It is known from both Franklin and Sterling Hill, but is an uncommon mineral.
Crystals are rhombohedral, mostly curved, and commonly tarnished.
It is light to dark brown, varying substantially within a given specimen, and easily oxidized. Composition Siderite is an iron carbonate mineral. The paucity of analytical data preclude even general statements about the range of solid solution among the rhombohedral carbonates at Franklin and Sterling Hill. Occurrence and paragenesis Siderite occurs in seams and vugs, sometimes forming masses, and is associated with calcite, pyrite, galena, and other common minerals. At Sterling Hill siderite occurs in vuggy areas between hematite and other carbonates.