SIDERITE
Siderite is an iron carbonate mineral - FeCO3.
It is moderately common in hydrothermal veins, some concretions, and in some
Precambrian-aged banded iron formations (BIFs). Siderite is usually light
brown to dark brown to reddish brown in color. It tends to form
distinctive rhombohedral crystals with curved crystal faces (see first pic
below; dolomite crystals do this as well). Siderite also has three
cleavage planes not at right angles (rhombohedral cleavage). Thanks to
its iron content, siderite is moderately heavy for its size.
Siderite
(field of view ~2.9 cm across) from the Vekol Mine near Casa Grande, Arizona,
USA.
Siderite
(3.4 cm across)
Siderite
from Dalnegorsk, Maritime Province, eastern Russia.
(Wayne State University collection, Detroit, Michigan,
USA)