Gahnite

Gahnite, ZnAl₂O₄, is a rare mineral belonging to the spinel group. It forms octahedral crystals which may be green, blue, yellow, brown or grey. It often forms as an alteration product of sphalerite in altered massive sulphide deposits such as at Broken Hill, Australia. Other occurrences include Falun, Sweden where it is found in pegmatites and skarns, Charlemont, Massachusetts; Spruce Pine, North Carolina; White Picacho district, Arizona; Topsham, Maine; and Franklin, New Jersey in the United States.[1][2]

Gahnite
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	ZnAl2O4
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Identification
Color	Dark green, bluish green, blue to indigo, yellow to brown
Crystal habit	Typically octahedra, rarely as dodecahedra also massive to granular
Twinning	Common on [111] produces striations
Cleavage	Indistinct parting on [111]
Fracture	Conchoidal, uneven
Mohs scale hardness	7.5–8.0
Luster	Vitreous
Streak	Grey
Diaphaneity	Translucent to nearly opaque
Specific gravity	4.38–4.60
Optical properties	Isotropic
Refractive index	n = 1.79–1.80
References	[1][2][3]

It was first described in 1807 for an occurrence in the Falu mine, Falun, Dalarna, Sweden, and named after the Swedish chemist, Johan Gottlieb Gahn (1745–1818), the discoverer of the element manganese.[2][3] It is sometimes called zinc spinel.