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 potassium 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. diagenesis 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 clay 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 muscovite-pyrophyllite 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 shale 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 bentonite (<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 + kaolinite (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 paragonite/smectite (or brammallite/ 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.