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Formula: Mg(H2O)6Al2(OH)2(H2O)8(V10O28).13H2O
Vanadate
Crystal System: Orthorhombic
Specific gravity: 2.226 calculated
Hardness: 2
Streak: Yellow
Colour: Golden yellow
Luminescence: No fluorescence under UV
Environments
Sedimentary environments
Hydrothermal environments
Localities
There are two co-type localities, the Blue Cap mine and the Vanadium Queen mine, both in the La Sal Creek Mining District,
San Juan County, Utah, USA. Postite was first found on sandstone
blocks in the Vanadium Queen mine, and subsequently
better crystals were found at the Blue Cap mine.
Postite is rare. Crystals are found growing as needles on
corvusite-montroseite-bearing
sandstone blocks. Other minerals found nearby include
baryte, calcite,
clausthalite, devilline,
dickthomssenite, hewettite,
lasalite, magnesiopascoite,
martyite, natrozippeite,
navajoite, paramontroseite,
pascoite, pyrite,
rossite, selenium,
sherwoodite, sulphur,
tyuyamunite, uranopilite and
zeunerite.
Postite forms from the oxidation of
corvusite-montroseite assemblages in
a moist environment. Mining operations have exposed unoxidised and oxidised phases. Under ambient temperatures and
generally oxidising near-surface environments, water reacts with pyrite in the deposit
to form aqueous solutions with a relatively low pH (acid). The various
secondary vanadate phases
that formed depend upon the acidity and presence of other cations such as Na+, Ca2+,
Mg2+ and Al3+
(CM 50.1.45-53).
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