2" Siderite and Calcite Association - Colorado

This specimen contains spherical aggregates of red-brown siderite, formed in association with calcite crystals. This specimen was collected from Septarian Ridge in Montrose County, Colorado.

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.
Siderite & Calcite
Septarian Ridge, Montrose County, Colorado
2" wide, 1.3" thick