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Clays and Clay Minerals, Vol. 33, No. 4, 265-274, 1985.

MINERALOGICAL A N D MORPHOLOGICAL EVIDENCE FOR THE FORMATION OF ILLITE AT THE

EXPENSE OF ILLITE/SMECTITE

RICHARD M. POLLASTRO

U.S. Geological Survey, Federal Center, Denver, Colorado 80225

Abstract--The conversion of smectite to illite by way of a mixed-layer illite/smectite (I/S) series was found to be the major depth-related reaction in clay-mineral assemblages from two cored sedimentary sequences in the Rocky Mountains. The I/S reaction occurred in both interbedded sandstone and shale of Upper Cretaceous and lower Tertiary age in the Green River basin, Wyoming, and in chalk and chalky shale of the Upper Cretaceous Niobrara Formation, Denver basin, Colorado. As the proportion of illite layers in I/S increased with depth in these rocks, the amount of I/S in the clay fraction decreased, and the amount of discrete illite increased. Scanning electron microscopy revealed that the morphologies of highly expansible, randomly interstratified I/S clay (samples from shallow cores) exhibited no distinctive intergrowth or overgrowth textures. In deeply buried rocks containing highly illitic, ordered I/S and abundant discrete illite, however, fibers or laths of illite were formed on earlier I/S substrates. Less commonly, I/S of low expandability shows morphological features of both smectite and illite whereby rigid laths of illite appear to have formed diagenetically from the wall surfaces of I/S honeycombs. This combined morphology suggests some dissolution and reprecipitation (or some reorganization) of materials from the I/S substrate as the substrate was transformed into a more illitic mixed-layer day.

These data suggest that some I/S clay was destroyed by the selective cannibalization of smectite layers in I/S to provide the components needed to make a more illitic I/S. Moreover, discrete illite and other minerals apparently were formed during the reaction. Also, coarser mineral phases, such as potassium feldspar and detrital micas, may not have been required to supply the chemical components in the reaction. The observations provide an explanation for late diagenetic I/S reactions that occurred in restricted or relatively closed geochemical systems, as in early cemented rocks having extremely low permeability and little or no potassium feldspar.

Key Words--Diagenesis, Illite, Illite/smectite, Morphology, Scanning electron microscopy, X-ray powder diffraction.

I N T R O D U C T I O N

Numerous studies of shales and related rocks have convincingly demonstrated depth-dependent mineralogical changes (e.g., Weaver, 1979; Hower, 1981). The most significant reaction within clay-mineral assemblages of deeply buried sedimentary sequences involves the conversion of smectite to illite through a mixed-layer or interstratified illite/smectite (I/S) series. Although researchers generally agree that diagenesis of clay-rich rocks involves a smectite-to-illite transformation, there is still disagreement on the exact nature of the reaction. This reaction has been assumed by some to be isochemical; i.e., all the chemical components necessary for the reaction were present in the original mineral assemblage of the rock (Hower et al., 1976). Other studies have convincingly demonstrated that diffusion of elements from one lithology to another is an active process in the conversion o f smectite to illite (Altaner et al., 1984).

In order for smectite to be converted to illite, K + must be added in the interlayer space and the amount of tetrahedral A13+ must be increased. This reaction might be simply expressed as

Copyright �9 1985, The Clay Minerals Society

Smectite + A13+ + K + = Illite + Si 4+

Accompanying this reaction is the release ofoctahedral FC + and Mg 2+ and tetrahedral Si 4+ from smectite. The released silicon is believed to form, at least in part, fine-grained quartz or quartz cement; Fe and Mg are believed to form, in part, reaction products (usually occurring as cements and/or pore fillings) that are more stable under the increased burial conditions, such as chlorite, dolomite, and/or ferroan carbonates (Hower et al., 1976; Boles and Franks, 1979; McHargue and Price, 1982). Interlayer Ca 2+ and/or Na + are also released as smectite layers are destroyed, and these elements may also be incorporated into mineral reaction products such as authigenic plagioclase.

Detailed mineralogical and chemical studies by Hower et aL (1976) found that the gain in illite layers in I/S with increasing burial depth coincided with the loss of K-feldspar in a sequence of shale from the early Tertiary Wilcox Group of the Gulf Coast of the Uni ted States. Assuming that the original smectite 2:1 layers remained intact, they concluded that the decomposi- tion of the K-feldspar, and perhaps detrital potassium micas, provided the necessary K and A1 for the smectite

265

266 Pollastro Clays and Clay Minerals

to be converted to illite. Simplified, this reaction, referred to below as Reaction (1), might be expressed as:

K-feldspar + / - K-mica + Smectite (or smectite layers in I/S) -, Illite (or illite layers in I / S ) + Quartz +

Dolomite, etc. (1)

In contrast, Boles and Franks (1979), studying the sandstones and shales of the Wilcox Group in southwest Texas, proposed that the reaction involved a selective decomposi t ion o f the smectite layers in the I/S itself, a conservation of A13+, and a concentration o f potassium already present in the I/S clay. This cannibalization of I/S clay, referred to below as Reaction (2) in their paper, provides most of the chemical components needed to make I/S with a relatively higher illite content. Any addit ional potassium needed for the reaction could come from K-feldspars. Reaction (2) might be expressed as:

N smectite layers in I/S ~ illite layer in US + quartz + dolomite, etc., (2)

where N is equal to the number of smectite layers required to balance the reaction. Reaction (2) would result in a significant reduction in the amount o f I/S clay present in the rock. Boles and Franks (1979) showed that Reaction (2) was a better fit for the mineralogical data of Hower et aL (1976). Hower (1981), in a review of the many interpretations of shale diagenesis, stated that the reaction suggested by Boles and Franks (1979) may be correct.

In the present report, X-ray powder diffraction (XRD) data and observations from scanning electron microscopy (SEM) studies support the hypothesis proposed by Boles and Franks (1979) that the progressive format ion of more illitic clays in these rocks may be at the expense of earlier, more expandable I/S clays during burial diagenesis. Little K + and AP + from coarser feldspar or micas, even i f available, is needed for the reaction to occur.

STUDY AREAS A N D M E T H O D O L O G Y

Detai led qualitative and semiquanti tat ive analyses by XRD and petrographic methods were performed on the whole-rock and clay-size fractions of core sam- pies o f some potential hydrocarbon-producing units from two basins within the Rocky Mountains region. One study involved the analysis o f core from interbedded, discontinuous, low-permeabil i ty sandstones and shales of lower Tertiary and Upper Cretaceous age from the E1 Paso Natural Gas (EPNG) Wagon Wheel No. 1 well, Green River basin, Wyoming (Figure 1). The sec- ond study was the analysis of chalk and chalky shale recovered in five cores of the Upper Cretaceous Nio- brara Format ion, Denver basin, Colorado (Figure 1).

The EPNG Wagon Wheel No. 1 well (sec. 5, T30N,

R108W, Sublette County, Wyoming), located on the crest of the Pinedale anticline in the northern Green River basin, Wyoming (Figure 1), was a site originally planned for the nuclear s t imulat ion of tight-gas sandstones. In anticipat ion of this test, 12 sections o f core, totaling about 275 m (900 ft), were taken at selected intervals over depths ranging from 1525 to 5485 m (5000 to 18,000 It). The approximate locations o f these cores in the well and units sampled are shown in Figure 1. The clay minerals from about 180 sandstone and shale samples were analyzed in the present study. Other aspects and details of the mineralogy, petrology, and stratigraphy of the Wagon Wheel core were described by Pollastro and Bader (1983), Pollastro and Barker (1984), and Law et al. (1985).

The present report also concentrates on the analysis o f 105 samples from the uppermost , t ime-correlat ive chalk unit of the Upper Cretaceous Niobrara Forma- tion, Denver basin, Colorado. The samples were taken from core recovered from 5 wells at three depths between 445 and 1767 m (1460 and 5800 It) and along a southest to northwest t rend on the eastern flank of the basin (locations A, B, and C in Figure 1). Detai ls on the locations and descriptions for these Niobrara cores are given in Pollastro and Scholle (1985).

Quali tat ive identification and semiquant i ta t ive calculated weight percentages of total clay and other minerals in sandstones, shales, carbonates, and the acid- insoluble residues of the carbonates were es t imated from X R D analyses of randomly oriented whole-rock and residue powders ground to pass a 44-urn (325 mesh) screen. Semiquant i ta t ive values were determined by comparison with several prepared mixtures of concen- trated mineral phases from these same rocks, insofar as possible, and by the procedures outl ined by Schultz (1964) and Hoffman (1976) with minor modification. Petrographic analysis o f impregnated and stained thin sections was also used to supplement the X R D analysis.

Oriented clay aggregates of the < 2-/zm and <0.5- tzm (equivalent spherical diameter) fractions were prepared using a modified f i l ter-membrane-peel technique (Pollastro, 1982) similar to that described by Drever (1973). Semiquant i ta t ive X R D analyses of the clay- sized fractions were made using the method o f Schultz (1964) with two modifications. One modif icat ion involved the use o f a digital planimeter to measure the peak areas and linear peak heights; the other incorporated the ratio o f the l inear peak intensities o f the kaolinite and chlorite doublet between 25 ~ and 26~ (CuKa radiation). Semiquant i ta t ive interpretat ions of the <2-g in clay-mineral analysis were then checked against the relative intensit ies of the clay-mineral re- flections in both the whole-rock and in termediate fractions to assure that interpretat ions were representat ive of the bulk rock (Towe, 1974; Carson and Arcaro, 1983),

Vol. 33, No. 4, 1985 Formation of illite at the expense of illite/smectite 267

EL P A S O W A G O N WHEEL

CORED " % . . . . . . . . INTERVALS ANTICLINE

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Location map and diagrammatic cross-sections of study areas. E1 Paso Natural Gas Wagon Wheel No. 1 well was drilled through Tertiary and Upper Cretaceous sandstones and shales to total depth of 5790 m (19,000 ft). Five cores of the Niobrara Formation were taken from the uppermost time-equivalent chalk unit immediately below the Pierre Shale at sites A (two wells), B (two wells), and C.

Composition and ordering of the I/S clay was determined on oriented, ethylene glycol-saturated specimens of both the < 2-/~m and < 0.5-urn fractions and by the methods of Reynolds and Hower (1970) and Schultz (1978). Ordering types were defined using the "Reichweite" (R) notation, as described by Reynolds (1980), where "R" signifies the most distant layer in an interstratified sequence that affects the probability of occurrence of the final layer. Most diffractograms of natural occurring I/S clay can be categorized as falling into one of three ordering types of interstratification (Reynolds and Hower, 1970). These three ordering types are (1) random I/S (R = 0), (2) allevardite or short- range ordered (R = 1,2), and (3) kalkberg or long-range ordered US (R >-- 3).

SEM and simultaneous energy-dispersive elemental X-ray (EDX) analyses of freshly fractured whole-rock samples were also used to characterize the morphology and chemical composition of the clay minerals, particularly I/S clays. Small samples of rock chip were mounted on a luminum stubs and sputter-coated with ~ 100 ~ of gold/palladium.

ANALYTICAL RESULTS

PC-ray powder diffraction and petrographic analyses of whole-rock samples

In the EPNG Wagon Wheel core, the Tertiary sandstones (1525 to 2255 m; 5000 to 7400 ft) are typically coarse-grained lithic arkoses and contain K-feldspar and K-micas. Interbedded shales also contain mod- erate amounts of K-feldspar and K-micas. Few of the Cretaceous rocks from the Lance, Ericson, and Rock Springs Formations (2255 to 4865 m; 7400 to 16,000 ft), however, contain more than a few weight percent K-feldspar as detected by XRD. In the Cretaceous sandstones and shales cored from 4865 to 5485 m (16,000 to 18,000 It) in the Wagon Wheel No. 1 well, K-feldspar amounts to as much as 4% of the whole- rock. Some K-feldspar, most of it partly replaced by carbonates, was identified in the Cretaceous intervals. The common replacement of K-feldspar by carbonate minerals indicates that K-feldspar probably was more abundant in these Cretaceous rocks prior to their diagenetic alteration. Detrital K-micas (muscovite, bio-


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Figure 2. Composition of mixed-layer illite/smectite (I/S) clay and calculated weight percent abundance of I/S and discrete illite clay from sandstones (A) and shales (B) of the Wagon Wheel No. 1 well vs. present burial depth. T = Ter- tiary rocks; K = Cretaceous rocks.

tite, and some discrete illite) are common throughout the entire section of core in the Wagon Wheel No. 1 well Late quartz overgrowths, iron carbonates (ferroan calcite and ankerite), and iron chlorite were also common in cores recovered below 2438 m (8000 ft).

In shaly chalk from the Cretaceous Niobrara For- mation, no K-feldspar was detected. Plagioclase, however, was found in most samples in amounts up to 6% of the acid-insoluble fraction (< 4% of the who/e-rock). The only potassium-bearing minerals identified in the insoluble residues were I/S clay and discrete illite.

X-ray powder diffraction analyses of clay-mineral samples

All samples from the Wagon Wheel core contained varying amounts of illite, I/S, and chlorite. Kaolinite was found in many of the samples throughout the core, but its occurrence was unpredictable. Most sample~ between 2134 and 2530 m (7000 and 8300 ft) als0 contained corrensite.

The proportion of illite layers in I/S increased progressively with depth in the day-sized fractions from both the Tertiary and Cretaceous interbedded sand-

NIOBRARA INSOLUBLE RESIDUES

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( < 2 # m F R A C T I O N ) in <2#ra C L A Y Id

Figure 3. Composition of mixed-layer illite/smectite (I/S) clay and weight percent ratio of I/S to discrete illite (I~) in shaly chalks of the Niobrara Formation. A, B, and C refer to drill locations shown in Figure 1.

stones and shales of the Wagon Wheel cores and in the shaly chalk of the Cretaceous Niobrara Format ion (Fig- ures 2 and 3). As the proportion of illite layers in US increased with depth in these rocks, the calculated percentage of I/S clay decreased and discrete illite increased in the clay-size fractions. Also, the range of the percentage of expandable layers of I/S was progressively more restricted with increased burial depth.

In the Wagon Wheel samples, the progressive increase in illite layers in I/S with depth was found to be associated with a progressive increase in ordering of the I/S clay (PoUastro and Barker, 1984). I/S clay in the Wagon Wheel sandstones and shales changed from a randomly interstratified phase (R = 0, Figure 4a), to an ordered, allevardite-like mineral (R --- 1, Figure 4b) at ~2375 m (7800 ft) depth. The transformation from allevardite-like I/S to the kalkberg-like (R -> 3, Figure 4c) I/S form (>85% illite layers in US) occurred at ~4570 m (15,000 ft).

Scanning electron microscopy and energy-dispersive analysis

SEM analyses of the rock sequences studied, particularly the sandstones from the Wagon Wheel No. 1 core, showed characteristic morphological features of I/S clay. At relatively shallow depths in the Wagon Wheel well (< 1829 m; 6000 ft), sandstone cements of highly expandable, randomly interstratified I/S clay (>70% expandable layers, R = 0) were observed as poorly developed wrinkled-sheet structures (Figure 5a) or as well-developed, simple honeycomb structures


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DEGREES 26 Figure 4. Typical X-ray powder diffraction patterns from Wagon Wheel samples showing changes in ordering types of mixed-layer illite/smectite (I/S) clay. Patterns produced from oriented, ethylene glycol-saturated mounts of the <0.5-gm fraction. A. Random I/S (R = 0); B. Short-range ordered I/S (R = 1); C. Long-range ordered I/S (R >- 3); Id = discrete illite; K = kaolinite; C = chlorite; CuKa radiation.

(Figure 5b). These clays exhibited little or no distinctive intergrowth or overgrowth textures superimposed on their primary morphologies. Further downhole in the Wagon Wheel cores (>3048 m; 10,000 ft), however, and in rocks containing low-expandable and ordered I/S clay (<35% expandable layers, R > 1), diagenetic overgrowths ofillite-like forms, usually as fibers or laths, were common on earlier, wrinkled-sheet or honeycomb substrates (Figures 5c, 5d, and 5e). Deeply buried, interbedded shales in the Wagon Wheel cores, having some microporosity and composed of discrete mica and ordered I/S (R -> 1) clay, also showed abundant overgrowths of authigenic illitic forms superimposed on the shale fabric (Figure 5t). Many of these overgrowth textures appeared to represent an exten- sion, perhaps as syntaxial overgrowths, of the (001) planes. Intergrowth features were also observed in illitic, ordered I/S clay. Authigenic I/S clay of low expandability in the Wagon Wheel sandstones locally showed a morphology characteristic of both smectite and illite where rigid laths or plates of illite appear to have formed as diagenetic growths from the wall surface of original I/S honeycombs (Figures 6a and 6b). High-magnification of honeycomb sheets revealed the development of a fabric consisting of interwoven, lath- like fibers in low expandable I/S (Figures 6c and 6d).

EDX analyses of I/S clay from the Wagon Wheel sandstones revealed the basic chemical relationships. The EDX chemistries of I/S are profiled downhole in the Wagon Wheel sandstones along with their corresponding morphologies in Figure 7. As the amount of illite layers in I/S and degree of ordering increased with depth in these rocks, the A1/Si ratio increased and the amount of K; Mg and Ca decreased. The amount of Fe varied with the increasing extent of the reaction (Figures 7a, 7b, and 7c). The morphologies showed an increasing amount of fibrous or lath-like texture with increasing illite layers in I/S and discrete illite as calculated from the XRD analysis.

DISCUSSION AND INTERPRETATION

Because no detailed chemical analyses or mass-balance equations have been derived for the samples studied, interpretations are limited to those suggested by mineralogical and textural observations. The clay-mineral XRD analyses in both the EPNG Wagon Wheel No. 1 and Niobrara studies show that, as the degree of ordering and the amount of illite layers in I/S increased with increased burial, the calculated weight percentage of I/S clay decreased and the amount of discrete illite increased (Figures 2 and 3). Boles and Franks (1979) noted that the mineralogical data of Hower et aL (1976) showed a similar trend. Foscolos and Powell (1980) also noted a "destruction of expandable 2:1 layer silicates" and an increase in discrete illite with depth in several cores of Tertiary to Permian


Figure 5. Scanning electron micrographs of mixed-layer illite/smectite (I/S) from the Wagon Wheel No. 1 cores. A, B. Highly expandable, randomly interstratified I/S cement in lower Tertiary sandstone (<1828 m; 6000 ft) showing no overgrowth or intergrowth textures. C, D, E. Low expandable, ordered US (> 35% smectite layers) cement in Cretaceous sandstones (> 3048 m; 10,000 ft) showing diagenetic overgrowths as fibers or laths (arrows) on honeycomb or smectite-like substrates. F. Lath- like overgrowths (arrow) on fabric of porous shale composed of ordered I/S and discrete illite.

rocks from the Canadian Northwest Territories. The fate of these minerals, however, was unknown.

The mineralogical data from the present studies suggest that more illitic I/S clays formed with progressive burial, perhaps at the expense of more expandable

I/S itself. Discrete illite also formed. These suggestions, however, are based on two assumptions: (1) that the initial ratio of I/S clay to discrete illite clay within each of the lithologies was about the same before any significant depth-diagenetic reactions occurred; and (2)


Figure 6. Scanning electron micrographs of ordered mixed-layer illite/smectite (I/S) from standstones of Wagon Wheel No. 1 cores showing intergrowth textures. Low magnification (A) and high magnification (B) of I/S cement showing combined morphology of both smectite and illite where rigid laths or plates ofillite appear to form as a diagenetic growth from the wall surface of original I/S honeycombs. C, D. High magnification of I/S honeycomb sheets showing fine textures of interwoven, lath-like fibers.

that the average composition of detrital I/S clay sup- plied to these depositional systems did not differ sig- nificantly up and down the stratigraphic sections. Al- though these assumptions are difficult to verify, the depth-dependent relationships are consistent among the various facies, ages, and areas for both the present study and for those studies described from the literature in the above paragraph. Detailed analyses of core and surface samples from stratigraphically equivalent chalk beds of the Niobrara Formation in the Denver basin have shown that a large variation in the clay- mineral composition of acid-insoluble residues exists within a group of samples over very small stratigraphic intervals (Pollastro and Scholle, 1985). Both the average composition of I/S and the average weight ratio of I/S to discrete illite from any one group sampled in the basin, however, appears to be more dependent on burial depth (or paleoburial depth) than on relation to provenance.

The depth-diagenetic I/S reaction in the Niobrara

cores apparently proceeded even though no K-feldspar was detected in the original rock. The source of the potassium and a luminum had to be from some other phase if, in fact, the reaction was isochemical. In deeply buried Niobrara rocks, the most likely source was the clay minerals, and probably the abundant , less stable, expandable clay. Mobility of elements need only have been minimal if the source was I/S, itself. Discrete illite was hardly the K § and AP § source for the illitization of I/S because the amount of discrete illite increased with depth in these rocks. In addition, under deep- burial conditions it is less reasonable to assume the destruction of a discrete mica (illite) phase to create a mixed-layer phase with a significant expandable component. Smectite layers are the most likely component to have broken down under the increased tempera- tures, and possibly the pressures that developed with the further burial of these rocks.

The smectite-to-illite reaction occurred in the shales and sandstones of the EPNG Wagon Wheel cores. Fac-


tors such as the lenticularity of the rocks, the early cementat ion by carbonates, and the low permeabi l i ty could have set some l imitat ions on mobil i ty of fluids and elements and, therefore, favored an in situ chemical source for the reaction. Except for the Ericson For- mation, however, the sandstones in the Wagon Wheel cores contained abundant rock fragments (particularly feldspar and volcanic and mafic grains) that could have contributed to chemical and mechanical instabili ty during burial diagenesis (Hayes, 1979). Because K-micas were abundant as lithic fragments, and because K-feldspar was likely present in significant quantities at some t ime throughout the Wagon Wheel core, it is reasonable that Reaction (1) could apply in part or in whole to these rocks. Reaction (1), however, can not explain the significant decrease in I/S clay with depth.

Textural observations and relationships from SEM studies, particularly the persistence of the honeycomb fabric of I/S through the max imum stage of i l l i t ization and the development of iUitic overgrowths and inter- growths on the honeycomb substrate, suggest that later illite-rich clays may have formed from earlier, more expandable I/S clays during burial diagenesis of the sandstones, shales, and shaly chalks of cores examined in this study. Photographs of I/S clay from other alia- genetic studies showing similar I/S morphologies with overgrowth textures have been published throughout the literature without this interpretat ion (see, e.g., A1- mon and Davies, 1977; Wilson and Pittrnan, 1977; Sommer, 1978; Wilson, 1982). These observations also suggest a process that may have involved a partial dissolution and consequent t ransformation of the I/S clay to a more illitic I/S. In this process, the resulting more illitic clay may have developed morphological characteristics of illite on a morphology that was more like smectite, or that was syntaxially precipitated on the earlier, but now more illitic, I/S substrate. Also, solid-state processes likely contributed to the overall chemical changes.

Figure 8 presents a simplified, graphic interpretat ion of the reaction proposed for the rocks studied in this report; it is not meant, however, to represent a chemical mass balance. In the reaction, previously described in part by Reaction (2) of Boles and Franks (1979), some smectite interlayers in I/S are selectively cannibalized, thereby conserving A13+ and also providing the necessary chemical constituents for the conversion o f other smectite layers to illite layers. Poorly developed illite layers in I/S may either by diagenetically annealed and rebuilt (aggraded) under deeper burial condit ions or, perhaps, break down to supply addit ional potassium and contribute to the decrease in the total amount of I/S clay. Boles and Franks (Reaction 2) calculated that a 24% weight loss in total I/S clay occurs in the conversion of an 80% expandable I/S clay to a 20% expandable I/S clay. This reaction suggests some solution

Figure 7. Energy-dispersive elemental X-ray analysis of mixed-layer illite/smectite (I/S) clay from sandstones in Wag- on Wheel No. 1 core with corresponding scanning electron micrographs showing their morphologies. A. Random I/S (70- 75% expandable layers); B. Ordered I/S (25-30% expandable layers); C. Ordered I/S (15-20% expandable layers).

and reconstruction of the I/S clay. Reaction products, i l lustrated in Figure 8, are likely to be authigenic mineral phases such as quartz, chlorite, Fe-Mg carbonates, and, perhaps, discrete illite. In this reaction, the original honeycomb morphology o f earlier authigenic I/S clay is retained to some degree throughout the entire course of the reaction; however, the honeycomb substrate may be modif ied during progressive i l l i t ization to the extent where it begins to inherit the morphological character or overgrowth and intergrowth textures of illite. Fibers or laths of illite or illitic I/S may form from earlier I/S substrates. It also appears that potassium from K-feldspars or other sources, i f available, contributes to the smectite-to-il l i te reaction (Hower et al., 1976) or to later authigenic grain-growth o f discrete illite fibers in porous rocks.


Figure 8. Simplified, diagrammatic interpretation of the smectite-to-illite reaction showing cannibalization of smectite by solution and reorganization of mixed-layer illite/smectite with increased burial depth (and temperature).

SUMMARY

The mineralogical data from the buried rock sequences of this report and data from Boles and Franks (1979), Hower et al. (1976), and Foscolos and Powell (1980) show that as the percentage ofillite layers in I/S increases progressively with increased depth, the amount of I/S clay decreases progressively and the amount of discrete illite increases correspondingly.

Observations of the I/S from the rocks of the present study using SEM reveal specific textural characteristics. Deeply buried rocks typically contain illitic, ordered I/S with morphological features characteristic ofillites superimposed on a substrate with a morphology characteristic of smectite. These textural characteristics suggest some inheritance, perhaps by solution and reprecipitation or recrystallization with only slight chemical modification, from the original I/S clay into the more illitic composition.

These data and observations support the hypothesis originally proposed by Boles and Franks (1979) that some I/S clay is destroyed by cannibalization of smec- rite layers to provide some or all (depending on the

extent of the reaction) of the chemical components needed to make I/S with a relatively higher illite content. In addition, some discrete illite may be formed in the reaction. Additional potassium and a luminum for the smectite-to-illite reaction or for later grain- growth of illitic clays may come from coarser potassium- or a luminum-bearing phases, such as K-feldspar and K-micas, if they are available. Such phases, however, may not be needed for the reaction to occur in some buried argillaceous sediments.

ACKNOWLEDGMENTS

This work was partly supported by the U.S. De- partment of Energy. The manuscript has benefited greatly from the critical reviews and discussions by Leonard G. Schultz, Paul C. Franks, C. Gene Whitney, James R. Boles, and Stephen G. Franks.

REFERENCES

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(Received 10 May 1984; accepted 24 November 1984; Ms. 1369)

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