leucite An important feldspathoid mineral KA1Si₂O₆; sp.gr.2.5; hardness 5.5–6.0; tetragonal; normally white or grey; vitreous lustre; crystals form very characteristic tetragonal trioctohedra; cleavage imperfect {110}; occurs in potassium-rich rocks which may be silica deficient, such as leucite basanites, leucite tephrites, lamproites, and leucitophyres or leucitites.

leucite Grey or white feldspar mineral, a potassium aluminium silicate, KAl(SiO₃)₂. Unstable at high pressures, it can be found in potassium-rich lava flows and volcanic plugs. Hardness 5.5–6; r.d. 2.5.