Gahnite

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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; and, in the United States, Charlemont, Massachusetts; Spruce Pine, North Carolina; White Picacho district, Arizona; Topsham, Maine; and Franklin, New Jersey.[2][3]

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Gahnite
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General
CategoryOxide minerals
Spinel group
Spinel structural group
FormulaZnAl2O4
IMA symbolGhn[1]
Strunz classification4.BB.05
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Identification
ColorDark green, bluish green, blue to indigo, yellow to brown
Crystal habitTypically octahedra, rarely as dodecahedra also massive to granular
TwinningCommon on [111] produces striations
CleavageIndistinct parting on [111]
FractureConchoidal, uneven
Mohs scale hardness7.5–8.0
LusterVitreous
StreakGrey
DiaphaneityTranslucent to nearly opaque
Specific gravity4.38–4.60
Optical propertiesIsotropic
Refractive indexn = 1.79–1.80
References[2][3][4]
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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.[3][4] It is sometimes called zinc spinel.

See also

References

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