3.5" Orange Aragonite on Scalenohedral Calcite - Mexico
This is a gorgeous specimen which features red-orange aragonite that formed after a scalenohedral calcite crystal. The rhombohedral cleavage of the calcite can be seen on what could be considered the underside of the specimen. This mineral association was collected from Chihuahua, Mexico.
Aragonite is a carbonate mineral with the chemical composition of CaCO3. It is a pseudomorph after calcite and varies in colors depending on impurities. It displays a translucent to white color when pure, and when impure can vary between yellow, green, pink, blue and brown. Typically forming in low temperature hydrothermal veins, It is also known to form in hot springs, as precipitates from chemicals in sedimentary rock, and in some cases as crystals in fossilized sea creatures.
Calcite, CaCO3, is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.