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Category Oxide minerals
Spinel group
Spinel structural group
(repeating unit)
Strunz classification 04.BB.05
Color Dark green, bluish green, blue to indigo, yellow to brown
Crystal habit Typically octahedra, rarely as dodecahedra also assive[clarification needed] to granular
Crystal system Isometric
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]

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]

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.