Skutterudite

What is skutterudite? Cobalt arsenide, cubic and tin-white

Skutterudite is a cubic cobalt arsenide, CoAs3 on the September 2026 IMA list, tin-white to silver-gray with a black streak. The Handbook of Mineralogy skutterudite sheet gives point group 2/m −3, crystals as cubes, octahedra and dodecahedra to 9 cm, rarely prismatic, in skeletal growth forms and distorted aggregates, and also massive, granular and dense. It twins on {112} as sixlings and complex shapes, also reported on {011}. Cleavage is distinct on {001} and {111}, fracture conchoidal to uneven, hardness 5.5 to 6, density 6.5 measured against 6.821 calculated. It tarnishes gray or iridescent.

What the IMA list records. Skutterudite, CoAs3, status G (grandfathered), 1845, Norway, first reference the Handbuch der bestimmenden Mineralogie (Wien, 1845); second reference the 1971 Acta Crystallographica structure paper. The sheet marks Skutterud, Modum, Norway, as the type locality and names the mineral for it; type material is at the Mining Academy, Freiberg, Germany, number 3910. More on the term is at what a type locality is.

Where the sheet places it. Typically in medium- to high-temperature hydrothermal veins with other Ni–Co sulfide minerals, associated with nickeline, cobaltite, arsenopyrite, silver, silver sulfosalts, bismuth, calcite, siderite, barite and quartz.

Why does the sheet write CoAs2−3? The arsenic-deficit ledger

The arsenic-deficit ledger: both analyses on the skutterudite sheet carry about 2.75 atoms of arsenic plus sulfur per metal atom, not the 3 of the IMA formula. Divide each weight percentage by its atomic weight and the Skutterud analysis gives 2.65 arsenic and 0.08 sulfur per metal, 2.74 together, with cobalt 87.0% of the metals; the Cobalt, Ontario microprobe analysis gives 2.60 and 0.17, 2.77 together, cobalt 82.9%. The sheet prints the same structural formulas, Σ = 2.73 and Σ = 2.77. Ideal CoAs3 would be 79.23% arsenic; the analyses have 76.41% and 75.7%.

What the newer source says. Schumer et al. 2017, American Mineralogist 102, 205–209 report that the IMA then defined skutterudite and nickelskutterudite as anion-deficient, BX3−x, after Palache et al. 1944. They found no anion deficiency in the nickelskutterudite crystal they analysed, which normalises to (Ni0.62Co0.28Fe0.12)Σ1.02(As2.95S0.05)Σ3.00, 2.94 anions per metal, and proposed (Ni,Co,Fe)As3. The September 2026 list writes CoAs3 and NiAs3, with no x.

What the ledger does not settle. Whether the sheet’s shortfall of 0.23 to 0.26 atoms is in the mineral or in the older analyses is not decided by the sources fetched here, and this page does not decide it. The ledger records that the IMA formula, the sheet header and the printed analyses disagree, and by how much.

Skutterudite or nickelskutterudite? The series and its four IMA rows

Skutterudite and nickelskutterudite cannot be told apart in the hand: their sheets print the same hardness, density, streak and colour. The skutterudite sheet says it forms a series with nickel-skutterudite; the nickelskutterudite sheet says the reverse, (Ni,Co,Fe)As3, hardness 5.5 to 6, density 6.5, tin-white to silver-gray, streak black. A 20.00 g piece reads 16.92 g in water as either. Which metal leads needs an analysis.

Four rows, by script. A field scan of the IMA list for formulas written as one metal plus As3 or Sb3, checked by a whole-line token scan, finds four: skutterudite CoAs3 (G, 1845, Norway), nickelskutterudite NiAs3 (Rn, 2007 s.p., Germany), ferroskutterudite FeAs3 (A, 2006-032, Russia) and kieftite CoSb3 (A, 1991-052, Sweden). Schumer et al. 2017 name the same four as the skutterudite group. The ferroskutterudite sheet gives hardness and density as n.d.

Documented failure: cobaltite by weight. cobaltite reads 16.84 g in water for 20.00 g at its density of 6.33, only 0.08 g from skutterudite, and its hardness, 5.5, sits inside skutterudite’s 5.5 to 6. Its sheet gives a perfect {001} cleavage and a grayish black streak, against skutterudite’s distinct cleavage and black streak. The density ladder for the silver-white arsenides is on the cobaltite page and is not repeated here. Methods: how to measure specific gravity at home and how to use a streak plate.

Where is skutterudite found, and what grows with it?

The skutterudite sheet’s Distribution field calls it an accessory mineral from many localities, rarely in fine specimens. It names Skutterud, Modum, Norway [TL]; Schneeberg, Annaberg and Freiberg, Saxony; Lölling, Carinthia, Austria; the valley of Gistain, Huesca, Spain; Riomanou, Hautes-Pyrénées, and Mount Chalanches, Isère, France; Talnotry, Kirkcudbrightshire, Scotland; a number of mines in Cornwall; Khovu-Akay, Tuva, Russia; well-crystallized from the Aghbar (Arhbar) and Irhtem (Ightem) mines, Bou Azzer district, Morocco; Chatham, Connecticut, and Mine la Motte, Missouri; and Cobalt, South Lorrain Township, Sudbury and Gowganda, Ontario.

The pink and green crusts. The erythrite sheet lists skutterudite among erythrite’s associates and places erythrite in the oxide zone of Co–Ni–As-bearing deposits; the annabergite sheet lists nickel-skutterudite among annabergite’s. The arsenates are on erythrite and annabergite.

Old labels and other uses of the name. Minerals.net calls smaltite the former name of the cobalt-rich end member and chloanthite a discredited name merged into nickelskutterudite, both still seen on old labels; the IMA list has no line containing either. Schumer et al. note that synthetic skutterudite-structure compounds are studied for thermoelectric properties; those are laboratory materials and out of scope here. No USGS or BGS text was fetched, so this page makes no ore claim. Magnetism: USGS Open-File Report 99-529, Table 1 left all 3 skutterudite samples behind above 1.70 amp, and its report text calls skutterudite nonmagnetic; see which minerals are magnetic. Arsenic: see toxic minerals in a collection; nothing here is safety advice. The rest is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

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