K-bentonites of the Middle Ordovician High Bridge Group along the Cincinnati arch are characterized by interstratified illite/smectite (I/S) clays with rectorite-type ordering. Approximately 20% of the layers are expandable. They are structurally similar to I/S formed at temperatures exceeding 100°C during burial diagenesis, however stratigraphic evidence and a color alteration index of < 1.5 for conodonts in associated carbonates reveals they have never been deeply buried or subjected to temperatures greater than 80°C.

Whole-rock samples of K-bentonites contain ∼8% K2O and ∼4% MgO, whereas the <0.1-µm size fraction contains 6–7% K2O and 5% MgO. By comparison with a hypothetical parent ash, these values represent a net gain of K and Mg and a net loss of Si, Fe, Ca, and Na during post-depositional alteration. K-fixation is accounted for by a layer charge imbalance arising primarily out of octahedral substitution of Mg+2 for Al+3, indicating that the interstratification evolved from a montmorillonite precursor. The chemical characteristics of I/S layers in K-bentonites developed early during the alteration of volcanic ash to montmorillonite. Relatively high contents of K and Mg probably reflect both seawater and parent material composition at the time of formation. The composition and ordered stacking in K-bentonites was determined by the composition of the original smectite rather than by the pressure-temperature conditions of burial diagenesis.

Exchange isotherms for the pairs Na-K and Na-Ca were measured by use of 0.1 N solutions at 5°, 35°, and 70°C in phillipsite from Tecopa, California (3.63 Al/32 oxygen unit cell), and Oki Islands, Shi-mane Prefecture, Japan (6.31 Al/32 oxygen unit cell). All isotherms except those for Na-Ca at 5°C were reversible. Free energy was evaluated for all reversible exchanges. The thermodynamic affinity sequences were K > Na > Ca in both phillipsites. The selectivity for K in competition with Na and that for Na competing with Ca became larger at the lower temperatures. The siliceous phillipsite preferred the larger cation more strongly for the Na-K system, and Na more strongly for the Na-Ca system than the aluminous phillipsite.

Statistical analyses of chemical data from the literature of corrensite minerals suggest a large compositional variability, more evident in octahedral than in tetrahedral coordination. Mg occupies 40–80% of the octahedral sites, with Al and Fe2+ making up the remainder. Approximately 15–30% of the tetrahedral sites are filled by Al. Despite this compositional variability, distinct fields for the several types of mixed-layer trioctahedral chlorite/trioctahedral swelling layer are not apparent. Statistical analyses of the composition of corrensite compared with saponite, vermiculite, and chlorite suggest that corrensite is an intermediate between trioctahedral chlorite and trioctahedral smectite. If Fe/(Fe + Mg) > 50%, chlorite alone is favored, but with increasing Mg, chlorite appears to transform into corrensite and then, by iron oxidation, into trioctahedral smectite. Despite the chemical variability between corrensite, chlorite, and saponite, corrensite appears chemically to be a well-defined species. On the other hand, corrensite cannot be characterized chemically on the basis of its swelling component. Thus, the current definition of corrensite as a regular 1:1 interstratification of trioctahedral chlorite and either trioctahedral smectite or vermiculite is appropriate.

Al-rich di, trioctahedral chlorite exists as the species cookeite and sudoite. Di,dioctahedral chlorite exists as the species donbassite. Cookeite has essential Li in its structure, sudoite has essential Mg, and donbassite has only small amounts of either element. To date, sudoite has been reported to have only IIb structural units and donbassite to have only Ia structural units. Cookeite is based primarily on Ia structural units, but IIb units are present in specimens from two localities. Most Al-rich chlorite species have regular-stacking “r” or “s” 2-layer stacking sequences, but 1-layer Ia-2 and Ia-6 polytypes also are known. The structural units (Ia or IIb) and the specific stacking sequences can be explained by a combination of local charge balance and minimization of cation-cation repulsion involving the interlayer and tetrahedral cations. X-ray powder diffraction data are adequate to differentiate Al-rich chlorite from trioctahedral chlorite and to identify the type of structural unit present, but single crystal study is necessary to identify the 2-layer and 1-layer sequences with certainty.

Phillipsite is a common constituent in diagenetically altered rhyolitic vitric tuffs of Cenozoic saline, alkaline-lake deposits where it occurs as spherulites or aggregates of prismatic crystals. This phillipsite has chemical and physical properties that differ from phillipsites that occur in rocks of other compositions and depositional environments, but previously it had not been completely characterized. Published analyses of five samples from four deposits in the southwestern United States show that the phillipsite is very siliceous and alkalic; the Si/(Al + Fe3+) ratio is 3.08–3.37, alkalis greatly exceed the divalent exchangeable cations, and the Na/K ratio is 1.05–3.05. The mean index of refraction (1.451–1.470) is much lower than that generally reported for the phillipsite group and can be correlated with the relatively high Si and alkali contents. Monoclinic unit-cell parameters show the following ranges: a = 9.931–9.991 Å, b = 14.142–14.155 Å, c = 8.650–8.706 Å, ß = 124.57–125.07°, and V = 1000.3–1007.4 Å3. The especially small b dimension is consistent with the high Si content.

Yucca Mountain is being studied as a potential site in southern Nevada for an underground, high-level nuclear waste repository. A major consideration for selecting this site is the presence of abundant zeolites in Miocene ash-flow tuffs underlying the region. Beneath Yucca Mountain four diagenetic mineral zones have been recognized that become progressively less hydrous with depth.

Zone I, the shallowest zone, is 375–584 m thick in the central part of Yucca Mountain, but 170 m thick to the north. Zone I contains vitric tuffs that consist of unaltered volcanic glass and minor smectite, opal, heulandite, and Ca-rich clinoptilolite. Zone II thins south to north from 700 to 480 m and is characterized by complete replacement of volcanic glass by clinoptilolite with and without mordenite, and by lesser amounts of opal, K-feldspar, quartz, and smectite. Zone III thins south to north from 400 to 98 m thick and consists of analcime, K-feldspar, quartz, and minor calcite and smectite. Heulandite occurs locally at the top of zone III in the eastern part of Yucca Mountain. Zone IV occurs in the deepest structural levels of the volcanic pile and is characterized by albite, K-feldspar, quartz, and minor calcite and smectite.

Clinoptilolite and heulandites in zone I have uniform Ca-rich compositions (60–90 mole % Ca) and Si:Al ratios that are mainly between 4.0 and 4.6. In contrast, clinoptilolites deeper in the volcanic sequence have highly variable compositions that vary vertically and laterally. Deeper clinoptilolites in the eastern part of Yucca Mountain are calcic-potassic and tend to become more calcium-rich with depth. Clinoptilolites at equivalent stratigraphic levels on the western side of Yucca Mountain have sodic-potassic compositions and tend to become more sodium-rich with depth. Despite their differences in exchangeable cation compositions these two deeper compositional suites have similar Si:Al ratios, generally between 4.4 and 5.0. Analcimes have nearly pure end-member compositions, typical of these minerals formed by diagenetic alteration of siliceous volcanic glass; however, K-feldspars are Si-rich compared to the ideal feldspar formula.

Bulk-rock contents of Si, Na, K, Ca, and Mg of zeolitic tuffs generally differ significantly from stratigraphically equivalent vitric tuffs, suggesting that zeolite diagenesis took place in an open chemical system. Both the whole rock and the clinoptilolite are relatively rich in Ca and Mg in the eastern part of Yucca Mountain and rich in Na in the western part. The Ca- and Mg-rich compositions of the zeolitized tuffs in the eastern part of Yucca Mountain may be due to cation exchange by the sorptive minerals with ground water partially derived from underlying Paleozoic carbonate aquifers.

Diagenetic zones become thinner and occur at stratigraphically higher levels from south to north across Yucca Mountain, probably due to a higher geothermal gradient in the northern part of the area. The diagenetic zones were established when the geothermal gradient was greater than it is today, probably during the thermal event associated with the development of the Timber Mountain-Oasis Valley caldera complex north of Yucca Mountain.

The structure of sepiolite from a piston core obtained on Ninetyeast Ridge in the Indian Ocean was modified by exposure to ethylene glycol vapor. With ethylene glycol, the sepiolite Oil X-ray powder diffraction peak expanded from 12.4 to 12.8 Å, and the 130 peak contracted from 4.53 to about 4.45 Å. The cell modification is consistent with concomitant expansion along the c-axis and contraction along the b-axis. This structural distortion is not permanent, however, inasmuch as the sepiolite returned to its original state 6 to 12 hr after it was removed from the ethylene glycol-saturated atmosphere.

Chemical analysis by X-ray fluorescence (XRF) and calculated structural formulae of clay-size fractions of smectites from Cretaceous bentonites and illitic clays from Cretaceous, Devonian, and Ordovician bentonites and Jurassic and Permian sandstones indicate the nature and extent of various types of ionic substitution. The determination of tetrahedral (Al, Si) and octahedral (Al, Mg, Fe) composition shows the variable chemistry of these materials. Structural formulae of the illitic clays show that they have tetrahedral charges between 0.4 and 0.8 per half unit cell, and can be divided into phengitic types having octahedral charges of 0.2-0.4 and muscovitic types having octahedral charges <0.2. Evaluation of the formulae in the light of X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) data shows that the occupancy of non-exchangeable interlayer sites (predominantly K) varies from 47% to 90% of that of ideal muscovite. In some minerals as much as 20% of these sites is occupied by ammonium ions (determined independently). The amount of surface silicate charge balanced by non-exchangeable cations versus that balanced by exchangeable cations has been examined in conjunction with TEM data and suggests that in most samples the charges are about equal. The octahedral composition of smectites in Cretaceous bentonites precludes their having served as transformation precursors for most of the Cretaceous illitic bentonites. The results suggest that these illitic clays originated by neoformation.

Mineralogical (XRD), morphological (transmission electron microscopy), chemical (major, rare-earth elements, and scanning-transmission electron microscopy), and isotope (Sr, O, H) measurements were made of marine detrital smectite from shales to study their reactions during early diagenesis. Albian, Aptian, and Palaeogene smectite samples were selected from Deep Sea Drilling Project drill cores taken in the Atlantic Ocean and from outcrops and drill cores from Belgium and northern France. Detrital, fake-like smectite particles seem to have adapted to their depositional environment by isochemical dissolution and subsequent crystallization of authigenic, lath-like particles. The major-element and rare-earth element compositions of both types of particles are similar. The Sr isotope chemistry suggests that the dissolution-crystallization process occurred soon after deposition in an almost closed chemical system. Except for slight changes in the amount of Fe and the oxygen isotope composition, the reaction took place without noticeable chemical exchange with the interstitial or marine environment. Such closed-system recrystallization of clay minerals may be a common diagenetic process if the water/rock ratio is small, as in shales.

The Rokle bentonite deposit is part of an accumulation of argillized volcanoclastic rocks in the Tertiary stratovolcanic complex of the Doupovské Mountains east of Karlovy Vary (Carlsbad), about 100 km westnorthwest of Prague, Czechoslovakia. The arenite basalt ash was originally composed of hyaloclasts and subordinate biotite. The following types of montmorillonite aggregates were produced during the alteration of the ash in a stagnant, lacustrine environment: (1) extremely fine-grained micro-crystalline aggregates that have honeycomb textures and that replace the original hyaloclasts, and (2) coarse crystalline aggregates that have more open honeycomb textures and that fill pores and cracks in altered hyaloclasts and in pumice vesicles. Both types of aggregates have the same chemical composition according to energy dispersive X-ray analysis.

Montmorillonite, separated as the <1-¼m size fraction from olive gray bentonite, has the following crystallochemical formula: (K0.09Na0.02Ca0.29Mg0.10) (Al2.43Fe3+1.05Fe2+0.005Mn2+0.005Mg0.50) (Si7.62Al0.38) O20(OH)4. Biotite was apparently stable during the alteration of the hyaloclasts. Anatase and possible accessory heulandite-clinoptilolite were also formed in small amounts, but were not observed by scanning electron microscopy. Goethite is the youngest oxidation product in some parts of the bentonite. Minute fragments of sodium-rich plagioclase, potassium feldspar, quartz, and muscovite are ubiquitous accessories of the original hyaloclasts. Together with kaolinite, they formed from the underlying fresh or kaolinized orthogneiss.

The first stage of lateritic weathering of pyroxenes in the Niquelandia area, Brazil, leads either to Fe-rich products or to a phyllosilicate clay. In relatively unfractured parent rock the phyllosilicate clay contains Ni-rich smectites, the atomic ratio of Ni: octahedral cations ranging from 0.3 to 0.5. These smectites were studied by polarized light microscopy, X-ray powder diffraction (XRD), transmission electron microscopy, and electron microprobe, and infrared, optical absorption, Mössbauer, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. The chemical composition of the smectite is constant on the optical microscope scale even to the smallest analyzed particles (3000 A in diameter and about 75 Å thick). From XRD data the mineral is principally a swelling, trioctahedral smectite; however, some kerolite-pimelite-like layers are present, and a weak 06,33 reflection indicates the presence of a small amount of a dioctahedral phase. Mössbauer results show that all Fe cations are Fe3+ in octahedral sites. The structural formula of the smectite is: (Ca0.01K0.05)(Al0.17Fe0.5Mg0.48Ni1.47Cr0.02)(Si3.92Al0.08)O10(OH)2

The results obtained from all the above methods suggest that in the smectites Ni, and, perhaps, a small amount of Mg are clustered in pimelite-like domains (or layers), whereas Fe and some Al are clustered in nontronite-like domains (or layers). Most selected-area electron diffraction patterns exhibit continuous or punctuated (hk) rings, indicating that particles contain several stacked layers. The patterns of some thin particles, however, suggest dioctahedral layers having trans-octahedral vacancies, such as in the Garfield, Washington, nontronite. Thus, the Ni-Fe-Mg-smectite, which seemingly is homogeneous, actually consists of mixed trioctahedral and dioctahedral layers or domains.

Multivariate analysis of variance and discriminant analysis were used to establish the crystal chemistry of several Al-rich smectites. The statistical analyses were carded out on 78 samples taken from the literature which were classified on the basis of their physicochemical properties. A strong discrimination exists between beidellites and montmorillonites, ‘non-ideal’ montmorillonites and ‘ideal’ montmorillonites, and Wyoming-type and Cheto-type montmorillonites. Of the Cheto-type montmorillonites, the Tatatilla-type samples are strongly discriminated, whereas the distinction between Chambers- and Otay-types is not strong. AlIV, AlVI, Fe, Mg, and Ca are generally important discriminating variables, whereas the tetrahedral portion of the layer charge, commonly used as a discriminating factor among these minerals, is only moderately significant.

Chemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ~0.55 per O10(OH)2 for illite layers in randomly interstratified I/S (R=0; >50% smectite layers) to ~ 1.0 per O10(OH)2 for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O10(OH)2; total charge = about -0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m2/g.

The Kimmeridgian-Volgian(-Ryazanian) clay stone is the principal oil source rock in the troughs of the North Sea. Randomly (R0) ordered mixed-layer illite/smectite (I/S) appears to have transformed to R1-(IS) or R3-(ISII) ordered simultaneously with oil generation. The proportion of illite layers in I/S increased to 95% during diagenesis in these claystones. Exceptions are three samples of I/S of probably bentonitic origin; these have apparently changed during diagenesis to R0-ordered I/S containing 40-50% illite layers. Fine fractions of the claystones dominated by I/S were analyzed by 27Al and 29Si magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray powder diffraction (XRD), and for total chemical composition. Si/Al ratios determined from MAS-NMR agree closely with those calculated from total chemical analysis; however, tetrahedral and octahedral Al occupancies were most accurately determined by NMR. An increase in the percentage of illite layers and in the ordering of the I/S was accompanied by fixation of K+ and NH4+ in the I/S, by tetrahedral Al-for-Si substitution, and by octahedral Al-for-(Mg + Fe) substitution, resulting both in an increase of charge in the 2:1 layers and in a migration of charge from octahedral to tetrahedral sheets. The I/S of probably bentonitic origin had a larger tetrahedral and a smaller octahedral charge than expected from its content of illite layers. MAS-NMR showed a significantly higher content of tetrahedral Al (most likely in smectitic sites) than expected from the percentage of illite layers calculated from XRD. Correspondingly, XRD of K+-saturated and glycolated specimens showed that several smectite layers possessed a significant charge. A constant b-dimension of the I/S and the presence of a significant charge in the smectite layers suggest that a transformation of smectite to illite layers in the I/S by tetrahedral Al-for-Si substitution followed by interlayer cation fixation and interlayer contraction is the most probable genesis for the I/S investigated.

Weathering of minerals often produces intimate mixtures of reactants and products in a smaller volume than that explored by an electron microprobe. As a result, many microanalyses are difficult to decipher because they are composite analyses of mixtures. A procedure to resolve such microanalyses into their component parts was applied here to data obtained in a study of the weathering of muscovite and biotite in granite gneiss and associated soils. A key element oxide content (K2O, in the examples demonstrated) was plotted against Si/Al ratio for all data. This achieved a spread of points having an external shape depending on the compositions of the pure phases present in the various mixtures analyzed. In all probability, some of the microanalyses consisted of only a single phase, or almost so, and these points were located at or near extremities of the body of data. The aim of subsequent treatment was to encompass the data within boundary curves, each of which corresponded to mixtures of any two of the pure phases present. The chemical compositions of some phases, for example, kaolinite and muscovite, are known, and boundary curves for such mixtures were calculated directly. The compositions of unknown or suspected phases, however, were estimated from the graph of K2O content against Si/Al ratio, in conjunction with graphs of the other element oxide contents plotted against Si/Al ratio. The unknown compositions were subsequently refined by successive approximations. Data in the vicinity of the boundary curves were adjusted for the presence of free iron oxides containing structural Al. Once a satisfactory set of intersecting boundary curves was developed to encompass the data, the compositions of the major phases in each system were obtained, and the diagram of boundary curves was used to estimate the relative proportions of these phases at each point analyzed by the microprobe.