Category Sulfate mineral Strunz classification 7.CC.65 Space group F1 | Formula
(repeating unit) K2Ca2Mg(SO4)4·2H2O Crystal system Triclinic | |
 | ||
Crystal class Pinacoidal (1)
(same H-M symbol) | ||
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.
Contents
It occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad Deposits of New Mexico. It is also present as a 2-3% contaminant of Himalayan salt.
Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria. The name comes from the German Polyhalit, which comes from the Ancient Greek words πολύς (polys) and ἅλς (hals), which mean "many" and "salt", and the German ending -it (which comes from the Latin ending -ites, which originally also came from Greek), which is used like the English ending -ite to form the names of certain chemical compounds.
Despite the similarity in names between it and halite (the naturally occurring form of table salt), their only connection is that both are evaporite minerals. The use of the Greek words for many and salt in polyhalite is due to polyhalite consisting of several metals that can form salts in the more general sense of the word salt used in chemistry.
Production
The only polyhalite mined in the world comes from a layer of rock over 1,000 m (3,300 ft) below the North Sea off the North Yorkshire coast in the UK. Deposited 260 million years ago, it lies 150–170 m (490–560 ft) below the potash seam at the Boulby Mine. Sirius Minerals Plc is developing a new mine in the area.
In 2010, the first mining operations of the polyhalite mineral commenced at Boulby Mine. The mine is currently the only producer of polyhalite which is marketed by Israel Chemicals Ltd as Polysulphate.
Composition and use
Polyhalite is used as a fertilizer since it contains four important nutrients and is low in chloride: