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Gahnite  (Zinc Spinel)
Current inventory:  1 gem
 

Chemistry:  ZnAl₂O₄

Discovered in 1807;   IMA status: Valid (pre-IMA; Grandfathered).
Gahnite is named after the Swedish chemist and mineralogist, Johan Gottlieb Gahn (1745-1818), who discovered Gahnite and manganese.

Classification

Mineral Classification:

Oxides

Strunz 8th Ed. ID:

4/B.01-40

4 : Oxides & Hydroxides
B : Oxides with metal : oxygen = 3:4 (spinel type M3O4 and related), Alumina-Spinels
01 : Spinel group, Spinel - Gahnite series

Crystal Data

Crystallography:

Isometric - Hexoctahedral

Crystal Habit:

Crystals typically octahedra, rarely as dodecahedra, may be modified, to 12 cm; also as exsolution lamellae in other minerals; granular, massive.

Twinning:

On [111] as both twin and composition plane, common

Physical Properties

Cleavage:

[111] Indistinct

Fracture:

Conchoidal also Irregular, Uneven

Tenacity:

Brittle

Moh's Hardness:

7.5 - 8.0

Density:

4.00 - 4.64 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Dark Bluish Green, Grayish Green, Deep Green, Greenish Black, Bluish Black

Transparency:

Transparent to Translucent

Luster:

Vitreous (glassy)

Refractive Index:

1.82  Isotropic  (1.790 - 1.820 due to impurities)

Birefringence:

0.00  Isotropic

Dispersion:

n/a

Pleochroism:

n/a

Occurances

Geological Setting:

An accessory mineral in granites and granite pegmatites; in medium- to high-grade metamorphic rocks and metamorphosed base-metal sulfide deposits; in diasporites, formed by the low-grade metamorphism of bauxites; a detrital mineral in placers. Gahnite is found in zinc deposites at many localities worldwide.

Type Locality:

Falu mine, Falun, Dalarna, Sweden

Year Discovered:

1807

View mineral photos:

Gahnite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Gahnite is one of the rarer members of the Spinel group of minerals. It is found at many zinc deposites worldwide. Crystals are typically very small, included and just translucent to opaque, but faceted gems are very collectable due to their rarity and color. Current sources are Australia, Brazil, Germany, Italy, Namibia, Nigeria, Sweden, and in the USA in Sussex County, New Jersey, Carroll County, Maryland, Mitchell County, North Carolina and Fremont County, Colorado.
 

  
Gahnite gems for sale: