Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K2Ca2Mg(SO4)4·2H2O. 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 is used as a fertilizer.
Museum specimen of polyhalite and anhydrite
An evaporite is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.
A cobble encrusted with halite evaporated from the Dead Sea, Israel (with Israeli ₪1 coin [diameter 18mm] for scale)
Anhydrite
Hopper crystal cast of halite in a Jurassic rock, Carmel Formation, southwestern Utah
Calcite