The mineral siderite

Chemical Formula

FeCO₃

Composition

Iron carbonate, usually containing some magnesium and calcium, sometimes magnesium, zinc, and cobalt. Combinations of all these elements do not exist; only particular combinations can exist. Nevertheless, the variable elements are all listed together in the "Variable Formula" field to avoid confusion. (See the Brownspar Group for more information.)

Variable Formula

(Fe,Mg,Ca,Mn,Zn,Co)CO₃

Color

White, light yellow, green, yellowish-brown, light to dark brown, reddish-brown, gray

Streak

White

Hardness

3½ - 4

Crystal Forms
and Aggregates

(Hexagonal) Occurs as rhombohedral crystals, usually with curved, saddle-like faces, or rounded faces. Seldom occurs in scalenohedral crystals. Most commonly in coxcomb and platy aggregates of curved crystals. Also massive, botryoidal, mammilary, stalactitic, oolitic, grainy, radial, fibrous, nodular, concretionary, and in rounded balls.

Transparency

Rarely transparent; usually translucent to nearly opaque

Specific Gravity

3.7 - 3.9

Luster

Vitreous, pearly

Cleavage

1,3 - rhombohedral

Fracture

Conchoidal to even

Tenacity

Brittle

Other Names

Chalybite

Varieties

Oligonite or Oligon Spar - Manganese rich variety of Siderite, containing a greater amount of iron over manganese. Formula = (Fe,Mn)CO₃. If the amount of manganese supercedes the iron, the mineral is now Mangansiderite, which is an iron rich variety of Rhodochrosite. Formula = (Mn,Fe)CO₃.

Sideroplesite or Magniosiderite - Magnesium rich variety of Siderite, containing a greater amount of iron over magnesium. Formula = (Fe,Mg)CO₃. If the amount of magnesium exceeds the iron, the mineral is now Breunnerite, which is an iron rich variety of Magnesite. Formula = (Mg,Fe)CO₃.}

Sphaerosiderite - Spherulitic variety of Siderite, forming rounded masses from radiating, fibrous crystals.

Clay Ironstone - Concretionary variety of Siderite.

In Group

Carbonates ; Calcite group

All About

Siderite belongs to the calcite group, a group of related carbonates that are isomorphous with one another. They are similar in many physical properties, and may partially or fully replace one another, forming a partial solid solution series. All members of the calcite group crystallize in the trigonal subdivision of the hexagonal system (as rhombohedrons and scalenohedrons), have perfect rhombohedral cleavage, and exhibit a strong double refraction in transparent rhombohedrons.

Siderite is easily altered to iron oxides. A brown Goethite replacement of Siderite is not uncommon. Limonite, an amorphous mineral, is commonly found in rhombohedral crystals, as it frequently pseudomorphs after Siderite.

Concretionary Siderite nodules are noted for containing a wide variety of flora and fauna fossils, especially in the open-pit coal mining operations of eastern Illinois and western Indiana, where these concretions are common in the overburden removed before the coal veins are exposed.

Uses

Siderite is an ore of iron in some countries.

Striking Features

Color and crystal forms, cleavage, and white streak

Complex Tests

1) Becomes attracted to magnetic fields when heated
2) Effervesces in hot hydrochloric acid

Popularity (1-4)

Prevalence (1-3)

Demand (1-3)

Distinguishing
Similar Minerals

Dolomite - lighter in weight and does not become attracted to magnetic fields when heated (otherwise very difficult to distinguish)
Calcite - lighter in weight
Sphalerite - different cleavage, doesn't effervesce in hydrochloric acid, does not become magnetic when heated

Commonly
Occurs With

Barite, Calcite, Chalcopyrite, Limonite, Quartz, Chalcedony, Rhodochrosite, Sphalerite, Galena, Stibnite, Fluorite, Cryolite

Noteworthy
Localities

Siderite is a common mineral and fine specimens have been obtained from many localities. Fantastic specimens were found in numerous places in Cornwall, England, especially in Tavistock and Devon. In France, a large deposit exists in Lorraine, and fine clusters were found in Isere and Allevard. Fine specimens have also come from Freiberg and the Harz Mountains in Germany, and from Panasqueira, Portugal. Good crystals are found in Western Australia at Broken Hill and Prospect Hill. In South America, excellent specimens are from Nova Lima and Ouero Prito, Minas Gerais, Brazil, from the Julcani District, Huancavelica, Peru. In Ivigtut, Greenland, crystals are associated with Cryolite.
In the U.S., much Siderite is from a vein in Roxbury, Connecticut, with many specimens fully altered to Limonite. It has also been found in the Eagle mine, Gilman Co., Colorado. Other localities include Bisbee, Cochise Co., Arizona; Kellogg, Shoshone Co., Idaho; the Jamestown mine, Tuolumne Co., California; Park City, Summit Co., Utah; and in Isle Royal National Park in Michigan.

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Links

Picture Links

1. Rounded rhombohedral crystals
2. Greenish-brown plates