Apophyllite is a beautiful mineral, with glossy, clear crystals that are well-formed and can grow quite large. It is one of the many sheet silicates (phyllosilicates) named so because they easily break into very thin “leaves”, plates or sheets. Their structure consists of alternating silicate sheets and layers of potassium, sodium, calcium, fluorine, and water. Apophyllite, however, has a unique structure. Most phyllosilicates have a silica backbone layer made up of interlocking 6-fold rings of silica tetrahedra with pseudo-hexagonal symmetry, while apophyllite’s T layer is made up of 4-fold and 8-fold rings of silica tetrahedra with genuine tetragonal symmetry.
Apophyllites are classified into three types based on their composition: KF, KOH and NaF. By far the most prevalent is KF (India). KOH is mostly found in South Africa. NAF is very rare.
Apophyllites are rather common across the world, being found in India, Germany, Canada, Norway, Scotland, Ireland, Brazil, Japan, and the United States, among other places. A large proportion of Apophyllites specimens are coming from the zeolite deposits in basalt quarries in India.
Apophyllites have been discovered in ancient lava and basalt flows. When the rock was molten, air bubbles produced solid chambers that the crystals grew in. Apophyllite can also be found in voids in limestones from the contact metamorphic zone.