Polyhalite

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For polyhalide, see Polyhalogen ions.
Polyhalite
History museum of Truskavets 081.jpg
Museum specimen of polyhalite and anhydrite
General
Category Sulfate mineral
Formula
(repeating unit)
K2Ca2Mg(SO4)4·2H2O
Strunz classification 07.CC.65
Crystal system Triclinic (pseudo-orthorhombic)
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
Crystal symmetry Triclinic pinacoidal Space group: P1
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 is from the Greek polys hals for many salts.[3]

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

Production[edit]

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 Ltd as Polysulphate.

Composition and use[edit]

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[edit]