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Formula: Na2Mg2V5+10O28.20H2O
Decavanadate, pascoite group
Crystal System: Monoclinic
Specific gravity: 2.38 measured, 2.362 calculated for the empirical formula
Hardness: 1
Streak: Yellow
Colour: Yellow to yellow-orange
Environments
Lasalite occurs as efflorescences in the oxidised portions of
uranium-vanadium deposits of the
Colorado Plateau type. Associated minerals include rossite,
dickthomssenite and
hewettite
(HOM).
Localities
At the type locality, the Vanadium Queen Mine, La Sal Creek Mining District, San Juan County, Utah, USA,
lasalite occurs in efflorescences on the sandstone walls of
the mine workings and along fractures in the sandstone. It forms by
oxidation of primary
corvusite by subsurface water and reaction with
dolomite and calcite cement of the
host sandstone. Evaporation leads to the formation of the
efflorescent assemblage that contains lasalite, rossite,
dickthomssenite and
hewettite.
Primary ore minerals occur as reduced oxides of
uranium and vanadium and
silicates, formed near local concentrations of organic matter or other reducing agents. The main ore minerals found
at the Vanadium Queen mine are montroseite,
corvusite, vanadium-bearing
hydromica (synonym of brammallite,
hydrobiotite and illite) and
uraninite. Oxidised ore consists largely of
vanadium-bearing hydromica (synonym of
brammallite, hydrobiotite
and illite) and tyuyamunite.
The lasalite-rich efflorescent crusts are commonly 1 to 3 mm thick. Some
sandstone surfaces are coated with isolated single crystals 1 to
2 mm in length. Lasalite is transparent, yellow to yellow-orange, with a yellow streak and an adamantine
lustre. Where lasalite forms a thin layer on dark sandstone, the apparent colour is brown. In thicker layers,
the yellow colour is more apparent, and even thicker crusts are yellow-orange. On prolonged exposure to conditions
of low humidity, the crystals dehydrate to a yellow powder
(CM 46.5.1365-1372).
Lasalite from the Vanadium Queen Mine -
Image
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