Millerite

Millerite
General
Category	Sulfide mineral
Formula (repeating unit)	NiS
IMA symbol	Mlr[1]
Strunz classification	2.CC.20
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) (same H-M symbol)
Space group	R3m
Unit cell	a = 9.607 Å, c = 3.143 Å; Z = 9
Identification
Colour	Pale brass-yellow to bronze-yellow, tarnishes to iridescence
Crystal habit	Typically acicular (needle-like) often in radial sprays – also massive
Cleavage	Perfect on {1011} and {0112} – obscured by typical form
Fracture	Uneven
Tenacity	Brittle; capillary crystals elastic
Mohs scale hardness	3–3.5
Luster	Metallic
Streak	Greenish black
Diaphaneity	Opaque
Specific gravity	5.3–5.5
Other characteristics	brittle and becomes magnetic on heating
References	[2][3][4][5][6]

Millerite or nickel blende is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates. It can be distinguished from pentlandite by crystal habit, its duller colour, and general lack of association with pyrite or pyrrhotite.