Siderite

Siderite is a mineral composed of iron(II) carbonate (FeCO3), with a name derived from the Ancient Greek word for iron. It is a valuable iron ore, consisting of 48% iron and lacking sulfur and phosphorus, though zinc, magnesium, and manganese commonly substitute for the iron. Siderite crystallizes in the trigonal crystal system, typically forming rhombohedral crystals with curved and striated faces, and occurs in colors ranging from yellow to dark brown or black, the darker shades often indicating manganese content. It has a Mohs hardness of 3.75 to 4.25, a specific gravity of 3.96, a white streak, and a vitreous or pearly luster. Its antiferromagnetic properties below its Néel temperature can aid in identification.

Siderite is commonly found in hydrothermal veins alongside barite, fluorite, and galena, and is also a frequent diagenetic mineral in shales and sandstones, where it can form concretions that sometimes preserve three-dimensional fossils. In sedimentary rocks, it typically forms at shallow burial depths, with its elemental composition reflecting the depositional environment. Recent studies have used the oxygen isotopic composition of sphaerosiderite, a soil-associated type, as a proxy for ancient meteoric water. Evidence of siderite on Mars suggests abundant water early in that planet’s history.

As a carbonate iron ore, siderite has been economically important for steel production but is far from ideal. Its hydrothermal mineralization creates small ore lenses along steep bedding planes, making opencast mining impractical and requiring expensive underground stoping. The ore is also harder to smelt than oxide ores, as the carbonate must be driven off as carbon dioxide in a preliminary roasting step to avoid killing a blast furnace. Early nineteenth-century developments by Sir Thomas Lethbridge and Charles Sanderson addressed this with a three-chambered roasting furnace. Despite these challenges, siderite’s richness in manganese and negligible phosphorus gave it a brief advantage during the early Bessemer steel process, when spiegeleisen—a ferromanganese ore—was needed to counteract phosphorus impurities. This demand encouraged mining, but the advent of the Gilchrist Thomas process, which used a basic liner to remove phosphorus as slag, ended that need, leading to the closure of many siderite mines by the 1880s.