The Antrim Shale (Devonian, Michigan basin) contains a large volume of authigenic quartz. The shale contains approximately 50% quartz by weight of which, in the >500 mesh-size fraction, 56% is polycrystalline. This is approximately twice the amount of quartz in most shales and 10 times the amount of polycrystalline quartz in the silt-size fraction of sandstones and shales. Scanning electron microscopy reveals an authigenic surface composed of hexagonal tabular plates which coalesce to form smooth grain surfaces. These plates have not been previously reported on quartz grains. Oxygen isotopes of quartz and carbonate phases are interpreted to indicate a gradual isotopic lightening of the pore fluids, from approximately -4 to -9 or -10 ppm. Most of the authigenic quartz h s a ^dgr¹⁸O ^approx 22 ppm (SMOW).

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