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

Metamorphic environments
Coal-seam fires

Dorrite is stable in strongly oxidising, high-temperature, low-pressure environments (Mindat).


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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