Siderite

Siderite is a mineral composed of iron(II) carbonate (FeCO₃). Its name comes from the Ancient Greek word σίδηρος (sídēros), meaning "iron". A valuable iron ore, it consists of 48% iron and lacks sulfur and phosphorus. Zinc, magnesium, and manganese commonly substitute for the iron, resulting in the siderite-smithsonite, siderite-magnesite, and siderite-rhodochrosite solid solution series.^[3]

Siderite has

Although carbonate iron ores, such as siderite, have been economically important for steel production, they are far from ideal as an ore.

Their hydrothermal mineralisation tends to form them as small

The recovered ore also has drawbacks. The carbonate ore is more difficult to smelt than a haematite or other oxide ore. Driving off the carbonate as carbon dioxide requires more energy and so the ore 'kills' the blast furnace if added directly. Instead the ore must be given a preliminary roasting step. Developments of specific techniques to deal with these ores began in the early 19th century, largely with the work of Sir Thomas Lethbridge in Somerset.^[12] His 'Iron Mill' of 1838 used a three-chambered concentric roasting furnace, before passing the ore to a separate reducing furnace for smelting. Details of this mill were the invention of Charles Sanderson, a steel maker of Sheffield, who held the patent for it.^[13]

These differences between spathic ore and haematite have led to the failure of a number of mining concerns, notably the

Spathic iron ores are rich in manganese and have negligible phosphorus. This led to their one major benefit, connected with the Bessemer steel-making process. Although the first demonstrations by Bessemer in 1856 were successful, others' initial attempts to replicate his method infamously failed to produce good steel.^[15] Work by the metallurgist Robert Forester Mushet showed that the reason for the discrepancy was the nature of the Swedish ores that Bessemer had innocently used; they were very low in phosphorus. Using a typical European high-phosphorus ore in Bessemer's converter gave a poor quality steel. To produce high quality steel from a high-phosphorus ore, Mushet realised that he could operate the Bessemer converter for longer, burning off all the steel's impurities including the unwanted phosphorus but also the carbon (which is an essential ingredient in steel), and then re-adding carbon, along with manganese, in the form of a previously obscure ferromanganese ore with no phosphorus, spiegeleisen.^[15] This created a sudden demand for spiegeleisen. Although it was not available in sufficient quantity as a mineral, steelworks such as that at Ebbw Vale in South Wales soon learned to make it from the spathic siderite ores.^[16] For a few decades, spathic ores were therefore in demand and this encouraged their mining. In time though, the original 'acidic' liner of the Bessemer converter, made from siliceous sandstone or ganister, was replaced by a 'basic' liner in the newer Gilchrist Thomas process. This removed the phosphorus impurities as slag produced by chemical reaction with the liner, and no longer required spiegeleisen. From the 1880s demand for the ores fell once again and many of their mines, including those of the Brendon Hills, closed soon after.

Gallery

• 
  Siderite from Redruth, Cornwall, England.
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  Siderite crystals with galena and quartz. Size: 6.2 cm × 4.1 cm × 3.6 cm (2.4 in × 1.6 in × 1.4 in).
• 
  Disc-shaped, brown siderite crystals perched upon chalcopyrites.
• 
  Cut siderite from Minas Gerais, Brazil. Size: 5 mm × 3.2 mm (0.20 in × 0.13 in).
• 
  Colorado siderite, with sharp blades of olive-brown and minor accenting quartz.
• 
  Fossiliferous siderite concretion from the Lower Carboniferous.

Notes

References

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