Gypsum

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A good example of fibrous compact gypsum, this is known as satin spar.The Isabella Quarry, located in Delta County, Michigan near Isabella and Nahma Junction, is an old local quarry site whose precise opening and closing dates are not historically documented. Geologically, the site is situated within the Paleozoic sedimentary rock formations of Michigan's Upper Peninsula, where it has been noted for localized occurrences of gypsum varieties like satin spar and selenite.Satin spar is a fibrous variety of gypsum distinguished by its silky, pearly luster and parallel, needle-like crystal structure, which produces a shimmering chatoyant effect when light reflects off its tightly packed fibers.