2.5" Calcite Encrusted Fibrous Aurichalcite Crystals - Mexico
This colorful specimen contains blue-green, fibrous aurichalcite crystal clusters that are encrusted in a layer of calcite. This beautiful specimen was collected from the Ojuela Mine in Durango, Mexico.
Aurichalcite is a secondary mineral that forms in the oxidation zones of copper and zinc ore deposits. It has the chemical formula (Zn,Cu)5(CO3)2(OH)6 and often forms as radiating acicular blue-green crystals and linings along cavity walls.
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.