8 Photos
Polyhalite
Chemical Formula	K2Ca2MgSO44·2H2O
Species	Sulfates
Crystal System	Triclinic
Mohs Scale	2-3
Specific Gravity	2.78
Color	Colourless, white, pink, red, grey; colourless in transmitted light.
Luster	Vitreous, Resinous
Refractive Index	n = 1.546 - 1.548 n = 1.558 - 1.562 n = 1.567
Diaphaneity	Transparent
Cleavage	Perfect, perfect
Crystal Habit：Crystals small and rare; tabular or elongated and exhibiting numerous forms. Commonly massive, fibrous to foliated.
Geological Setting：Oceanic salt deposits; rarely as a volcanic product.

Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K₂Ca₂Mg(SO₄)₄·2(H₂O). Polyhalite crystallizes in the triclinic system although crystals are very rare. The normal habit is massive to fibrous. It is typically colorless, white to gray, although it may be brick red due to iron oxide inclusions. It has a Mohs hardness of 3.5 and a specific gravity of 2.8.

It occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad Deposits of New Mexico.

Polyhalite was first described in 1818 for specimens from its type locality in Salzberg, Austria. The name is from the Greek polys hals for many salts.

It has no connection to the sodium halide mineral halite other than that both are evaporite minerals.