Skutterudite

(mineralogy) (Co,Ni)As₃ A tin-white mineral with metallic luster composed of cobalt and nickel arsenides; crystallizes in the isometric system but commonly is massive; hardness is 5.5-6 on Mohs scale, and specific gravity is 6.6; it is a minor ore of cobalt and nickel.

A mineral with composition (Co,Ni)AS₃, an ore of cobalt and nickel. Commonly the mineral is massive with metallic luster and tin-white color. The hardness is 5½–6 on Mohs scale and the specific gravity 6.6. Skutterudite is found at Freiberg, Annaberg, and Schneeberg in Germany, and at Cobalt, Ontario. See also Cobalt; Nickel.

Environment

This series occurs in medium-temperature veins lining walls, associated with silver and related silver-hued cobalt and nickel minerals, often interlocked and all looking much alike.

Usually massive and granular. Crystals may develop, particularly on surfaces in contact with a calcite vein filling, but they are dull and uneven. Cube and octahedron faces are most common, sometimes with dodecahedrons and pyritohedrons.

Tin-white. Luster metallic; hardness 5Ɖ-6; specific gravity 6.1-6.8; fracture granular; cleavage none. Brittle.

Triarsenides of cobalt, nickel, and iron. Arsenic amounts to 75% of the weight; the balance is made up by the metals. This series was formerly known under the name smaltite-chloanthite and was considered to be completely isomorphous with a third iron triarsenide. Subsequent studies indicate that only the cobalt triarsenide actually exists in a pure state and cobalt, iron, and nickel are always present in the others. This suggests that skutterudite should be the name for the high-cobalt end-member (formerly smaltite), and the others should be known (depending upon their analyzed composition) as nickelian skutterudite (instead of chloanthite) and ferroan skutterudite (instead of the discredited "arsenoferrite").

Skutterudite fuses on charcoal, giving an arsenic (garlic) odor and forming magnetic ball. A cobalt-rich skutterudite ball dissolves in nitric acid to form a pink solution, though iron and nickel are usually present in sufficient abundance to mask the color. Tests on analyzed samples showed that a nitric acid solution of (cobalt) skutterudite neutralized by ammonia becomes red-violet and a red-violet precipitate settles out. Nickel skutterudite under the same conditions gave a blue-violet solution and a pale green precipitate. Rarer ferroan skutterudite gives a strong brown iron-hydroxide precipitate that will mask either of the other elements.

From a collector's standpoint, this series is of marginal significance; distinguishing its specifics requires too-sophisticated tests. Likely to be confused with arsenopyrite, which gives no cobalt or nickel test, and with cobaltite and a series of diarsenides (loellingite and rammelsbergite), which cannot be distinguished by methods available to amateurs.

Valuable ores of cobalt and nickel. In North America this series is abundant and of great economic importance only at Cobalt, Ontario; rare elsewhere. It was important only in minor veins in Germany, France, Spain, Morocco, and Chile. In recent years, the best crystal source of high-cobalt skutterudites has been Bou Azzer, Morocco.

Weathering of this triumvirate of minerals results in the formation of pink and green secondary minerals (erythrite and annabergite) on outcrops and near the surface. These colors are good guides to the presence of minerals of this and the related group. They are rarely pure and commonly have closely locked zones with differing compositions so that it is only possible to analyze pure samples with a microprobe.

Skutterudite is a naturally occurring cobalt arsenide mineral. The crystal structure of this mineral has been found to have important technological uses for several compounds isostructural with the mineral.

Contents 1 The skutterudite mineral 2 The skutterudite crystal structure 3 References 4 See also

The skutterudite mineral

Skutterudite from Bou Azzer, Morocco.

Skutterudite is a cobalt arsenide mineral that has variable amounts of nickel and iron substituting for cobalt with a general formula: (Co,Ni,Fe)As₃. Some references give the arsenic a variable formula subscript of 2-3. High nickel varieties are referred to as nickel-skutterudite, previously chloanthite. It is a hydrothermal ore mineral found in moderate to high temperature veins with other Ni-Co minerals. Associated minerals are arsenopyrite, native silver, erythrite, and annabergite. It is mined as an ore of cobalt and nickel with a by-product of arsenic.

The mineral has a bright metallic luster, and is tin white or light steel gray in color with a black streak. The specific gravity is 6.5 and the hardness is 5.5-6. Its crystal structure is isometric with cube and octahedron forms similar to that of pyrite. The arsenic content gives a garlic odor when heated or crushed.

It was discovered in Skuterud, Norway, in 1845. Smaltite is a synonym for the mineral. Notable occurrences include Cobalt, Ontario , Canada, Skuterud, Norway, Franklin, New Jersey in the United States, and in other cobalt and nickel mining areas around the world. The rare arsenide minerals are classified in Dana's sulfide mineral group, even though it contains no sulfur.

The skutterudite crystal structure

The skutterudite unit cell.

The crystal structure of the skutterudite mineral was determined in 1928 by Oftedahl to be cubic, belonging to space group Im-3 (number 204). The unit cell can be considered to consist of eight smaller cubes made up of the Co atoms. Six of these cubes are filled with (almost) square planar rings of As, each of which is oriented parallel to one of the unit cell edges, see image on the right. The As atoms then form octahedra with Co in the centre.

In crystallographic terms, the Co atoms occupy the 8c sites, while the As atoms occupy the 24g sites. The position of the Co atoms within the unit cell is fixed, while the positions of the As atoms are determined by the parameters x and y. It has been shown that for the As-rings to be fully square, these parameters must satisfy the Oftedahl relation x+y=1/2. Any deviation from this relation yields a rectangular configuration of the As atoms. Indeed, this is the case for all known compounds with this structure, and it should be noted that the As atoms then do not form perfect octahedra.

Together with the unit cell size and the assigned space group, these two parameters fully describe the crystal structure of the material which is often referred to as the skutterudite structure.

References

• Dana's Manual of Mineralogy ISBN 0-471-03288-3
• A. Kjekshus and T. Rakke. Compounds with skutterudite type crystal-structure .3. Structural data for arsenides and antimonides. Acta Chemica Scandinavica Series A 28 (1): 99-103 1974.
• Mineral galleries
• Mindat.org
• Webmineral.com

See also

• List of minerals

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