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
IMA symbol	Cli[1]
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[2]
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[3]
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.[2]
Other characteristics	Not radioactive, not fluorescent
References	[4][5][6][7]

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.^[6]