This calcite is recrystallized from a limestone in contact with a granitic intrusion. The brown color is unusual and is derived from iron in the limestone that was released by the metamorphism. It effervesces in dilute hydrochloric acid (muriatic acid is the same) as does all calcite.
The crystals of this calcite are interlocking, so it does not cleave into large rhombs, though individual specimens show many cleavage faces in several directions. The blue, green and variously wild-colors of the calcite you see being sold at rock shows and shops are an artificial dye. The brown in this calcite is entirely natural.
Brown calcite is an interesting challenge to students who have been introduced to common white or light gray calcite. The rhombohedral cleavage is less important than the effervescence in dilute hydrochloric acid, and its Mohs scale hardness of 3 - you can't scratch it with a fingernail, but you can scratch it with a penny to confirm the identification.
Collected at the West End Chemical Company limestone quarry in the Slate Range in Inyo County at the north end of Searles Valley. The dolomitic limestone was burned to produce carbon dioxide for the carbonation process at the company's plant on Searles Lake, San Bernardino County, California. The quarry is now inactive. The plant produced various salts from brines pumped from below the surface of Searles Lake.
USDA Farm Service Agency image of brine concentration ponds on Searles Lake in 2009. Click to select, click again to enlarge.
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