Apophyllite - Encyclopedia

    Class : Silicates
    Subclass : Phyllosilicates
    Crystal System : Tetragonal
    Chemistry : (K,Na)Ca₄Si₈O₂₀(OH,F) . 8H₂O
    Rarity : Uncommon

Apophyllite is actually a group of 3 minerals with similar compositions : fluorapophyllite, hydroxyapophyllite and natroapophyllite. Despite a high water content, its particular structure removes it from zeolites to which it is sometimes wrongly related. The minerals in this group are secondary and present in the vacuoles of the basaltic rocks that provided the finest and largest crystals in the world. Apophyllite is also present in hydrothermal veins and as a final hydrothermal alteration in skarns. It is also reported in the travertines of thermal springs. It owes its name to the Greek apophullos (which loses its leaves) because of its exfoliation under the heat of a blowtorch, its pearly white appearance, a little similar to boiled fish eyes, had formerly been called ichthyophthalmite (Greek words for "fish" and "eye"). Apophyllite occurs in different forms : in tetragonal prismatic crystals, usually short, terminated by an octahedron rarely clustered in flowers or balls, less often in flattened tabular crystals. The faces of the prism are striated vertically. It is also found to be massive and lamellar in foliaceous or grayish aggregates of a white or greyish color, sometimes pink or water green (fluorapophyllite). It is a mineral exclusively destined to the collection, which forms sets of aesthetic and sought-after crystals.

A twin is known on {111} but it is rare.

Assemblages and collages are common for this species due to its overabundance in the Indian mineral market.