APOPHYLLITE (from Greek words "apo" (from) and "phyllon" (leaf), according to the ability to split into leaflets within the flame of the blow torch * EN: apophyllite, fisheye stone; DE: Apophyllit; FR: apophyllite; ES: apofilita; RU: апофиллит) is the rock-forming mineral of the sub-class of layered silicates, KCa4[Si4O10]2F•8H20. It usually contains the impurity of Na, which replaces part of K, and hydroxyl, which replaces fluorine during weathering. It crystallizes into the tetragonal crystal system; the crystal structure is sub-layered. It forms prismatic (up to acicular ones), isometric, lamellar, and other crystals, drusen, foliated and columnar aggregates, dense masses. Pure apophyllite is colourless, watery-transparent, changed one is white. The impurities colorate apophyllite with yellowish, greenish, reddish colours, which are often distributed sectorally. The cleavage is very perfect along one direction. The hardness is 4.5-5. The density is 2350±20 kilograms per cubic metre.

Apophyllite is the low-temperature hydrothermal mineral. It is typical for amygdaloidal basic effusives (including traps), within the voids of which it may be found together with zeolites, Iceland spar, agate, and so on, less often it may be found within the voids and along the cracks within granites, nephelinic syenites, metamorphic rocks, limestones and skarns, and also within the certain ore deposits. It is known within the modern depositions of mineral water sources. Beautiful lumps and drusen of apophyllite are valuable collectible material.