A hydrous calcium potassium silicate containing fluorine. The composition is variable but approximates to KFCa₄(Si₂O₅)₄· 8H₂O. It resembles the zeolites, with which it is sometimes classified, but differs from most zeolites in having no aluminum. It exfoliates (swells) when heated, losing water, and is named from this characteristic; the water can be reabsorbed. The mineral decomposes in hydrochloric acid, with separation of silica. It is essentially white, with a vitreous luster, but may show shades of green, yellow, or red. The symmetry is tetragonal and the crystal structure contains sheets of linked SiO₄ groups, and this accounts for the perfect basal cleavage of the mineral (see illus.). It occurs as a secondary mineral in cavities in basic igneous rocks, commonly in association with zeolites. The specific gravity of apophyllite is about 2.3–2.4, the hardness is 4.5–5 on Mohs scale, the mean refractive index is about 1.535, and the birefringence is 0.002.