Mirabilite

mirabilite

thenardite

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Formula: Na₂(SO₄).10H₂O
Hydrated sulphate
Crystal System: Monoclinic
Specific gravity: 1.464 measured, 1.467 calculated
Hardness: 1½ to 2
Streak: White
Colour: Colourless, white
Solubility: Soluble in water
Environments:

Evaporite Deposits

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 (Dana). It quickly dehydrates to thénardite in dry air (Mindat).

Localities

At Boron, Kramer district, Kern county, California, USA, thénardite pseudomorphs after mirabilite form as dehydration products (KL p187).

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 (MN).

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