Siderite

For the type of meteorite, seeIron meteorite.

Siderite
General
Category	Carbonate mineral
Formula (repeating unit)	FeCO3
IMA symbol	Sd
Strunz classification	5.AB.05
Dana classification	14.01.01.03
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol:(32/m)
Space group	R3c
Unit cell	a= 4.6916 c= 15.3796 [Å];Z= 6
Identification
Color	Pale yellow to tan, grey, brown, green, red, black and sometimes nearly colorless
Crystal habit	Tabular crystals, often curved; botryoidal to massive
Twinning	Lamellar uncommon on{0112}
Cleavage	Perfect on {0111}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scalehardness	3.75–4.25
Luster	Vitreous, may be silky to pearly
Streak	White
Diaphaneity	Translucent to subtranslucent
Specific gravity	3.96
Optical properties	Uniaxial (−)
Refractive index	nω= 1.875 nε= 1.633
Birefringence	δ= 0.242
Dispersion	Strong
References

Sideriteis amineralcomposed ofiron(II) carbonate(FeCO₃). Its name comes from theAncient Greekwordσίδηρος(sídēros), meaning "iron". A valuableiron ore,it consists of 48%ironand lackssulfurandphosphorus.Zinc,magnesium,andmanganesecommonly substitute for the iron, resulting in the siderite-smithsonite,siderite-magnesite,and siderite-rhodochrositesolid solutionseries.

Siderite hasMohs hardnessof 3.75 to 4.25, aspecific gravityof 3.96, a whitestreakand avitreous lustreor pearlyluster.Siderite isantiferromagneticbelow itsNéel temperatureof 37 K (−236 °C) which can assist in its identification.

It crystallizes in thetrigonal crystal system,and arerhombohedralin shape, typically with curved and striated faces. It also occurs in masses. Color ranges from yellow to dark brown or black, the latter being due to the presence of manganese.

Siderite is commonly found inhydrothermalveins,and is associated withbarite,fluorite,galena,and others. It is also a commondiageneticmineral inshalesandsandstones,where it sometimes formsconcretions,which can encase three-dimensionally preservedfossils.Insedimentary rocks,siderite commonly forms at shallow burial depths and its elemental composition is often related to thedepositional environmentof the enclosing sediments.In addition, a number of recent studies have used theoxygen isotopic compositionof sphaerosiderite (a type associated withsoils) as aproxyfor theisotopiccomposition ofmeteoric watershortly after deposition.

Carbonate iron ore

Althoughcarbonateiron 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 smallore lenses,often following steeplydippingbedding planes.This makes them not amenable toopencast working,and increases the cost of working them by mining with horizontalstopes.As the individual ore bodies are small, it may also be necessary to duplicate or relocate the pit head machinery,winding engineand pumping engine, between these bodies as each is worked out. This makes mining the ore an expensive proposition compared to typicalironstoneorhaematiteopencasts.

The recovered ore also has drawbacks. The carbonate ore is more difficult tosmeltthan a haematite or other oxide ore. Driving off the carbonate as carbon dioxide requires more energy and so the ore 'kills' theblast furnaceif 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 ofSir Thomas LethbridgeinSomerset.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.

These differences between spathic ore and haematite have led to the failure of a number of mining concerns, notably theBrendon Hills Iron Ore Company.

Spathic iron ores are rich in manganese and have negligible phosphorus. This led to their one major benefit, connected with theBessemer 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.Work by the metallurgistRobert Forester Mushetshowed 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.This created a sudden demand for spiegeleisen. Although it was not available in sufficient quantity as a mineral, steelworks such as that atEbbw Valein South Wales soon learned to make it from the spathic siderite ores.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 organister,was replaced by a 'basic' liner in the newerGilchrist Thomas process.This removed the phosphorus impurities asslagproduced 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 theBrendon Hills,closed soon after.

Gallery

• 
  Siderite fromRedruth,Cornwall, England.
• 
  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.