Polyhalite

Polyhalite
Museum specimen of polyhalite and anhydrite
General
CategorySulfate mineral
Formula
(repeating unit)
K2Ca2Mg(SO4)4·2H2O
IMA symbolPlhl[1]
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[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 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.

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Polyhalite on Mindat.org
  3. ^ Polyhalite data on Webmin
  4. ^ Handbook of Mineralogy