Leucite is a member of the feldspathoid group, forming directly from cooling lava in volcanic rock. It forms in low-silica environments where little Quartz is present. When Leucite forms at high temperatures, it crystallizes as isometric crystals in trapezohedral crystal form. As it cools, its crystal structure becomes unstable, and transforms into tetragonal form while retaining the original isometric shape. Though Leucite crystals are usually equant, the crystal faces may be slightly uneven due to the alteration.
When in the oxidized zone, Leucite often alters to Pseudoleucite, which is a chalky-white combination of Orthoclase, Nepheline, and Analcime. Extensive oxidation may further break it down into Kaolinite or clay.