Class : Oxides and hydroxides
    Subclass : Oxides
    Crystal system : Cubic
    Chemistry : ZnAl₂O₄
    Rarity : Quite common

Gahnite is a zinc-bearing aluminous spinel typical of certain rocks of contact metamorphism (marbles in particular), of replacement deposits containing zinc, and more rarely of granitic pegmatites. It is also a mineral that can be found in neoformation in foundry slag. It is a so-called “normal” spinel, the bivalent ions being all in tetrahedral sites. Gahnite forms two continuous series, one towards the ferrous term, hercynite, the other towards the aluminous term, spinel in the strict sense. It was named in honor of the Swedish mineralogist and chemist Johan Gottieb Gahn who discovered the mineral and isolated the element manganese. Gahnite is usually presented in octahedra, sometimes in dodecahedra, rarely in cubes. It can also be massive or in irregular, rounded grains. Its color is dark green to greenish black, sometimes bluish black or brownish.

Main photo : Gahnite from Franklin, Sussex County, New Jersey, USA © Modris Baum

A twin is known on {111}.

No fakes recorded for this mineral species.