SIDERITE-RHODOCHROSITE with CALCITE, CARNIC, RUMANIA (small cabinet)

Siderite is a mineral made up of iron(II) carbonate (FeCO3), deriving its name from the Ancient Greek term for iron, σίδηρος (sídēros). This mineral is an important iron ore, containing approximately 48% iron while being free of sulfur and phosphorus. Iron can be substituted by elements such as zinc, magnesium, and manganese, leading to the formation of various solid solution series, including siderite-smithsonite. siderite-magnesite and siderite-rhodochrosite. With a Mohs hardness ranging from 3.75 to 4.25 and a specific gravity of 3.96, siderite exhibits a white streak and a vitreous to pearly luster. It becomes antiferromagnetic below its Néel temperature of 37 K (−236 °C), which aids in its identification. Siderite crystallizes in the trigonal system, typically forming rhombohedral crystals with curved and striated faces, and can also appear in massive forms. Its color varies from yellow to dark brown or black, with the darker hues often attributed to manganese content. Commonly found in hydrothermal veins, siderite is associated with minerals such as barite, fluorite, and galena. It frequently occurs as a diagenetic mineral in shales and sandstones, where it can form concretions that may encase well-preserved fossils. In sedimentary environments, siderite typically forms at shallow burial depths, and its elemental makeup often reflects the conditions of the surrounding sediments. Recent research has also utilized the oxygen isotopic composition of sphaerosiderite, a soil-associated variant, as an indicator of the isotopic composition of meteoric water shortly after its deposition.