Nichromite

Nichromite (Ni,Co,Fe)(Cr,Fe,Al)₂O₄^[1] is a black cubic metallic mineral and member of the spinel group.^[5] Nichromite was originally reported from the Bon Accord nickel deposit in Barberton District, South Africa.^[1] Occurring naturally in a nickel deposit, nichromite is named for chromite with dominant nickel.^[2]

Nichromite
General
Category	Spinel group
Formula	(Ni,Co,Fe)(Cr,Fe,Al)2O4
Strunz classification	4.BB.05
Crystal system	Isometric
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Unit cell	a = 8.32 Å; V = 573.86 Å3; Z = 8
Identification
Color	Dark green, black
Crystal habit	Granular, anhedral to subhedral
Fracture	Conchoidal
Mohs scale hardness	6-6+1⁄2
Luster	Metallic
Streak	Greyish green
Diaphaneity	Opaque
Specific gravity	5.10
Optical properties	Isotropic
References	[1][2][3][4]

The atomic arrangement of the spinel group is a commonly studied structure and characteristically has four closely packed oxygen atoms. The nickel atoms are organized corresponding to a "normal" spinel arrangement.^[6]

The mineral has only been found in the Bon Accord Nickel Deposit in South Africa where it is formed by replacing chromite and rimmed by trevorite.^[4]