A leucite crystal and crystal structure.
Leucite is a gray or white feldspar mineral – a potassium aluminum silicate (KAl(SiO3)2). Unstable at high pressures, it has a restricted occurrence. Leucite can be found in potassium-rich lava flows and volcanic plugs. It forms crystals having 24 equal faces. Hardness 5.5–6, relative density 2.5.