Polyhalite

Polyhalite
Museum specimen of polyhalite and anhydrite
General
Category	Sulfate mineral
Formula	K2Ca2Mg(SO4)4·2H2O
IMA symbol	Plhl[1]
Strunz classification	7.CC.65
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	F1
Unit cell	a = 6.95 Å, b = 8.88 Å, c = 6.95 Å; α = 104.06°, β = 113.94°, γ = 101.15°; Z = 4
Identification
Color	Colorless, white, gray; often salmon-pink to brick-red from included iron oxide
Crystal habit	Typically fibrous, foliated, massive; rarely as tabular crystals; pseudo-orthorhombic
Twinning	Characteristically polysynthetic on {010}, {100}
Cleavage	Perfect on {101}; parting on {010}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3.5
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.78
Optical properties	Biaxial (−)
Refractive index	nα = 1.546 – 1.548 nβ = 1.558 – 1.562 nγ = 1.567
Birefringence	δ = 0.021
2V angle	Measured: 60° to 62°
Solubility	Soluble in water, with precipitation of gypsum and perhaps syngenite
References	[2][3][4]

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 valuable fertilizer.

Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria.^[2] 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. The only deposit currently being mined lies under North Yorkshire, UK, extending under the adjacent North Sea.

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.^[5][4]

Despite the similarity in names between polyhalite 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.

The only polyhalite mined in the world comes from a layer of rock over 1,000 m (3,300 ft) below North Yorkshire and extending off the North Sea coast in the UK, deposited 260 million years ago.

In 2010, the first mining operations of this polyhalite mineral layer commenced at Boulby Mine, and the mine is in 2023 the only producer of polyhalite, marketed by ICL Group Ltd. as Polysulphate. In 2016, Sirius Minerals announced plans for the Woodsmith Mine in the area.^[6] In March 2020, the Woodsmith project was taken over by Anglo American plc and construction of two 1,500 m (4,900 ft) shafts to reach the 70 m (230 ft) mineral seam was underway in 2015. These will reach a mineable area of around 25,200 ha (62,000 acres) and Woodsmith Mine will be the deepest in Europe.^[7] The Woodsmith project was originally slated to begin production in 2027; development has since been slowed, and Anglo American now expects to ramp up construction from 2027, with a new first production date yet to be announced.^[8][9][10]

Polyhalite is used as a fertilizer since it contains four important nutrients and is low in chloride:

• 48% SO₃ as sulfate
• 14% K₂O as from sulfate of potash
• 6% MgO as from magnesium sulfate
• 17% CaO as from calcium sulfate

Polyhalite has a variety of other uses, including:

Soil amendment: Polyhalite can help to improve the drainage and fertility of soil. It can also help to reduce the acidity of soil.

Water treatment additive: Polyhalite can help to remove impurities from water, such as sulfates and chlorides. It can also help to soften water and make it less corrosive.

Animal feed additive: Polyhalite can be added to animal feed to improve the nutritional value of the feed.

Industrial applications: Polyhale can be used in a variety of industrial applications, such as making glass, soap, and paper.