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Formula: ZnAl2O4
Multiple oxide, spinel subgroup,
forms a series with hercynite and with
spinel
Crystal System: Isometric
Specific gravity: 4.62 calculated
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, columbite-(Mn)
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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