Gahnite Dimensions: 55 mm x 40 mm x 30 mm; largest crystal size 7 mm Locality: Rosendal pegmatite, Kemiö Island (Kimito Island), Southwestern Finland Region, Fi — Photo: Jyrki Autio · Public domain
Gahnite
Also known as: Zinc spinel.
Zinc aluminate spinel — green-to-blue octahedral crystals often formed where sphalerite alters in massive sulfide deposits.
Intro adapted from Wikipedia (CC BY-SA).
Gahnite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula ZnAl₂O₄.
It crystallizes in the cubic crystal system (space group Fd3m). It is a member of the Spinel structural group.
Key properties: Mohs hardness 7.5–8; specific gravity 4.38–4.6; typically green or blue; Gray streak; Vitreous luster.
IMA symbol Ghn, grandfathered status — known before the IMA’s founding, named in 1807.
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.
It is found associated with Rhodonite, franklinite, calcite, andradite, willemite (Franklin, New Jersey, USA); corundum, pyrrhotite, högbomite, nigerite, phlogopite, staurolite.
The name refers to the Swedish chemist and mineralogist, Johan Gottlieb Gahn (1745–1818), who discovered the mineral.
Collections: Collector Favorites, Rare & Remarkable.