Polyhalite

Polyhalite
Museum specimen of polyhalite and anhydrite
General
Category	Sulfate mineral
Formula (repeating unit)	K2Ca2Mg(SO4)4·2H2O
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	[1][2][3]

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 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.^[1] 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.^[4][3]

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.

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. 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 as Polysulphate. In 2016, Sirius Minerals announced plans for the Woodsmith Mine, a new polyhalite mine in the area.^[5] In March 2020, the project was taken over by Anglo American plc.

Composition and use[]

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

Crystal structure of polyhalite

References[]