Kinetics of illite formation

DENNIS EBERL Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 JOHN HOWER Department of Geology, Case Western Reserve University, Cleveland, Ohio 44106

ABSTRACT companying increase in positive charge in the interlayer position through the concen- An activation energy of 19.6 ± 3.5 tration of cations such as K+. When the kcal/mole was found for conversion of charge on a layer reaches about 0.7 equi- synthetic beidellite with the composition valents per O10(OH)2 (see Hower and Al2Si3.66Alo.3401o(OH)2K().34 to mixed-layer Mowatt, 1966), water is expelled from the illite/smectite. The size of this activation interlayer and is fixed, convert- energy and the rate constants suggest that ing the expandable smectite structure into (1) the alteration of smectite to illite during that of an illite. involves the breaking of chemi- Perry and Hower (1970) suggested that cal bonds in the 2:1 layers; (2) either an the occurrence of this reaction is directly re- equilibrium or a kinetic interpretation for lated to temperature. For example, data the range of mixed layering found in burial from a typical Gulf Coast well is shown in diagenetic sequences is compatible with the Figure 1. Mixed-layer from this well is T° C kinetic data; and (3) the formation of illite initially 80 percent expandable (that is, Figure 1. Relationship between expandability from smectite on the ocean floor will not be crystallites contain 80 percent smectite and and temperature for I/S from well "E" of Perry seen, even if the reaction is favored ther- 20 percent illite layers). Reaction begins at and Hower (1970). modynamically, because the reaction rate is a depth at which the temperature reaches too slow. 50°C and ceases at 100°C, where the clay is depth at which specific diagenetic reactions 20 percent expandable. With 35 percent take place. The effect of possible differences INTRODUCTION expandable layers, an allevardite-type or- in pore-water composition between wells dering develops. was likewise unknown. Hydrothermal experiments were under- Clay sequences from other wells studied In order to study the effect of time and taken to determine the rate of illite forma- by Perry and Hower are similar, but they temperature on the conversion of smectite tion from smectite. This reaction, which in- provide evidence that the reaction may be to illite, a portion of the system A1203- cludes the progressive alteration of smectite somewhat dependent on pressure or reac- Si02-K20-Na20-H20 was chosen for hy- to a mixed-layer illite/smectite (hereafter, tion time as well as on temperature. They drothermal experimentation at a water I/S), has been shown to take place ubiqui- studied two sets of wells: one set from Gal- pressure of 2 kb. The work of Velde (1969) tously with increasing depth of burial in the veston and Harris Counties, Texas, in sed- on the system - at Gulf Coast by Burst (1959, 1969), Perry iment of Oligocene-Miocene age and one low temperatures encouraged us to hope and Hower (1970), and Weaver and others set of offshore Louisiana wells of Pliocene- for petrologically meaningful results. (1971). Similar trends were found by Pleistocene age. The Texas wells are not Dunoyer de Segonzac (1965) in the Cre- only in older sediment, but they are also in EXPERIMENTAL TECHNIQUES taceous strata of the African Cameroun, by stratigraphic sections where the geothermal Long and Neglia (1968) for the Miocene- gradient is higher. Perry and Hower could Glass starting compositions listed in Pliocene strata of the Po Valley, and by not, therefore, unequivocally distinguish Table 1 were prepared by the Ludox gel Foscolos and Kodama (1974) for the Lower between the effects of age (kinetics), temp- method of Luth and Ingamells (1965). Cretaceous of northeastern British erature, and pressure in determining the Natural samples of sodium- and potas- Columbia. These studies indicate that an understanding of the smectite-to-illite transformation is central to understanding TABLE 1. STARTING COMPOSITIONS burial metamorphism in argillaceous sedi- Composition Atomic proportions of Mineral with ment. no. the elements analogous composition The alteration of smectite layers to illite Si Al Na K layers in a mixed-layer clay results from an increase in the net negative charge on the II 1.0 0.64 0.00 0.093 K-beidellite smectite 2:1 layers. This negative charge V 1.0 0.64 0.056 0.038 Na0.6-K0 4 beidellite may result from the substitution of a triva- VII 1.0 0.64 0.093 0.000 Na-beidellite 3+ lent cation, such as Al , for tetravalent sili- XIV-K API4 standard no. 26. K-saturated Wyoming con in the tetrahedral sheet and (or) from Initially 100% expandable (<1 /u,m) the substitution of a divalent cation, such as XrV-Na API4 standard no. 26. Na-saturated Wyoming 2+ Mg , for trivalent aluminum in the oc- Initially 100% expandable bentonite (<1 /xm) tahedral sheet. Electrical neutrality is pre- served during these substitutions by an ac- * American Petroleum Institute.

Geological Society of America Bulletin, v. 87, p. 1326-1330, 4 figs., September 1976, Doc. no. 60914.

1326

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1326/3418592/i0016-7606-87-9-1326.pdf by guest on 29 September 2021 KINETICS OF ILUTE FORMATION 1327

sium-saturated Wyoming bentonite were Hower (1970). The expandabilities re- composition II reacted to form randomly also used as starting materials. Two types ported are accurate to within ±5 percent. interstratified I/S and finally ordered 1/S. of hydrothermal experiments were per- Expandability was inversely proportional formed: (1) those carried out in small REACTION OF THE GLASS to run time. Thus, a sequence of reactions welded gold tubes at a water pressure of 2 STARTING COMPOSITIONS similar to those that appear during burial kb and (2) those run in large-volume diagenesis (Fig. 1) could be followed in the (25-ml) teflon reaction vessels at a water The conversion of natural smectite into laboratory. The reaction can be sum- pressure of about 5 b. mixed-layer clay occurs below 150°C in the marized as follows:

The gold-tube runs were prepared by Gulf Coast (see Fig. 1). Laboratory experi- (1) . (2) putting 30 mg of the powdered starting ments performed at these temperatures glass —> 100% expandable smectite —> material into 1.90-cm-long gold tubes (3.05 would take a long time to reach equilib- I/S + (or pyrophyllite) + quartz. mm O.D., 2.54 mm I.D.) containing 35 ¡x\ rium. In fact, glasses run at 152°C showed With sodium in the interlayers, reaction of distilled water that had been introduced little tendency to crystallize even after 100 at 260°C proceeded in a manner analogous into the tube with a microsyringe. (The rel- days (runs 105, 111, 116, Table 2). Crystal- to the above reaction, except that mixed- atively large water/solid ratio was found to lization did occur at 260°C, at least 100° layer /smectite (or / be important for obtaining well-crystallized higher than Gulf Coast temperatures. Kine- smectite) presumably formed rather than run products.) The tube was then welded tic data found at these elevated tempera- illite/smectite. At higher temperatures, shut and placed in a stainless steel rod-type tures were used to calculate the rate con- however, the progressive formation of reaction vessel. The water pressure in the stants that would prevail during diagenesis. paragonitelike layers was interrupted by the bomb was controlled at 2 kb by equilibra- Glass compositions II, V, and VII (Table appearance of a highly expandable beidel- tion with a 2-1 reservoir connected to the 1) were run for varying lengths of time at lite (D. Eberl and J. Hower, in prep.), which vessel, and the bomb was sealed from the 260°, 300°, 340°, 390°, and 490°C. The complicated reaction kinetics. Thus, kinetic reservoir when the bomb had reached the ratios of the elements for these starting calculations (Table 3) are limited to the desired temperature. Temperatures were glasses are those of a beidellite-type smec- potassium composition II. maintained in horizontal resistance fur- tite, the structural formula for which can be The conversion of glass composition II naces by on-off regulators; the temperature written Al2(Si3.6(iAl0.M)O10(OH)2(K, Na)0.34 into beidellite in the first step of the above was measured by an external thermocouple after MacEwan (1961). It was found that in reaction goes to completion in several days. located in a well in the side of the vessel. most runs these glasses reacted according to The second step, however, takes place over Temperatures were corrected for differ- Ostwald's step rule, which, simply stated, months or years, depending on the temper- ences between the exterior and interior of predicts that the easiest phase to form will ature. Thus, the time required for the first the bomb and are accurate to within ±5°C. form first, and then equilibrium will gradu- step can be ignored when calculating the The teflon-bomb runs were prepared by ally be approached through a series of rate constants. putting 100 mg of starting material and 25 steps. The initial phase to crystallize was a The appearance of illite layers in the sec- ml of solution into reaction vessels that had 100 percent expandable smectite. With in- ond step of the equation can be described been manufactured by boring out solid creasing run time, the smectite then reacted with a first-order kinetic equation. The in- teflon rods. These vessels were sealed with along a path determined by the interlayer tegrated form of this equation is teflon screw tops and reinforced with hose cation. clamps. The bombs were then placed in With potassium as the only interlayer cat- large boiling flasks that contained mixtures ion, smectite initially synthesized from of ethylene glycol and water that boiled at the desired temperatures. The flasks were TABLE 2. KINETIC DATA FOR THE CONVERSION OF topped with reflux condensers and held at SMECTITE INTO ILLITE/SMECTITE boiling temperatures on hot plates. The solid run products were identified Run Composition Temperature Run time Percent Other run with a General Electric XRD6 diffraction no. (°C) (days) expandable products unit using Ni-filtered copper K^, radiation. of I/S phase The final teflon-bomb solutions were 105T II 152 101 No reaction analyzed for sodium and potassium by 1 II 260 99 85 Kao flame emission with a model 303 Perkin- 2 II 260 266 65 Kao Elmer atomic absorption unit. X-ray pat- 3 11 300 31 85 Kao, Q, fspar terns of randomly oriented samples were 4 11 300 88 70 Kao, fspar made of the run products; the products 6 II 343 5 90 None were also x-rayed after orientation by set- 7 II 343 23 80 Kao 8 II tling from suspension onto glass slides. Dif- 343 88 35 (ord) Q, fspar 9 II 343 99 25 (ord) fractograms of the oriented specimens, both Q(?) 11 II 393 3 70 Q air-dried and ethylene glycol—treated, were 12 II 393 14 70 Q(?), py(?) made at 2° 20/min. In addition, the spacings 13 II 393 23 35 (ord) Q, py(?), fspar of the (001)/(002)lo/17 and the (002)/ 14 II 393 169 15 (ord) Q. py (003) or the (002)/(005) 10/2? reflections 10/17 HIT V 152 82 No reaction of the mixed-layer phases were determined 28 V 260 99 90 Kao at 0.4°/min. The phases synthesized were 29 V 260 266 75 Kao, Q identified in part by peak positions given 116T VII 152 101 No reaction in Brown (1961). In addition, the compo- 54 VII 260 92 90 Kao sition of the mixed-layer phase was deter- 55 VII 260 259 70 Kao mined by comparison between patterns of oriented, ethylene glycol—treated specimens Note.- Q = quartz, py = pyrophyllite, fspar = , Kao = Kaolinite, T = teflon bomb run, ord and patterns calculated by Reynolds and = allevardite ordering.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1326/3418592/i0016-7606-87-9-1326.pdf by guest on 29 September 2021 1328 EBERL AND HOWER

1 1 - 1 I T -I COMPOSITION U COMPOSITION n Ea= 19.6* 3.5 KCAL /Mole (1st ORO. REACT)

M *-2 es o

-3 -

80 160 240 1 i i i i 1.4 1.5 1.6 1.7 1.9 DAYS 1.8 3 Figure 2. First-order kinetic plots of data found in Table 2 for the re- (l/T) x |0 action of composition II. Error is maximum error. Figure 3. Arrhenius plot of the rate constants found in Table 3 for the reaction of composition II.

In this study, a is the initial concentration of Rate constants determined from Figure 2 The potassium-saturated formed smectite, which is taken to be 100 since the are given in Table 3. illite layers much faster than did synthetic clay is initially 100 percent smectite layers. The Arrhenius equation is smectites run at comparable temperatures. The x equals the amount of smectite that The reaction mechanism for illite formation A login k has reacted to form illite after i days. There- 2.303 R , in the potassium bentonites is not under- fore, (a — x) equals the percentage of smec- A 1 IT stood, and we are reluctant to apply these tite layers (or the expandability) of the I/S results to shale petrology. where Ea is the activation energy, R is the synthesized, and the equation reduces to gas constant, and T is the absolute tempera- The presence of sodium in the system ture. A plot of the log of the rate constants greatly affected the reaction rate. Table 5 100 ln- kt. (Table 3) against the reciprocal of the abso- shows that the appearance of illite was % expandable lute temperatures is a straight line, the slope slowed by an increasing amount of sodium of which is a function of the activation relative to potassium. Purely sodium- The first-order rate constant k for a specific energy. Such a plot is shown in Figure 3. saturated bentonites showed little tendency -1 temperature is given in terms of (days) . If The activation energy determined from this to react below 379°C; here reaction was the reaction is of first order, a plot of the plot for the formation of illite layers from probably stopped by the appearance of the left side of this equation against time for a beidellite is 19.6 ± 3.5 kcal/mole. highly expandable beidellite mentioned specific temperature should yield a straight previously. line that passes through the origin, the slope REACTION OF THE NATURAL of which is k. Such plots, together with the STARTING COMPOSITIONS APPLICATION OF THE estimated error, are shown in Figure 2. The KINETIC RESULTS error in determining the rate constant is re- Data for the conversion of 100 percent ported as maximum error and is attributa- expandable Wyoming bentonite into I/S is The activation energy of 19.6 ± 3.5 ble to the error in determining expandabil- shown in Table 4. The Wyoming bentonite kcal/mole found for the conversion of smec- 2+ ity- is a smectite containing Mg substituting tite to illite probably represents the energy 3+ For composition II, the reaction is of first for Al in the octahedral layer (Kerr and required to break chemical bonds in the tet- order at all temperatures below 400°C, as is others, 1951). Samples of this bentonite rahedral sheet so that aluminum can substi- shown by the reaction-rate lines that pass were supersaturated with KC1 and NaCl. tute for , thereby building a negative through the origin (Fig. 2). At 490°C com- position II crystallized directly from the glass to an ordered structure with 30 to 40 TABLE 4. CONVERSION OF WYOMING BENTONITE INTO ILLITE / SMECTITE percent expandable layers. Therefore, this (GOLD-TUBE RUNS) temperature is not included in the study. Run Starting Temperature Run time Percent Other run no. composition (°C) (days) expandable products TABLE 3. EXPERIMENTALLY of I/S phase DETERMINED RATE CONSTANTS AND 4 MAXIMUM ERROR FOR THE 90 XIV-K 215 167 35 (ord) Q(?), KC1 FORMATION OF ILLITE LAYERS IN 91 xrv-K 250 167 20 (ord) KC1 BEIDELLITE FORMED FROM 93 XIV-K 285 74 35 (ord) Q(?), KC1 COMPOSITION II 94 XIV-K 330 74 30 (ord) Kao, Q, KC1 96 XIV-Na 250 167 100 NaCl Temperature Rate constant 97 XrV-Na 285 74 90 NaCl 1 (°C) (days" ) 98 XIV-Na 330 74 90 Kao, Q, NaCl 100 379 50 (ord) 3 XIV-Na 169 Kao, Q, fspar 260 1.2 ± 0.5 x 10~ (rectorite) 300 4.3 ± 1.2 x IO-3 340 13.0 ± 3.0 x 10"3 Note: Mineral abbreviations as in Table 2. 390 45.0 ± 7.0 x 10"3 '' Allevardite-type ordering.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1326/3418592/i0016-7606-87-9-1326.pdf by guest on 29 September 2021 KINETICS OF ILUTE FORMATION 1329

charge on the 2:1 layers. Complementary be read from this figure. The lower curve evidence for the strength of bonds in clay assumes an activation energy of 19.6 crystals comes from the acid dissolution of kcal/mole. The upper curve, with an activa- clay minerals. Osthaus (1956) found an ac- tion energy of 23.4 kcal/mole, assumes tivation energy of 17 to 18 kcal/mole for the maximum error in determining the reaction release of octahedral and rate and gives a maximum reaction time. by the acid dissolution of According to the upper curve of Figure 1 and . Abdul-Latif and Weaver and at a typical Gulf Coast temperature of (1969) treated with acid and 80°C, composition II will convert to a 20 found an activation energy of 18.4 percent expandable I/S in less than 1 m.y. In kcal/mole for the release of magnesium, Perry and Hower's Gulf Coast well (Fig. 1), aluminum, and iron, and Thompson and illite/smectite collected at 80°C is more than Hower (1973) found an activation energy 40 percent expandable and has been buried of 19 kcal/mole for the release of fixed to its present depth since Miocene time. If potassium from illite interlayers by the acid the expandability of Gulf Coast clay is a dissolution of . function of reaction rate and the kinetics of Burst (1969) has suggested that the for- the chemically simple system studied here mation of illite from smectite during are applicable to the complex natural sys- diagenesis involves the simple compaction tem, then I/S of this expandability should TEMPERATURE °C and dehydration of smectite layers by the have disappeared long ago. The high ex- pressure of the overburden. Perry and pandabilities prevailing in the Gulf Coast Figure 4. Relationship between temperature Hower (1970), however, contended that would then indicate that I/S is stable and and the time it would take to form a 20 percent expandable I/S from a 100 percent expandable the chemistry of the smectite 2:1 layers that its expandability is determined by pre- vailing P-T conditions, rather than by the smectite formed from composition II. Ea = 19.6 must change before illite can form. The ac- kcal/mole is the activation energy for the reac- length of time the clay has had to react. tivation energy here determined supports tion. E„ = 23.4 kcal/mole assumes maximum the mechanism of Perry and Hower. Addi- But can these experimental rate constants error. tional evidence for this mechanism comes be applied to the Gulf Coast system? Table from Weaver and others (1971) and Fos- 5 showed that the presence of sodium slows tion of the interlayers with potassium. (For colos and Kodama (1974). They found that the reaction rate for the Wyoming bento- a summary of this problem, see Grim, increasing amounts of aluminum substitute nite, and Blatter (1974) showed that mag- 1968.) for silicon in the tetrahedral sheet as burial nesium and calcium will also slow the reac- Figure 4 shows that even in the fast- diagenesis proceeds. tion. In addition, the reaction for composi- reacting experimental system, the reaction Another petrologically important ques- tion II previously written is probably not of beidellite to illite by tetrahedral substitu- tion is whether the relationship between the reaction that occurs during diagenesis, tion would proceed so slowly at ocean- temperature and expandability (Fig. 1) rep- since kaolinite does not appear to increase bottom temperatures that illite so formed resents chemical equilibrium or reaction with depth and in fact has been shown to would not be detectable even if the reaction kinetics. If it represents equilibrium, then disappear (Dunoyer de Segonzac, 1965). were favored thermodynamically. For ex- the expandability of I/S in ancient sedimen- The natural reaction might be Al3+ + K+ + ample, using an activation energy of 19.6 tary rock can be used as a sensitive indi- smectite —» I/S + Si02. This equation, like kcal/mole at 0°C, it would take more than cator of diagenetic grade. Figure 4 is de- the experimental reaction, involves the tet- 250 m.y. to convert beidellite into a 20 per- rived from rate constants, obtained at ex- rahedral substitution of aluminum for sili- cent expandable I/S. In order to form such perimental temperatures, extrapolated to con. In this reaction, however, kaolinite an I/S, beidellite on the floor of the Atlantic lower temperatures with the Arrhenius does not form, because there is excess Ocean would have had to begin reacting equation. The time necessary at a given aluminum and potassium in the system. during Paleozoic time, before the present temperature to form a 20 percent expanda- These cations could come from the decom- Atlantic Ocean basin was formed. ble I/S from a 100 percent expandable position of detrital feldspar and muscovite. There is little reason to suppose that oc- beidellite formed from composition II can We do not know what effect this excess tahedral substitution occurs more readily potassium and aluminum would have on than tetrahedral substitution; acid dissolu- the rate constant. Because of these unde- tion experiments mentioned previously TABLE 5. DEPENDENCE OF THE RATE termined factors that may slow reaction in show that activation energies for the re- OF ILLITE FORMATION ON THE the natural system, the kinetic data pre- moval of cations from the two sites are SOLUTION COMPOSITION FOR THE sented here are consistent with either an REACTION OF WYOMING BENTONITE nearly identical. Therefore, the formation equilibrium or a kinetic interpretation for of illite from smectite by octahedral or tet- Figure 1. Run no. K/Na (ppm Percent rahedral substitution is probably limited to of final expandable A third application of the kinetic results the elevated temperatures of burial solution) after 78 days concerns the formation of illite from smec- diagenesis and does not occur on the ocean tite on the ocean floor. There have been floor. There is another mechanism, how- 150 0.000 100 many studies that purport to show the ever, by which illite could form at low 153 0.042 80 diagenetic conversion of smectite to illite at temperature. A charge could be built on the 152 0.38 70 the interface between sediment and sea 2:1 lattice by the reduction of octahedral 146* 6.4 60 water. An alternative explanation is that ferric iron. This reaction, which would be 151 10.4 60 some illite enters the sea in a degraded state dependent on Eh-Ph conditions rather than Note: The runs were made in teflon bombs and as with a full illitic charge on on temperature, could be responsible for lasted 78 days at 152°C. the 2:1 layers. The transformation back to forming minor amounts of illite layers in * Eighty-two days. illite would then simply involve reconstitu- minerals such as glauconite.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1326/3418592/i0016-7606-87-9-1326.pdf by guest on 29 September 2021 1330 EBERL AND HOWER

ACKNOWLEDGMENTS (Cameroun): Problèmes de diagenèse: Soc., p. 143-207. Alsace-Lorraine Service Carte Géol. Bull., v. Osthaus, B. B., 1956, Kinetic studies on mont- We thank T. F. Anderson and N. Giiven 17, p. 287-310. morillonite and nontronite by the acid- for their helpful comments. This research Foscolos, A. E., and Kodama, H., 1974, dissolution technique: Clays and Clay Min- was funded through National Science Diagenesis of clay minerals from Lower erals, v. 4, p. 301-321. Cretaceous of northeastern British Foundation Grant GA-1269. Perry, E., and Hower, J., 1970, Burial diagenesis Columbia: Clays and Clay Minerals, v. 22, in Gulf Coast pelitic sediments: Clays and p. 319-336. Clay Minerals, v. 18, p. 165-178. Grim, R. E., 1968, Clay mineralogy: New York, Reynolds, R. C., Jr., and Hower, J., 1970, The REFERENCES CITED McGraw-Hill Book Co., 576 p. nature of interlayering in mixed-layer Hower, J., and Mowatt, T. C., 1966, Mineralogy illite-: Clays and Clay Abdul-Latif, N., and Weaver, C. E., 1969, Kine- of the illite-illite/montmorillonite group: Minerals, v. 18, p. 25-36. tics of acid-dissolution of palygorskite (at- Am. Mineralogist, v. 51, p. 821-854. Thompson, G. R., and Hower, J., 1973, An ex- tapulgite) and : Clays and Clay Kerr, P. F., Hamilton, P. K., Pill, R. J., Wheeler, planation for low radiometric ages from Minerals, v. 17, p. 169-178. G. V., Lewis, D. R., Bakhardt, W., Reno, glauconite: Geochim. et Cosmochim. Acta, Blatter, C. F., 1974, Interaction of clay minerals D., Taylor, G. L., Mielenz, R. C., King, v. 37, p. 1473-1491. with saline solutions at elevated tempera- M. E., and Schultz, N. C., 1951, Analytical Velde, B., 1969, The compositional join tures [abs.]: Clay Minerals Conf., 23rd, data on reference clay minerals, preliminary muscovite-pyrophillite at moderate pres- Cleveland 1974, Abs., p. 18. report #7: Am. Petroleum Inst. Proj. 49, sures and temperatures: Soc. Française Brown, G., 1961, The x-ray identification and 160 p. Minéralogie et Cristallographie Bull., v. 92, crystal structure of clay minerals: London, Long, G., and Neglia, S., 1968, Composition de p. 360-368. Mineralog. Soc., 544 p. l'eau interstitielle des argiles et diagenèse Weaver, C. E., Beck, K. C., and Pollard, C. O., Burst, J. F., Jr., 1959, Post-diagenetic clay min- des minéraux argilleaux: Inst. Français Pét- 1971, Clay water diagenesis during burial: eral environmental relationships in the Gulf role Rev., v. 25, p. 53-69. How mud becomes gneiss: U.S. Geol. Sur- Coast Eocene: Clays and Clay Minerals, v. Luth, W. C., and Ingamells, C. O., 1965, Gel pre- vey Prof. Paper 134, p. 1-78. 6, p. 327-341. paration of starting materials for hydro- 1969, Diagenesis of Gulf Coast clayey sedi- thermal experimentation: Am. Mineralo- ments and its possible relation to petroleum gist, v. 50, p. 255-260. MANUSCRIPT RECEIVED BY THE SOCIETY JULY migration: Am. Assoc. Petroleum MacEwan, D.M.C., 1961, The montmorillonite 16, 1975 Geologists Bull., v. 53, p. 73-93. minerals (montmorillonoids), in Brown, G., REVISED MANUSCRIPT RECEIVED NOVEMBER 10, Dunoyer de Segonzac, G., 1965, Les argiles du ed., X-ray identification and structure of 1975 crétacé supérieur dans le basin de Douala clay minerals, Chap. 4: London, Mineralog. MANUSCRIPT ACCEPTED DECEMBER 17, 1975

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1326/3418592/i0016-7606-87-9-1326.pdf by guest on 29 September 2021