siderite

1. An ore of iron, FeCO₃.
2. A meteorite consisting mainly of iron and nickel.

For more information on siderite, visit Britannica.com.

A mineral (FeCO₃) with the same space group and hexagonal crystal system as calcite (CaCO₃). Siderite has a gray, tan, brown, dark brown, or red color, has rhombohedral cleavages, and occasionally may show rhombohedral crystal terminations. It may display curved crystal faces like dolomite (CaMg[CO₃]₂), but more commonly is found as massive, compact, or earthy masses. It has a high specific gravity of 3.94, a medium hardness of 3.5–4, and a high index of refraction, 1.88. See also Carbonate minerals; Crystal structure; Dolomite.

Siderite, a widespread mineral in near-surface sediments and ore deposits, occurs in hydrothermal veins, lead-silver ore deposits, sedimentary concretions formed in limestones and sandstones, and Precambrian banded iron formations that precipitated under acidic conditions. Famous localities for siderite are found in Styria (Austria), Westphalia (Germany), Cornwall (Britain), Wawa (Northern Ontario, Canada), Minas Gerais (Brazil), and Llallagua and Potosi (Bolivia). These and other occurrences have provided locally significant quantities of siderite as iron ore. See also Iron; Magnetite; Metamorphism; Ore and mineral deposits.

An old name for a loadstone or magnet. The term has also been variously used to indicate a steel-colored stone (possibly sapphire), a blue-colored quartz, carbonate of iron, and meteorites containing iron.

Environment

Rarely forms metamorphosed sedimentary strata, commonly in crystals in ore veins, sometimes in pegmatites.

Most commonly in rhombohedrons, often very acute, sometimes in scalenohedrons. Massive, in granular, crystalline cleavable aggregates, and earthy. Sometimes forms fibrous radial knobs (sphaerosiderite) or saddle-shaped rhombohedron crusts like dolomite.

Brown, white to gray. Luster vitreous to pearly; hardness 3Ɖ-4; specific gravity 3.8-3.9; fracture conchoidal; cleavage rhombohedral. Brittle; transparent to translucent.

Iron carbonate (62.1% FeO, 37.9% CO ₂ ), usually with some magnesium and calcium replacing part of the iron.

Fragments become magnetic after being heated on charcoal. Dissolves in hot acid with effervescence.

Ease of scratching and the cleavage show it to be a carbonate; usually the brown color, which is often only a thin surface layer, suggests iron carbonate. Magnetism following heating is then sufficient.

Very common in low- and medium-temperature ore veins, in which it is often associated with calcite, barite, and the sulfides. Also characteristic of sedimentary rocks, where it frequently forms concretionary masses ("clay ironstone"). Sometimes used as an ore of iron (France and Germany). Also in pegmatites associated with phosphates.

Some of the best, and usually very dark, crystals came from the Cornwall (England) mines, where they show quite a variety of forms. Allevard, France, and Erzberg, Styria (Austria), are notable as a source of fine clusters of crystals. Panasqueira, Portugal, and the Morro Velho mine at Nova Lima, Minas Gerais, Brazil, are prominent sources of specimen crystals. Large brown embedded cleavages were associated with the Greenland cryolite.

There are no truly notable American examples. Found at many localities in the U.S., including some old mines at Roxbury, Connecticut (where it forms great cleavable masses and free crystals, often altered to limonite), in Vermont, in New York State, in New England pegmatites, and in good crystals in Colorado in ore veins. As a rule, siderite crystals are not large, though very large (if dull) crystals have lately been found at St. Hilaire, near Montreal, Quebec.

It is not surprising that siderite, since it is easily altered, is commonly changed to limonite pseudomorphs that preserve the original shape of the crystal.

Siderite is also the name of a type of iron meteorite.

Siderite
Siderite from Poland
General
Category	Carbonate mineral
Chemical formula	FeCO3
Strunz classification	05.AB.05
Dana classification	14.01.01.03
Identification
Color	Pale yellow to tannish, grey, brown, green, red, black and sometimes nearly colorless
Crystal habit	Tabular crystals, often curved - botryoidal to massive
Crystal system	Trigonal - Hexagonal scalenohedral (3 2/m)
Twinning	Lamellar uncommon on{0112}
Cleavage	Perfect on on{0111}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	3.75 - 4.25
Luster	Vitreous, may be silky to pearly
Streak	White
Diaphaneity	Translucent to subtranslucent
Specific gravity	3.5
Optical properties	Uniaxial (-)
Refractive index	nω = 1.875 nε = 1.633
Birefringence	δ = 0.242
Dispersion	Strong
References	[1][2][3]

Siderite is a mineral composed of iron carbonate FeCO₃. It takes its name from the Greek word sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Both magnesium and manganese commonly substitute for the iron.

Siderite has Mohs hardness of 3.5-4, a specific gravity of 3.8, a white streak and a vitreous or pearly luster.

Its crystals belong to the hexagonal system, and are rhombohedral in 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 (sometimes called manganosiderite).

Siderite is commonly found in hydrothermal veins, and is associated with barite, fluorite, galena, and others. It is also a common diagenetic mineral in shales and sandstones, where it sometimes forms concretions. In sedimentary rocks, siderite commonly forms at shallow burial depths and its elemental composition is often related to the depositional environment of the enclosing sediments.^[4] In addition, a number of recent studies have used the oxygen isotopic composition of sphaerosiderite (a type associated with soils) as a proxy for the isotopic composition of meteoric water shortly after deposition.^[5]

References

Siderite

1. ^ http://rruff.geo.arizona.edu/doclib/hom/siderite.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-3647.html Mindat
3. ^ http://www.webmineral.com/data/Siderite.shtml Webmineral data
4. ^ *Mozley, P.S., 1989, Relation between depositional environment and the elemental composition of early diagenetic siderite: Geology, v. 17, p. 704- 706
5. ^ *Ludvigson, G.A., Gonzalez, L.A. Metzger, R.A., Witzke, B.J., Brenner, R.L., Murillo, A.P.and White, T.S., 1998, Meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology: Geology, v. 26, p. 1039-1042

• The Complete Book of Science, American Education Publishing, Columbus, Ohio 2005

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