Great Salt Lake

Formula: Na2(SO4).10H2O
Specific gravity: 1.490
Hardness: 1½ to 2
Streak: White
Colour: Colourless, white
Solubility: Soluble in water

Sedimentary environments

Mirabilite occurs in salt lakes, playas and springs, and as an efflorescence on alkali soils. At Great Salt Lake, Utah, USA, it crystallises during winter due to decreased solubility at low temperatures.

Alteration: Mirabilite is unstable in dry air, when it loses its water and alters to thénardite, either as pseudomorphs after mirabilite, or just crumbled to a loose powder.

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