Siderite
“Iron carbonate that rusts from within”
Siderite is an iron carbonate mineral that forms in reducing (low-oxygen) environments — swamps, bogs, hydrothermal veins, and deep marine sediments. When exposed to air, it slowly weathers to form a coating of iron oxides (rust), transforming from its original pale yellow-brown to dark brown limonite.
iron carbonate (FeCO₃)
Cornwall (England), Erzberg (Austria — massive deposit), Brosso (Italy), Minas Gerais (Brazil), Panasqueira (Portugal), Roxbury (Connecticut, USA)
3.5-4.5
Austria's Erzberg ('Ore Mountain') has been mined for siderite iron ore for over 1,300 years and is still actively mined today — its massive open-pit terraces carved into the mountainside resemble a giant staircase visible from space.
Test Your Knowledge
What does the formation of siderite (iron carbonate) instead of hematite or goethite (iron oxides) indicate about the depositional environment?
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