sideriitin

Siderite, known in Finnish as sideriitti, is a mineral with the chemical formula FeCO3. It is a carbonate mineral that crystallizes in the rhombohedral crystal system and commonly forms rhombohedral crystals, though it more often occurs as massive nodules in sedimentary rocks. The name derives from the Greek sideros meaning iron.

Appearance and properties: Colors range from colorless to pale yellow-brown; crystals can be brownish owing to iron oxidation. It has a vitreous to resinous luster, a Mohs hardness of 4, and a specific gravity around 3.9–4.0. It displays perfect rhombohedral cleavage and effervesces with cold dilute acids due to its carbonate content.

Occurrence and formation: Siderite forms in reducing, carbonate-rich environments, commonly in sedimentary rocks and iron ore deposits. It occurs as nodules or disseminations in coal measures, shales, and limestones, and is often associated with calcite, dolomite, quartz, goethite, magnetite, and pyrite. It can form diagenetically from iron-bearing fluids.

Economic significance: Siderite can be a source of iron in certain ore deposits, with iron content about 48% by weight. It is not as widely mined as some other iron ores, but has contributed to iron production where it forms high-grade deposits.

Weathering and alteration: Under surface conditions, siderite weathers to iron oxides such as goethite or limonite, and can transform to iron oxides.

---