Polyhalite
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| Polyhalite | |
|---|---|
Museum specimen of polyhalite and anhydrite
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| 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[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 as polysulphate. In 2016, Sirius Minerals announced plans for the Woodsmith Mine, 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