Leucite

Leucite is common in some volcanic rocks in which it crystallises with a cubic crystal structure at high temperature (ca. 900°C). Upon cooling to 700-600°C, it transforms into a tetragonal modification which is stable at room temperature and forms characteristic polysynthetic twin lamellae. The transformation is reversible. The mineral is usually included as a feldspathoid by petrologists, as it is a common primary rock-forming mineral in some high-potassium rocks but, despite being anhydrous, structurally it also has a topological relation to zeolites, especially analcime, and thus is typically included in that family also.