Spinel (Gahnite) Amity, New York, USA Thumbnail, 3 x 2.8 x 2.5 cm Photo Copyright © Arkenstone

Chemical Formula: ZnAl2O4
Locality: Falun, Sweden.
Name Origin: Named after the Swedish chemist and mineralogist, J. G. Gahn (1745-1818).

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.

Physical Properties

Cleavage: {111} Indistinct
Color: Bluish green, Grayish green, Deep leek green, Greenish black, Bluish black.
Density: 4 – 4.6, Average = 4.3
Diaphaneity: Subtransparent to opaque
Fracture: Uneven – Flat surfaces (not cleavage) fractured in an uneven pattern.
Hardness: 8 – Topaz
Luminescence: Non-fluorescent.
Luster: Vitreous (Glassy)
Magnetism: Nonmagnetic
Streak: gray

Photos :

Gahnite Davis Pyrite Mine, Charlemont, Franklin Co., Massachusetts, USA Size: 6.5 x 5.0 x 2.5 cm (small cabinet) © danweinrich
GAHNITE Locality: Erik Mattsgruvan Stope, Falu Copper Mine, Falun, Dalana, Sweden Collected before 1900 Specimen Size: 2.5 x 2 cm (thumbnail) Largest crystal: 1.5 cm.  © Mineral Classics
Gahnite with Quartz Mina Victoria, Arres, Val d’Aran, Lleida  Spain (09/2009) Specimen size: 3.8 × 2.1 × 2 cm = 1.5” × 0.8” × 0.8” Main crystal size: 1 × 1 cm = 0.4” × 0.4”  © Fabre Minerals
Franklin Mine, Franklin, Franklin Mining District, Sussex Co., New Jersey, USA © Rob Lavinsky