Apophyllite

Apophyllite is a hydrated potassium calcium phyllosilicate mineral (KCa₄Si₈O₂₀(F,OH)·8H₂O), a sheet silicate prized for its glassy, often pyramid-tipped tetragonal crystals. Apophyllite crystallizes in volcanic rock cavities (vesicles) from low-temperature hydrothermal fluids - the same environment that produces zeolites, with which it almost always co-occurs. The Deccan Traps of India, massive basalt flows covering much of western India, provide ideal conditions for apophyllite formation.

The crystals grow as water-rich silicate solutions percolate through the porous basalt over millions of years. Individual crystals can reach remarkable transparency and size - Indian specimens with crystals exceeding 10 centimeters are not uncommon. The characteristic crystal form is a square prism terminated by a pyramid, giving crystals a distinctive geometric appearance.

Green apophyllite gets its color from trace iron, while the common colorless variety has exceptional transparency that rivals optical-grade calcite. The pearly luster on the basal cleavage face (the flat end of the crystal) is a distinctive identification feature.

Apophyllite is identified by its square prismatic crystal habit with pyramidal terminations, strong pearly luster on the basal face, and association with zeolite minerals in basalt cavities. At hardness 4.5-5, it can be scratched by a knife.

Distinguish from quartz (hexagonal, much harder at 7), calcite (rhombohedral, fizzes in acid), and fluorite (cubic, different cleavage). Apophyllite's combination of square cross-section, pearly basal cleavage, and occurrence with zeolites is diagnostic.