Formula: FeCO3

Colour: Yellowish-brown to greyish-brown, pale yellow to tannish, grey, brown, green, red, black and, rarely, colourless; tarnished iridescent at times; colourless to yellow and yellow-brown in transmitted light.

Lustre: Vitreous, Silky, Pearly

Hardness: 3½ – 4½

Specific Gravity: 3.96

Crystal System: Trigonal

Member of: Calcite Group, Magnesite-Siderite Series, Rhodochrosite-Siderite Series, Siderite-Smithsonite Series.

Name: Named in 1845 by Wilhelm Karl von Haidinger from the Greek “σίδηρος” (sideros), “iron”, in allusion to its composition.

Crystals typically found as brown to tan rhombohedrons in clusters, faces often curved or composites; more often found as medium to dark brown massive fine-grained material or as massive crystalline material with exposed curved cleavage surfaces. Fungi like Lichenothelia may oxidize iron to produce siderite, which is then a biomineral.