What is Siderite?
Siderite is an iron carbonate with brown to honey-coloured rhombohedra, an important iron ore and an attractive collector mineral.
Mineralogy and crystal structure
Reference formula: FeCO3. Siderite is trigonal and belongs to the calcite group. Iron-rich colour and alteration can produce brown or rusty surfaces, while manganese and magnesium substitutions may shift the appearance. Siderite is classified with carbonates and has a trigonal crystal system. Typical hardness is 3.5–4.5 on the Mohs scale and specific gravity is approximately 3.8–4.0.
Colour, lustre and habit
Typical colours include brown, yellow-brown, grey, green, white. The usual rhombohedral, spheroidal, granular habit and vitreous, pearly, resinous lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Siderite forms in or is associated with hydrothermal veins, sedimentary iron formations, coal measures and carbonate replacement deposits. Important documented localities include Brazil, the United Kingdom, Germany, Austria, the United States and China.
Honey-brown rhombohedra with ore history
Siderite bridges industrial geology and collecting: compact ore records iron formation, while sharp honey-coloured crystals can be highly decorative.
Collecting and care
Avoid acids and hard impact. Clean gently with mild water and preserve fresh crystal faces and matrix.
