Echinoderm fragment (large dark area with mottled interior like a fingerprint) with cement overgrowths (irregular extensions from the central area). Echinoderm ossicles are single crystals of calcite and they tend to be overgrown with cement in optical continuity with the original crystal. This object is photographed with crossed polarized light, and the specimen is oriented so the light canceled out the colors that would otherwise be observed. Thin-section photomicrograph, Mississippian, North Alabama, field of view 1 mm wide.
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