Potassium magnesium sulfate is a double salt that can crystallise in a number of forms that occur as minerals.
- Langbeinite K<sub>2</sub>Mg<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>
- Leonite K<sub>2</sub>Mg(SO<sub>4</sub>)<sub>2</sub>÷4H<sub>2</sub>O
- Picromerite K<sub>2</sub>Mg(SO<sub>4</sub>)<sub>2</sub>÷6H<sub>2</sub>O