Gahnite

Gahnite
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General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	ZnAl2O4
Strunz classification	04.BB.05
Identification
Color	Dark green, bluish green, blue to indigo, yellow to brown
Crystal habit	Typically octahedra, rarely as dodecahedra also assive 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

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.

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. It is sometimes called zinc spinel.

File:Gahnit-G-EmpireTheWorldOfGems.jpg

Faceted Gahnite, 1.34 ct, from Argentina