Langbeinite
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Langbeinite is a potassium magnesium sulfate mineral with formula: K2Mg2(SO4)3. Langbeinite crystallizes in the isometric - tetartoidal system as transparent colorless or white with pale tints of yellow to green and violet crystalline masses. It has a vitreous luster. The Mohs hardness is 3.5 to 4 and the specific gravity is 2.83. The crystals are phosphorescent and piezoelectric.
The mineral is an ore of potassium and occurs in marine evaporite deposits in association with carnallite, halite and sylvite.
It was first described in 1891 and named for A. Langbein of Leopoldshall, Germany.
