Lithium potassium sulfate

Formula: KLiSO₄

Properties

Crystal system: hexagonal! The rarest crystal system in my collection.
Crystal shape: straight faceted hexagonal prism.
Color: colorless.
Stability on air: stable (anhydrous)

Preparation

Double salt can be prepared by dissolving individual sulfates in water and crystallizing the solution:

K₂SO₄ + Li₂SO₄ = 2KLiSO₄

However, the double salt does not crystallize from a stoichiometric solution, containing equal amounts of both ions. According to the literature, double salt crystallizes at room temperature, when K₂SO₄ : Li₂SO₄ mass ratio lies between 0.05 and 0.4. This corresponds to molar ratio range from 0.03 to 0.43. Thus, more than twofold excess of Li₂SO₄ must be present in the solution in order to crystallize the double salt.

Growing

Slow evaporation method was used. Growing is complicated by the non-stoichiometric composition of the solution. Concentration of K gradually drops as the double salt crystallizes. When it reaches the lower bound (approximately when half of the solution has evaporated), elongated crystals of lithium sulfate start growing instead; they can ruin the seed.