Abundant authigenic clay minerals (illite, kaolinite, chlorite), detrital mica grains (biotite, muscovite), and phyllosilicate-bearing lithic fragments (mudstone, phyllite) have been identified in the Mississippian age Marshall and Michigan Stray Sandstones of the Michigan Basin. These phyllosilicates were identified and analyzed with optical petrography, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Many of these phyllosilicates exhibit morphologies and textural relationships indicating extensive alteration (replacement, partial dissolution, neoformation) resulting from changing pore fluid conditions. Illite replacement of precursor kaolinite and illite neoformation are observed. Illite cement formation typically occurs at high temperatures (deep burial), but occurs in these strata at depths shallower than most illite-bearing sandstones. This cement formation requires an external source of potassium. Multiple hypotheses have been proposed to account for this potassium. Here these hypotheses are evaluated to determine the cause of illite cement formation in these Mississippian sandstones. Illite formation is made thermodynamically possible by the transport and mixing of deep saline brines in the Michigan Basin with Marshall pore fluids. Illite crystallinity data indicate that authigenic illite formed at temperatures exceeding 200° C. Converging evidence suggests that hot brines from deeper in the Michigan Basin mixed with Mississippian formation waters to initiate kaolinite illitization and the neoformation of illite. These brines likely moved vertically along reactivated faults and fractures in the basin.
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