Leucite

Leucite (KAlSi₂O₆) is a potassium aluminum silicate placed by petrologists in the feldspathoid group, alongside nepheline, sodalite, hauyne and lazurite. Feldspathoids are minerals with chemistries near those of the alkali feldspars but poor in silica content. Leucite crystallises with a cubic structure at high temperature around 900°C, but upon cooling to 700-600°C it transforms into a tetragonal modification stable at room temperature, forming characteristic polysynthetic twin lamellae, a transformation that is reversible. Its most common habit is trapezohedral crystals, though it can also occur granular or as euhedral, pseudo-cubic crystals, and twinning is common though hard to see because of leucite's low birefringence. The name comes from the Greek "leucos" for white, describing its typical white-to-grey, vitreous crystals, which register 5.5-6 on the Mohs scale with poor cleavage and conchoidal fracture.

Mount Vesuvius and Monte Somma in Campania form the historic type locality, with further fine Italian material from the Alban Hills near Rome (Villa Senni, Frascati, Ariccia), Roccamonfina, and Vulcano in the Lipari Islands. Beyond Italy, notable leucite localities include the Laacher See and Eifel district of Germany; the Leucite Hills of Sweetwater County, Wyoming, the Bear Paw Mountains of Montana, and Magnet Cove, Arkansas in the USA; the West Kimberley district of Western Australia; Kilimanjaro in Tanzania; and the Ruwenzori and Kariya areas of Uganda. Mount Nyiragongo in Congo's Kivu Province adds another classic African source. These occurrences are hosted in potassium-rich, silica-undersaturated volcanic rocks such as leucitite, tephrite, and leucite-phonolite, where leucite crystallizes with clinopyroxene, sanidine, and nepheline.

Collectors prize leucite for its sharp, pseudo-icositetrahedral crystals perched on dark lava matrix, a striking contrast rarely matched among rock-forming silicates. Although abundant in various lava rocks, leucite is extremely rare in gem-quality form and often has a milky or cloudy look, so faceted stones and well-shaped crystals are especially prized by collectors. To date only Italy has produced transparent, facetable leucite, with the Alban Hills near Rome yielding colorless crystals up to about 1 cm. Fine matrix specimens showing the mineral's diagnostic twinning and pseudo-cubic outline, alongside associated clinopyroxene, sanidine and titanaugite, remain a benchmark for systematic feldspathoid and Vesuvian mineral collections.