calcite - teaching hand specimen of unusual naturally-brown calcite
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 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 it's Mohs scale hardness of 3, which means you can't scratch it with a fingernail, but you can scratch it with a penny to confirm the identification.
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