STILBITE covered with APOPHILLITE

Stilbite refers to a group of tectosilicate minerals within the zeolite family. Originally classified as a distinct mineral species until 1997, it was redefined by the International Mineralogical Association to encompass a series, which includes the specific types Stilbite-Ca and Stilbite-Na. Stilbite-Ca, the more prevalent variant, is a hydrous silicate composed of calcium, sodium, and aluminum, represented by the formula NaCa4(Si27Al9)O72·28(H2O). In contrast, Stilbite-Na features a higher sodium content compared to calcium. These two species appear identical to the naked eye, and the term stilbite continues to be used in contexts where detailed testing has not been conducted.

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.