Textural, chemical, and mineralogical analyses of authigenic clays in anthracite-rank coal and associated shale from eastern Pennsylvania have allowed a better understanding of the parameters controlling diagenesis and perhaps coalification in this region. Minerals in anthracite occur in distinct assemblages associated with the following microenvironments: coal matrix, two orthogonal joint sets (termed systematic and nonsystematic cleat), and a third joint set. Mineralogical differences among microenvironments allow inferences about clay-mineral origins and the parameters that controlled mineral authigenesis. Kaolinite occurs in the shale, and all microenvironments within the coal seam. Authigenic minerals that replaced preexisting kaolinite during the latest stage of coalification (anthra-citization) at T > 200 °C include NH4-rich illite, pyrophyllite, and the following minerals, which are primarily restricted to the systematic cleat set: tosudite (R1-ordered, dioctahe-dral, mixed-layer chlorite/smectite), sudoite (di, trioctahedral chlorite) and rectorite (Rl-ordered, mixed-layer paragonite/smectite). Alteration of smectite to illite may be responsible for formation of authigenic illite and Na-bearing illite, which are present only in the shale and coal matrix, and Fe-, Al-rich chlorite and quartz in the third joint set. The chemical components for authigenesis appear to have multiple sources: A1 and Si from preexisting kaolinite and quartz, N from local organic matter, Mg and Na (for tosudite, sudoite, and rectorite) largely from metasomatic hydrothermal fluids, Fe and Mg for Fe-, Al-rich chlorite from smectite illitization.

Minerals in the shale, the coal matrix, and the nonsystematic cleat set are interpreted to represent authigenesis in a low-permeability environment (closed-system alteration); however, the assemblage sudoite + tosudite + rectorite in the systematic cleat set is interpreted to be the result of one or two stages of hydrothermal alteration (open-system alteration). We suggest that differences in minerals between the two nearly perpendicular cleat sets are the result of permeability differences which were maintained by an anisotropic lateral stress field created by plate convergence during the Alleghanian orogeny. Hydrothermal alteration may be related to large-scale basinal flow induced by Alleghanian-age uplift; such migrating fluids could also have transported heat from depth and thereby significantly increased the rate and rank of coalification in this region.

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