Polyhalite
| Polyhalite | |
|---|---|
|
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.
The only polyhalite mined in the world comes from a layer of rock over 1000m below the North Sea off the North Yorkshire coast in the UK. Deposited 260 million years ago, it lies 150-170m below the potash seam at the Cleveland Potash Boulby Mine.
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.
Polyhalite is used as a fertilizer since it contains four important nutrients and is low in chloride. It is an ideal natural source for all crops, especially brassicas, cereals, pulses, field vegetables, clover-rich grassland leys and silage crops.
Available in its natural form, polyhalite (marketed as Polysulphate™) has a low carbon footprint and low environmental impact. It is mined, crushed, screened and bagged, involving no chemical separation or other industrial processes. Being a natural mineral, it is suitable for its use in organic agriculture.
Polyhalite contains 4 essential plant nutrients:
- 48% SO3 as sulphate
- 14% K2O as from sulphate of potash
- 6% MgO as from magnesium sulphate
- 17% CaO as from calcium sulphate
