Bannisterite

Coarse-grained, intergrown, platy masses of dark brown bannisterite, nearly free of other minerals except for local, almost microscopic, patchy films of a gray mineral, probably calcite.  This specimen of bannisterite, unlike most others, is color-mottled in shades of medium to dark brown, and the cleavage faces have a subvitreous to locally dull luster, possibly due to alteration.  This is specimen FMM-1132 of the Franklin Mineral Museum and came to that institution through donation of the SPEX/Gerstmann collection.

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