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Formula: MgCr2O4
Multiple oxide, spinel subgroup, forms a complete series with
chromite and with spinel
Varieties
Picrochromite is a variety of magnesiochromite
Specific gravity: 4.2
Hardness: 5½
Streak: Brown
Colour: Black
Common impurities: Fe,Al
Environments
Igneous environments
Metamorphic environments
Magnesiochromite occurs as an accessory mineral in ultramafic rocks,
such as dunite,
serpentinite and
kimberlite. More rarely it is found as xenocrysts in
lamprophyre and mid-ocean
basalt. It may be detrital. Associated minerals include
olivine, augite,
magnetite, plagioclase and
pigeonite
(HOM).
Localities
At the type locality, the Schwarzenberg mining district, Erzgebirgskreis, Saxony, Germany, magnesiochromite
occurs in serpentinite
(Dana).
At Dun Mountain, Nelson Region, New Zealand, magnesiochromite occurs in
dunite
(Dana).
At the Clear Creek District, Kern county, California, USA, ferroan magnesiochromite is common both as masses and
as octagonal crystals in the serpentinite. Many of the grains have
been fractures and filled with cinnabar. Hydrothermal fluids have altered the
magnesiochromite to eskolaite, which, on further reduction, provided
the chromate that formed the mercury-bearing chromates
(Minrec 36.4.354).
At the Cripple Creek Mining District, Teller county, Colorado, USA, magnesiochromite has been reported as
inclusions in olivine in
monchiquite,
phonolitic-tephrite and
tephritic phonolite
(Minrec 36.2.173).
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