Carrollite

Carrollite
Carrollite from Kambove, Katanga. This specimen is 4.3 cm wide, with a 1.2 cm carrollite crystal partly covered by pyrite, between calcite crystals.
General
Category	Sulfide mineral Thiospinel group (Spinel structural group)
Formula (repeating unit)	CuCo2S4
Strunz classification	2.DA.05
Dana classification	2.10.1.2
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Identification
Color	Light to dark gray, rarely tarnishes to copper red or violet gray
Crystal habit	Octahedral and cubic crystals, also massive, granular or compact
Twinning	{111} Polysynthetic or spinel twins
Cleavage	Imperfect on {001}
Fracture	Conchoidal, subconchoidal or uneven
Tenacity	Very brittle
Mohs scale hardness	4.5 to 5.5
Luster	Metallic
Streak	Grey black
Diaphaneity	Opaque. R is 43% to 45% for lambda = 560 nm
Specific gravity	4.5 to 4.8 measured, 4.83 calculated
Refractive index	n is not determined for an opaque mineral
Solubility	Minerals of the linneite group are partly etched by nitric acid, with slight effervescence.
Other characteristics	Not radioactive, not fluorescent
References

Carrollite, CuCo₂S₄, is a sulfide of copper and cobalt, often with substantial substitution of nickel for the metal ions, and a member of the linnaeite group. It is named after the type locality in Carroll County, Maryland, US, at the Patapsco mine, Sykesville.

Space group: Fd3m. Unit cell parameters = a = 9.48 Å, Z = 8. Unit cell volume: V = 851.97 Å³ (calculated from unit cell parameters).

The linnaeite group is a group of sulfides and selenides with the general formula AB₂X₄ in which X is sulfur or selenium, A is divalent Fe, Ni, Co or Cu and B is trivalent Co, Ni or, for daubréelite, Cr. The minerals are isometric, space group Fd3m and isostructural with each other and with minerals of the spinel group.

The structure of the linnaeite group consists of a cubic close packed array of X (X is oxygen in the spinels and sulfur or selenium in the linnaeite group). Within the array of Xs there are two types of interstices, one type tetrahedrally co-ordinated and one type octahedrally co-ordinated. One eighth of the tetrahedal sites A are typically occupied by 2⁺ cations, and half of the octahedral sites B by 3⁺ cations. Charnock et al. confirmed that carrollite contains Cu wholly within the tetrahedral sites. Thus, the ideal formula one would expect for a spinel like carrollite is Cu²⁺Co³⁺₂S²⁻₄, but as in the case of copper sulfides in general the oxidation state of the copper atom is 1+, not 2+. An assignment of valences as Cu⁺Co³⁺₂S^1.75−₄ is more appropriate; this was confirmed in a study of 2009. The one missing electron per four sulfur atoms is delocalized, leading to metallic conductivity and even superconductivity at very low temperatures, combined with a complicated magnetic behavior.