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Skutterudite

Cobalt arsenide mineral

Skutterudite

Cobalt arsenide mineral

FieldValue
nameSkutterudite
categoryArsenide mineral
imageSkuttérudite.jpg
captionSkutterudite from Bou Azzer, Morocco
formulaCoAs3
IMAsymbolSkt
strunz2.EC.05
systemCubic
classDiploidal (m)
H-M symbol: (2/m )
symmetry*I*m
unit cella = 8.204 Å, Z = 8
colorTin-white to silver-gray, tarnishes gray or iridescent; in polished section, gray, creamy or golden white
habitCrystals are cubes, octahedra, dodecahedra, rarely prismatic; in skeletal growth forms, distorted aggregates; also massive, granular
twinningOn {112} as sixlings and complex shapes
cleavageDistinct on {001} and {111}; in traces on {011}
fractureConchoidal to uneven
mohs5.5–6
lusterMetallic
streakBlack
diaphaneityOpaque
gravity6.5
references

H-M symbol: (2/m ) Skutterudite is a cobalt arsenide mineral containing variable amounts of nickel and iron substituting for cobalt with the ideal formula CoAs3. 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, annabergite, nickeline, cobaltite, silver sulfosalts, native bismuth, calcite, siderite, barite and quartz. It is mined as an ore of cobalt and nickel with a by-product of arsenic.

The crystal structure of this mineral has been found to be exhibited by several compounds with important technological uses.

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.

Origins

Skutterudite has been known since the Middle Ages, when it was used in the production of smalt. It was discovered in the Skuterud Mines, Modum, Buskerud, Norway, in 1845. Smaltite is an alternative name for the mineral. Notable occurrences include Cobalt, Ontario and Franklin, New Jersey, in the United States.

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 (number 204).

The unit cell of a skutterudite consists of a total of 32 atoms, arranged in eight smaller cubes composed of cobalt atoms, which form octahedra with cobalt at the center. Six of these cubes are filled with planar square rings of arsenic, each oriented parallel to one of the unit cell's edges. In its structure, at the 2a Wyckoff position, there are two large structural voids—each approximately five angstroms in size—that can be filled with impurity atoms. Together with the unit cell size and the assigned space group, the parameters mentioned above fully describe the crystalline structure of the material. This structure is commonly referred to as a skutterudite structure.

Applications

Materials with a skutterudite structure are studied as a low cost thermoelectric material with low thermal conductivity. These materials have been synthesized with a thermoelectric figure of merit (ZT) close to 1 at 800 kelvin. A relatively high dimensionless figure of merit has been observed in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter but heavy ytterbium atoms partially occupy the voids in the CoSb3 host structure, resulting in low thermal conductivity values while the favorable electronic properties are not substantially disrupted by the addition of ytterbium.

References

References

  1. Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
  2. (2025). "Skutterudite". Mineral Atlas.
  3. (29 January 1990). "Handbook of Mineralogy". Mineralogical Society of America.
  4. (2025). "Skutterudite". Hudson Institute of Mineralogy.
  5. (1985). "Manual of Mineralogy (after James D. Dana)". J. Wiley and sons.
  6. Kraft, Alexander. (August 2019). "Dorothea Juliana Wallich (1657–1725) and Her Contributions to the Chymical Knowledge about the Element Cobalt". World Scientific.
  7. (November 2021). "Glass-based pigments in painting: smalt blue and lead–tin yellow type II". Archaeological and Anthropological Sciences.
  8. Uher, Ctirad. (2021-04-27). "Thermoelectric Skutterudites". CRC Press.
  9. Walker, T. L.. (1921-03-01). "Skutterudite from Cobalt, Ontario1". American Mineralogist.
  10. (1984). "The nickel-arsenide assemblage from Franklin, New Jersey: description and interpretation". Neues Jahrbuch für Mineralogie - Abhandlungen.
  11. (1999). "SKUTTERUDITES: A Phonon-Glass-Electron Crystal Approach to Advanced Thermoelectric Energy Conversion Applications". Annual Review of Materials Science.
  12. (1926). "The crystal structure of skutterudite and related minerals". Norsk Geologisk Tidsskrift.
  13. (January 2020). "Characterization of multiple-filled skutterudites with high thermoelectric performance". Journal of Alloys and Compounds.
  14. Uher, Ctirad. (2022). "Semiconductors and Semimetals". Elsevier.
  15. (June 2014). "Search for Resonant-Like Impurity in Ag-Doped CoSb3 Skutterudite: Theoretical and Experimental Study". Journal of Electronic Materials.
  16. (2012-06-22). "Electronic, vibrational, and transport properties of pnictogen-substituted ternary skutterudites". Physical Review B.
  17. (June 2009). "Synthesis, crystal structure and transport properties of skutterudite-related CoSn1.5Se1.5". Journal of Alloys and Compounds.
  18. (5 Oct 2012). "Thermal to Electrical Energy Conversion of Skutterudite-Based Thermoelectric Modules". Journal of Electronic Materials.
  19. (1996). "The effect of rare-earth filling on the lattice thermal conductivity of skutterudites". Journal of Applied Physics.
  20. (17 Mar 2014). "Low thermal conductivity and rapid synthesis of n-type cobalt skutterudite via a hydrothermal method". Journal of Materials Chemistry C.
  21. (1996-05-31). "Filled Skutterudite Antimonides: A New Class of Thermoelectric Materials". Science.
  22. (2000-09-18). "High figure of merit in partially filled ytterbium skutterudite materials". Applied Physics Letters.
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