Apophyllite crystals. Apophyllite is a hydrothermal mineral found filling cavities in igneous rocks such as basalt. It is a general term for the three separate minerals fluorapophyllite, hydroxyapophyllite, and natroapophyllite. All apophyllites consist of hydrated calcium, potassium and sodium silicates, fluorides and hydroxides. The most common form has a tetragonal structure (shown here). Specimen found in Paterson, New Jersey, USA.