Leonite
| Leonite | |
|---|---|
Leonite
|
|
| General | |
| Category | Sulfate mineral |
|
Formula (repeating unit) |
K2Mg(SO4)2·4H2O |
| Strunz classification | 7.CC.55 |
| Dana classification | 29.03.03.01 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/m |
| Unit cell | a = 11.78, b = 9.53 c = 9.88 [Å]; β = 95.4°; Z = 4 |
| Identification | |
| Formula mass | 366.69 g/mol |
| Color | White to colorless, yellow |
| Crystal habit | Tabular crystals |
| Twinning | {100} |
| Cleavage | none |
| Fracture | conchoidal |
| Mohs scale hardness | 2.5 - 3 |
| Luster | Vitreous or Waxy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.201 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.479 nβ = 1.482 nγ = 1.487 |
| Birefringence | δ = 0.008 |
| 2V angle | Measured: 90° Calc: 76° |
| Dispersion | none |
| Fusibility | easy |
| Other characteristics | Leonit, 钾镁矾, Leonita, Леонит, Kalium-Astrakanit, Kalium-Blödit |
| References | |
Leonite is a hydrated double sulfate of magnesium and potassium. It has the formula K2SO4·MgSO4·4H2O. The mineral was named after Leo Strippelmann, who was director of the salt works at Westeregeln in Germany. The mineral is part of the blodite group of hydrated double sulfate minerals.
Leonite has a bitter taste.
When leonite is analyzed for elements, it is usually contaminated with sodium and chloride ions, as it commonly occurs with sodium chloride.
In the mineral family of leonite, the lattice contains sulfate tetrahedrons, a divalent element in an octahedral position surrounded by oxygen, and water and univalent metal (potassium) linking these other components together. One sulfate group is disordered at room temperature. The disordered sulfate becomes fixed in position as temperature is lowered. The crystal form also changes at lower temperatures, so two other crystalline forms of leonite exist at lower temperatures.
The dicharged metal cation (magnesium) is embedded in oxygen octahedra, four from water around the equator, and two from sulfate ions at the opposite poles. In the crystal there are two different octahedral environments. Each of these octahedra are joined together by potassium ions and hydrogen bonds.
The sulfate occurs in layers parallel to the (001) surface. In the room temperature form, the sequence is ODODODODOD with O=ordered, and D=disordered. In the next form at lower temperatures, the disordered sulfate appears in two different orientations giving the sequence OAOBOAOBOAOBOAOB. At the lowest temperatures, the sequence simplifies to OAOAOAOAOAO.
The first phase transition happens at -4 °C. At 170 K (−103 °C), the crystals have space group I2/a, lattice parameters a = 11.780 Å, b = 9.486 Å, c = 19.730 Å, β = 95.23°, 8 formula per unit cell, and a cell volume of V = 2195.6 Å3. The c dimension and unit cell volume are doubled due to the presence of four sulfate layers rather than two as in the other forms. The next phase change happens at -153 °C. At 100 K (−173 °C), the space group is P21/a, a = 11.778 Å, b = 9.469 Å, c = 9.851 Å, β = 95.26°, 4 formula per unit cell, and a cell volume of V = 1094.01 Å3.
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