Apophyllite designates a specific group of phyllosilicates, which are a category of minerals. Initially, this term referred to a single mineral but was redefined in 1978 to encompass a class of minerals with similar chemical compositions that form a solid solution series, including fluorapophyllite-(K), fluorapophyllite-(Na), and hydroxyapophyllite-(K). The term apophyllite is derived from the Greek word apophyllízo (ἀποφυλλίζω), meaning 'it flakes off,' highlighting the tendency of these minerals to disintegrate upon heating due to water loss. Exfoliation can also occur through acid treatment or simple abrasion. These minerals are commonly found as secondary formations within vesicles of basalt or other volcanic rocks.
