Calcite (bi-pyramidal)

Here we have an historic specimen with a very rare twin form. The piece comes with a Mackay School of Mines label indicating Samuel Brady collection. It also passed through the Martin Jensen collection before landing with Kenny Gay. A small handwritten label provides more detail, and notes it was mined prior to 1923.  The twinning appears to be bi-pyramidal, which would be a first for me in a Michigan calcite.

Locality: Michigan MIne, Rockland, Michigan

Composition: calcium carbonate