leucite

{{Infobox mineral | name = Leucite | boxwidth = | boxbgcolor = | image = Leucite in rock w- nepheline Potassium aluminum silicate Albano Hills Italy 1927.jpg | imagesize = | alt = | caption = Leucite crystals in a rock from Italy | category = Tectosilicate minerals | group = Feldspathoid group | formula = KAlSi2O6 | IMAsymbol = Lct | strunz = 9.GB.05 | dana = | system = Tetragonal | class = Dipyramidal (4/m) (same H-M symbol) | symmetry = I41/a | unit cell = a = 13.056, c = 13.751 [Å]; Z = 16 | molweight = | color = White to grey | colour = | habit = Commonly as euhedral, pseudocubic crystals; rarely granular, massive | twinning = Common and repeated on {110} and {101} | cleavage = Poor on {110} | fracture = Conchoidal | tenacity = Brittle | mohs = 5.5–6 | luster = Vitreous | streak =White | diaphaneity = Transparent to translucent | gravity = 2.45–2.50 | density = | polish = | opticalprop = Uniaxial (+) | refractive = nω = 1.508 nε = 1.509 | birefringence = δ = 0.001 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = }}

Leucite (from the Greek word leukos meaning white) is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by Gerhard vom Rath in 1873 led him to refer the crystals to the tetragonal system. Optical investigations have since proved the crystals to be still more complex in character, and to consist of several orthorhombic or monoclinic individuals, which are optically biaxial and repeatedly twinned, giving rise to twin-lamellae and to striations on the faces. When the crystals are raised to a temperature of about 500 °C they become optically isotropic and the twin-lamellae and striations disappear, although they reappear when the crystals are cooled again. This pseudo-cubic character of leucite is very similar to that of the mineral boracite. Leucite is commonly found in igneous rocks, especially at Mt. Vesuvius