#tetragonal crystal
- Apophyllite — A group of minerals from the silicate class (specifically hydrated potassium calcium phyllosilicates containing fluorine), characterized by transparent or white tetragonal or pseudo-cubic crystals with a pronounced pearly or vitreous luster; a mineral belonging to this group. Its name derives from its specific tendency to exfoliate into flakes when heated under a blowpipe due to the loss of crystallization water.