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.
Properties 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.
Crystal structure 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 divalent 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.
Phase changes 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.
Temperature effects As temperature increases, the cell volume gradually increases for the I2/a and C2/m phases; however, the a dimension decreases with increasing temperature. The change in a dimension is −11×10−6 K−1. Birefringence drops as temperature rises. It varies from 0.0076 at −150 °C down to 0.0067 at 0 °C and 0.0061 at 100 °C. At the lower phase transition, birefringence steps down as the temperature drops; for the upper phase transition, it is continuous but not constant. At the upper phase transition, −4 °C, latent heat is released, and the heat capacity changes. This transition has a fair bit of hysteresis. At the lower phase transition, heat capacity stays the same, but latent heat is released. Leonite starts to lose water at 130 °C, but only really breaks down at 200 °C:
K2Mg(SO4)2·4H2O(s) → K2Mg(SO4)2·2H2O(s) + 2H2O(g). At even higher temperatures, langbeinite and arcanite (anhydrous potassium sulfate) and steam are all that remain:
2K2Mg(SO4)2·4H2O(s) → K2Mg2(SO4)3(s) + K2SO4(s) + 8H2O(g).
Other physical properties The logarithmic solubility product Ksp for leonite is −9.562 at 25 °C. The equilibrium constant log K at 25 °C is −3.979. The chemical potential of leonite is μj°/RT = −1403.97. Thermodynamic properties include ΔfGok = −3480.79 kJ mol−1; ΔfHok = −3942.55 kJ mol−1; and ΔCop,k = 191.32 J K−1 mol−1. The infrared spectrum of sulfate stretching modes shows peaks in absorption at 1005, 1080, 1102, 1134 and 1209 cm−1. Sulfate bending mode causes a peak at 720, and lesser peaks at 750 and 840 cm−1. An OH stretching mode absorbs at 3238 cm−1. When temperatures reduce, the peaks move and/or narrow, and additional peaks may appear at phase transitions. When leonite is stored for exhibition, it must not be in a place with too much humidity, otherwise it hydrates more.
Formation Starting in 1897, Jacobus Henricus van 't Hoff investigated how different salts were formed as sea water evaporated in different conditions. His purpose was to discover how salt deposits are formed. His research formed the basis for the studies of the conditions in which leonite is formed. Leonite can form when a water solution of potassium sulfate and magnesium sulfate is concentrated between the temperature range of 320–350 K (47–77 °C). Above this temperature range, langbeinite (K2Mg2(SO4)3) is formed. Below 320 K (47 °C), picromerite (K2Mg(SO4)2·6H2O) crystallises. For solutions with more than 90% proportion MgSO4, hexahydrite (MgSO4·6H2O) crystallises preferentially, and below 60%, arcanite (K2SO4) forms. In mixtures of potassium chloride, potassium sulfate, magnesium chloride and magnesium sulfate at 35 °C in water, leonite can crystallise out in a certain composition range. The plot of the system forms boundaries of leonite with potassium chloride, potassium sulfate, and picromerite. As magnesium is enriched, a quadruple point with kainite exists. In salt (NaCl) saturated brine, leonite can be deposited from magnesium and potassium sulfate mixtures as low as 25 °C. The 25 °C isotherm of the system has leonite surrounded by sylvine, picromerite, astrakanite, epsomite, and kainite. Sodium chloride saturated brines are formed by seawater evaporation, though seawater does not contain enough potassium to deposite leonite this way. Leonite is precipitated in series solar ponds at the Great Salt Lake. When picromerite is heated to between 85 and 128 °C, it gives off steam to give leonite:
K2Mg(SO4)2·6H2O(s) → K2Mg(SO4)2·4H2O(s) + 2H2O(g).
Reactions When leonite is dissolved in nitric acid and then crystallised, an acid potassium magnesium double sulfate is formed: KHMg(SO4)2·2H2O. Leonite heated with hydrated magnesium sulfate in an equimolar ratio at 350 °C produces langbeinite:
K2Mg(SO4)2·4H2O(s) + MgSO4·xH2O(s) → K2Mg2(SO4)3(s) + (4 + x)H2O(g). Potassium chloride solution can convert leonite to solid potassium sulfate:
2KCl(aq) + K2Mg(SO4)2·4H2O(s) → 2K2SO4(s) + MgCl2(aq). More potassium sulfate can be precipitated by adding ethylene glycol. Fluorosilicic acid in water reacts with leonite to produce insoluble potassium fluorosilicate and a solution of magnesium sulfate and sulfuric acid:
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