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Formula: Ca4[Mg3Fe3+9]O4[Si3Al8Fe3+O36 ]
Inosilicate (chain silicate), rhönite group,
sapphirine supergroup
Crystal System: Triclinic
Specific gravity: 3.059 calculated
Hardness: 5½
Streak: Brownish grey
Colour: Red-brown, brown-black, dark brown
Common impurities: Ti,Cr,Mn,Na,K
Environments
Metamorphic environments
Coal-seam fires
Dorrite is stable in strongly oxidising, high-temperature, low-pressure environments
(Mindat).
Localities
Réunion is an island in the Indian Ocean that is an overseas department and region of France. The mineral that we now
call dorrite has apparently been described twice previously. One report was from a
basalt-limestone contact
on Réunion Island. There it forms small, nearly opaque grains coexisting with
melilite and titanium-bearing
fassaite
(AM 73.1440-1448).
At the type locality, the Durham Ranch paralava occurrence, Reno Junction, Campbell county, Wyoming, USA, the rocks
form mesas with two distinct horizons. The lower horizon is composed of unaltered sedimentary rocks, generally
siltstone, sandstone
and conglomerates. This horizon is overlain by a thick capping of
baked and reddened sedimentary rocks. Between the two horizons is a small zone of incompletely combusted coal and
paralava (pyrometamorphic melt rock) containing coal-ash
(AM 73.1440-1448).
The dorrite is a product of oxidising, high-temperature, low-pressure metamorphism of alkalic rocks, in the
pyrometamorphic zone between the horizons. Associated minerals include
esseneite, titanium-bearing
andradite, magnetite -
magnesioferrite - spinel,
plagioclase, gehlenite -
åkermanite, wollastonite,
ulvöspinel, nepheline,
apatite and ferroan
sahamalite
(HOM).
The Southern Powder River Basin Mining District, Converse county, Wyoming, USA. The mineral that we now call
dorrite has apparently been described twice previously. One report is from an investigation of a paralava from
the Powder River Basin that identified a mineral as Fe3+-rich
melilite, but the chemistry and optical properties of this "iron-melilite"
are quite diferent from those of the melilite group and are similar to
those of the rhönite-like phase that is dorrite
(AM 73.1440-1448).
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