Gahnite

gahnite

pyrrhotite

spinel

hercynite

Images

Formula: ZnAl2O4 Multiple oxide, spinel group, forms a series with hercynite and with spinel
Specific gravity: 4.57
Hardness: 7½ to 8
Streak: Grey
Colour: Dark blue-green, yellow, brown, black
Common impurities: Fe,Mg
Environments:

Pegmatites
Metamorphic environments

Gahnite occurs in crystalline schist, in granite pegmatites especially those rich in lithium, and in contact metamorphosed limestone. It also occurs in high temperature replacement ore deposits in schist or marble (Dana).
It is formed by the low-grade metamorphism of bauxite (HOM).

Localities

At the Geco Mine, Gemmell Township, Thunder Bay District, Ontario, Canada, gahnite is associated with corundum, pyrrhotite, ferronigerite, phlogopite, staurolite, cordierite, pyrite and chalcopyrite (HOM).

The type locality is the Falun mine, Falun, Dalarna County, Sweden.

The Pulsifer pegmatite, West Mount Apatite Mining District, Auburn, Androscoggin county, Maine, USA, is a rare-element granitic pegmatite that intrudes upper amphibolite facies metapelites and biotite schists that are locally interbedded with calc-silicate rocks. The pegmatite exhibits five distinct zonations.
The border zone consists of fine to mediumgrained equi-granular quartz and plagioclase, and minor biotite and almandine.
The wall zone consists of slightly graphic K-feldspar, quartz, biotite and almandine.
The first intermediate zone is characterised by coarse-grained graphic feldspar and plumose muscovite-quartz aggregates. Almandine and schorl are found as accessory minerals, and rare beryl has been observed.
The first intermediate zone grades irregularly into a coarse-grained plagioclase plus quartz plus muscovite second intermediate zone.
The pocket zone assemblage, which lies below the second intermediate zone and immediately above the garnet seam, consists primarily of cleavelandite and quartz, although locally blocky K-feldspar and muscovite are also abundant. Gem tourmaline, beryl, fluorapatite, hydroxylherderite, gahnite, almandine, manganocolumbite and cookeite are among the minerals that have been found within the pocket zone.
The garnet seam is a 1 to 5 cm thick layer of 2 mm to 3 cm euhedral almandine plus anhedral smoky quartz.
Just below the garnet seam lies a zone of undetermined thickness that is composed primarily of blocky graphic albite.
Gahnite is found in the unit immediately above the garnet seam (R&M 97.1.8-11).

At Franklin, Franklin Mining District, Sussex County, New Jersey, USA, gahnite is associated with rhodonite, franklinite, calcite, andradite and willemite (HOM).

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