Siderite, in its crystallized form, is a common vein mineral associated with various metallic ores containing silver minerals, pyrite, chalcopyrite, tetrahedrite, and galena. Siderite is also a common constituent of banded Precambrian iron deposits, as in the Lake Superior region. Uses - A minor ore of iron. Important in Great Britain, but unimportant elsewhere. It is more often massive to granular.
Rhodocrosite crystals are some of the most highly sought after specimens in the world.
Occurrence - Rhodocrosite is a relatively rare mineral occurring in hydrothermal silver, lead and copper veins. Its rarity suggests the rather unusual conditions necessary for formation, high P CO2 and Mn concentrations. Uses - none, except as a mineral specimen. This is the aragonite structure. CaCO 3 occurs in both the calcite and aragonite structure types.