Abstract--Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 ~, to 2 ~tm) matrix of a Silurian lower anchizonal mudrock from central Wales: one parallel to bedding and one parallel to cleavage that is approximately 30*-50 ~ to bedding. Bedding-parallel micas consist of small (50-200 ~ thick) deformed packets (IMd polytype) and larger (100 ~-2 t~m) strain-free grains (2M~ polytype). All strained micas and some strain-free grains have compositions varying from Mus~Pg~4 to The data suggest that bedding-parallel metastable micas with disordered interlayer K and Na were initially derived from alteration of smectite during burial diagenesis. They subsequently underwent dissolution, with crystallization of more evolved bedding-parallel micas during deep burial. Discrete grains of stable muscovite and paragonite then crystallized in the cleavage orientation through tectonic stressinduced dissolution of bedding-parallel matrix micas. Combined XRD and TEM/AEM data further show that the so-called 6:4 ordered mixed-layer paragonite/muscovite actually corresponds to cation-disordered, homogeneous mica of intermediate composition.

Chlorite-mica stacks (grains of intergrown chlorite and white mica) in a matrix of fine-grained white mica and chlorite have been studied using XRD, SEM, EMPA, TEM and AEM methods. The stacks occur in a weakly cleaved Llandoverian mudstone, central Wales, which has a white mica (illite) crystallinity index of 0.35 ° A20 corresponding to the lower anchizone. White mica occurs as packets (100/~ to 8 #m thick) interleaved with dominant chlorite packets in stacks, with both apparent coherent interfaces or cross-cutting small angle boundaries with chlorite layers. It is well-crystallized 2M~ polytype with phengitic composition and low paragonite component [Na/(Na+K)-< 0-0.07]. Chlorite in stacks is Fe-rich and relatively homogeneous. TiO2 crystals surround stacks and occur within chlorite, and white mica is Ti-rich compared to matrix white mica. Fine-grained white mica and chlorite in the matrix have two different orientations: one parallel to bedding and one parallel to cleavage, which is approximately 30-50 ° to bedding. Matrix white mica is predominantly a 2M1 polytype, but some cleavage-parallel white mica is 3 T and some bedding-parallel white mica is 1Md. It is Na-rich [Na/(Na+ K)-0.14-0.42] and relatively heterogeneous; some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation. Matrix white mica and chlorite each contain less Fe than corresponding white mica and chlorite in stacks. Both matrix and stack chlorite are one-layer polytypes. The data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite and other ferromagnesian minerals (probably via intermediate expandable trioctahedral phyllosilicates). Most intergrowths of chlorite and mica in stacks formed by mica replacement of chlorite and altered biotite along cleavage fissures. Subsequent deformation caused further modification of pre-existing chlorite-mica stacks whereas partial dissolution of stacks and bedding-parallel matrix phyllosilicates resulted in the formation of cleavage-parallel phyllosilicates.

Abstract--Smectite and mixed-layer illite/smectite (US) in Triassic heulandite-rich bentonite from Kaka Point, New Zealand, have been investigated by scanning electron microscopy (SEM), transmission electron microscopy/analytical electron microscopy (TEM/AEM) and X-ray diffraction (XRD) for comparison with matrix phyllosilicates in closely associated siltstones and analcimized tuff. Samples that were treated to achieve permanent expansion showed that some smectite in bentonite occurs as curved packets of wavy 10.5-to 13-A layers enveloping relict glass shards, the centers of which consist of an amorphous clay precursor. The dominant clay minerals in bentonite are smectite-rich randomly disordered (R0) I/S with variable proportions of 10-A illite-like interlayers, only locally organized as 1:1 ordered (R1) US. R0 US was also observed in separate packets retaining the detailed texture of packets that replaced shards. Such relations are consistent with a "solid-state"-like, layer-by-layer replacement of original smectite layers by illite-like layers with partial preservation of the primary smectite texture, in contrast to textures observed elsewhere, such as in Gulf Coast mudstones. The smectite, as in other examples in marine sediments, has K as the dominant interlayer cation, suggesting that precursor smectite may be a major K source for reaction to form illite.Only a small proportion of illite (35%) occurs in mixed-layer smectite-rich US in bentonite and the dominant trioctahedral phyllosilicate is disordered high-Fe berthierine, implying that little mineralogical change has occurred with burial. This contrasts with observations of closely associated siltstones and analcimized tuff, which contain well-defined packets of illite and chlorite but which have no detectable matrix smectite component. These data imply that the rate of transformation of smectite to illite is much slower in bentonites than in associated sediments of the same buriai depth and age. Such relations emphasize the significance of factors other than temperature, (e.g., organic acids, permeability and pore fluid compositions) in affecting the rate and degree (and perhaps mechanism) of transformation of smectite to illite.

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Abstract--The Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the stratiform copper deposit at White Pine, Michigan, consists of 200 m of principally dark grey clastic sediments which contain detritus obtained dominantly from underlying mafic to intermediate volcanic rocks. Clay minerals from samples collected from the mine area and drill holes up to 100 km away have been studied using SEM, EMPA, TEM and AEM. Two morphologies of phyUosilicates, both including white mica and chlorite, occur in the 'lower' Nonesuch Formation: (1) detrital-shaped and (2) matrix. D~trital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as packets typically <200 .~ thick and as pore-filling cement.TEM images of detrital-shaped chlorite generally display 14-A periodicity, although 24-A corrensitelike units occur locally. Most dgtdtal-shaped chlorite from the mine area samples have a relatively restricted range of Fe/(Mg+Fe) ratios from 0.52 to 0.58, but the Fe/(Fe+Mg) ratios of detrital-shaped chlorite outside the mine area range from 0.27 to 0.64. TiO2 crystals occur within and surrounding the detrital-shaped chlorite. Matrix chlorite has Fe/(Fe+Mg) ratios of 0.47 to 0.63, indicating that it is relatively homogeneous and enriched in Fe compat~ to detrital-shaped chlorite.Detrital-shaped white mica occurs as a 2M~ polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a 1Ma polytype, is K and AI deficient relative to end-member muscovite and contains significant Fe and Mg.The data are consistent with homogenization of detrital-shaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineralization. The TiO2 crystals and corrensite-like units in detritalshaped chlorite imply that it is at least in part derived from alteration of biotite. The presence of immature 1Ma illite-rich I/S and a one layer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consistent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sedimentation and ore deposition occurred prior to 1 Ga.