Siderite

Lore & Background

Siderite is commonly found in hydrothermal veins, associated with barite, fluorite, and galena, and also occurs as a diagenetic mineral in shales and sandstones, where it sometimes forms concretions that can encase three-dimensionally preserved fossils. In sedimentary rocks, siderite forms at shallow burial depths, and its elemental composition often relates to the depositional environment. Recent studies have used the oxygen isotopic composition of sphaerosiderite as a proxy for meteoric water shortly after deposition, and evidence of siderite on Mars is interpreted as a possible indicator of abundant water early in that planet's climate history.

Reader's Guide

Although carbonate iron ores such as siderite have been economically important for steel production, they are far from ideal as an ore. Their hydrothermal mineralization tends to form small ore lenses, often following steeply dipping bedding planes, making them unsuitable for opencast working and increasing mining costs. The carbonate ore is more difficult to smelt than haematite or other oxide ores, requiring a preliminary roasting step to drive off carbon dioxide. Siderite's one major benefit came from its negligible phosphorus content, which made it valuable for producing spiegeleisen, a ferromanganese ore used in the Bessemer steel-making process to remove phosphorus. However, after the development of the Gilchrist Thomas process with a basic liner, which removed phosphorus as slag, demand for spathic ores fell, leading to the closure of many mines, including those of the Brendon Hills.

Did You Know?

• It consists of 48% iron and lacks sulfur and phosphorus.
• Siderite can form concretions that encase three-dimensionally preserved fossils.
• Evidence of siderite on Mars is interpreted as a possible indicator of abundant water early in that planet's climate history.