i e & R a h m a n , 1987; M a c k e n z i e

et al., 1988; H e l l e r - K a l l a i & M a c k e n z i e , 1987), d i s t o r t i o n o f t he ca lc i te e n d o t h e r m is c a u s e d

m a i n l y by v a r i a t i o n in t he ra te o f CO2 evo lu t i on , a n d the n a t u r e a n d d e g r e e o f d i s t o r t i o n

d e p e n d s on, inter alia, par t i c l e size, s a m p l e p a c k i n g , p r e s e n c e or a b s e n c e o f m o i s t u r e ,

a t m o s p h e r e a n d the kao l in i t e s a m p l e used . T h e effect o f m o i s t u r e h a s a l r e a d y b e e n e x p l a i n e d ,

b u t t he o t h e r effects c a n n o t be d i rec t ly r e l a t ed to a n y o n e o b v i o u s fac tor . Y e t t he o b s e r v a t i o n

t h a t s o m e d i s t o r t i o n o c c u r s w h e n the kao l in i t e a n d calci te a re p h y s i c a l l y s e p a r a t e d b u t

vo la t i les evo lved by the kao l in i t e o n h e a t i n g h a v e access to t he ca lc i te sugges t s s o m e

c o r r e l a t i o n w i t h evo l ved volat i les . C e r t a i n l y , t he v a r i a t i o n s s h o w n in T a b l e 3 a re insuf f ic ien t

to e x p l a i n the l a rge d i f f e rences de tec ted , b u t it m u s t be r e m e m b e r e d t h a t n o t all vola t i les w e r e

t r a p p e d , a n d t h a t s o m e m a y n o t e s c a p e b u t be s t r o n g l y s o r b e d o n o r r eac t w i t h pa r t i c l e

Kaolinite-calcite interactions on heating 2 0 1

0

8

8

"c

f ~

0

0

0

o I

....,

0

~ v

V v

V ~ V ~

V V V

V ~

V

V V V

V V V

V V ~

v

V V V V

<

202 R. C. Mackenzie et al.

surfaces. This would be particularly true for acids, such as HF and HCI (Heller-Kallai et al., 1988). The fact that small samples produce no distortion tends to support this view, as here the volatiles would escape cleanly into the atmosphere without having to traverse the sample. Moreover, the activity of kaolinite dehydroxylated at 600~ and the inactivity of the same kaolinite dehydroxylated at 750~ (Mackenzie & Rahman, 1987) could be accounted for by active species on the metakaolinite surface being rendered inactive by incorporation in the structure at the higher temperature. As calcite starts to dissociate before the dehydroxylation of kaolinite is complete, the CaO then formed may react with or be chemisorbed on fresh metakaolinite surfaces in contact with it, and be incorporated in the structure on further heating. Certainly, if, as suggested above, migration of CaO into metakaolinite occurs, the amount incorporated is likely to depend on the nature of the metakaolinite surface and how that surface is affected by volatile species. Moreover, the possibility that distorted calcite endotherms are related to adsorption and desorption of CO2 on metakaolinite surfaces closely associated with calcite (Mackenzie & Rahman, 1987) cannot be excluded, as the magnitude of such an effect would depend on the kaolinite used and on the manner in which its metakaolinite surface is modified by the species produced on heating.

C O N C L U S I O N S

All available evidence points to the distortion of calcite dissociation in the presence of kaolinite being associated in some manner with particle surfaces and with the volatiles formed when kaolinite is heated. The location and release of such volatiles must remain a matter for further study, but possible mechanisms for their effect on calcite can be formulated. For example: A. 1. Volatiles are evolved from kaolinite on heating. 2. Some escape but some are sorbed on,

or react with, particle surfaces. 3. Metakaolinite surfaces so modified could, where in close contact with dissociating calcite, react with or chemisorb non-crystalline CaO and, at different sites, adsorb COz. 4. On further heating, the sorbed volatiles and CaO would migrate into the metakaolinite structure, destroying the surface activity and desorbing the COz.

B. 1, 2. As above. 3. Acid volatiles from the kaolinite would react with the calcite, evolving amounts of CO2 too small to be detected, but leaving salts. 4. The salts formed (probably at the corners and edges of the crystals - Mackenzie & Rahman, 1987) could catalyse dissociation of neighbouring CaCO3 to give measurable COz release before the main dissociation.

These two possible mechanisms are not mutually exclusive and could act simultaneously. But they are not necessarily the only possibilities and further detailed investigation will be required to establish the sequence of events and to determine whether other mineral mixtures display related phenomena.

ACKNOWLEDGMENTS

The authors wish to thank Dr I. Miloslavski of the Hebrew University of Jerusalem and Dr L. Halicz of the Geological Survey of Israel for collection and analysis of the evolved volatiles, and Dr D. C. Bain of the Macaulay Land Use Research Institute, Aberdeen, for the analysis of C-1.

Kaolinite-calcite interactions on heating 203

R E F E R E N C E S

GUILLEM VILLAR M. C. & GUILLEM MONZONIS C. (1984) Kinetics and mechanism of formation of the amorphous phase from barium carbonate and kaolin. Thermochim. Acta 73, 67-78.

HAYES J. B. (1963) Kaolinite from Warsaw geodes, Keokuk region, Iowa. Iowa Acad. Sci. 70, 261-272. HELLER-KALLAI L. & MACKENZIE R. C. (1987) Effect of volatiles from kaolinite on calcite dissolution: DTA

evidence. Clay Miner. 22, 349-350. HELLER-KALLAI L., MILOSLAVSKI I. & AIZENSHTAT Z. (1986) Volatile products of clay mineral pyrolysis.

Naturwiss. 73, 615-617. HELLER-KALLAI L., MILOSLAVSKI I. & AIZENSHTAT Z. (1987) Volatile products of clay mineral pyrolysis

revealed by their effect on calcite. Clay Miner. 22, 339-348. HELLER-KALLAI L., MILOSLAVSKI I., AIZENSHTAT Z. & HALICZ L. (1988) Chemical and mass spectrometric

analysis of volatiles derived from clays. Am. Miner. (in press). KELLER W. D. (1986) Composition of condensates from heated clay minerals and shales. Am. Miner. 71,

1420-1425. KELLER W. D., PICKETT E. E. & REESMAN m. L. (1966) Elevated dehydroxylation temperature of the Keokuk

geode kaolinite--a possible reference material. Proc. Int. Clay Conf. Jerusalem, 1, 75-85. MACKENZIE R. C. & RArlMAN A. A. (1987) Interaction of kaolinite with calcite on heating. I. Instrumental and

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