Polyhalite

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Polyhalite
History museum of Truskavets 081.jpg
Museum specimen of polyhalite and anhydrite
General
CategorySulfate mineral
Formula
(repeating unit)
K2Ca2Mg(SO4)4·2H2O
Strunz classification7.CC.65
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupF1
Unit cella = 6.95 Å, b = 8.88 Å,
c = 6.95 Å; α = 104.06°,
β = 113.94°, γ = 101.15°; Z = 4
Identification
ColorColorless, white, gray; often salmon-pink to brick-red from included iron oxide
Crystal habitTypically fibrous, foliated, massive; rarely as tabular crystals; pseudo-orthorhombic
TwinningCharacteristically polysynthetic on {010}, {100}
CleavagePerfect on {101}; parting on {010}
FractureConchoidal
TenacityBrittle
Mohs scale hardness3.5
LusterVitreous to resinous
StreakWhite
DiaphaneityTransparent
Specific gravity2.78
Optical propertiesBiaxial (-)
Refractive indexnα = 1.546 - 1.548 nβ = 1.558 - 1.562 nγ = 1.567
Birefringenceδ = 0.021
2V angleMeasured: 60° to 62°
SolubilitySoluble 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[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. In 2016, Sirius Minerals plc announced plans for a new polyhalite mine in the area.[5]

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]

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