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 halite (the naturally occurring form of table salt) and it, 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 Ltd as polysulphate. In 2016, Sirius Minerals plc announced plans for a new polyhalite mine in the area.[5]

Composition and use

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

Crystal structure of polyhalite

References

  1. 1 2 Polyhalite on Mindat.org
  2. ↑ Polyhalite data on Webmin
  3. 1 2 Handbook of Mineralogy
  4. ↑
  5. ↑ Topf, Andrew (1 September 2016). "Banks backstop Sirius Minerals for $2.9B UK potash mine". mining.com. Retrieved 1 December 2016.
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