LEUCITITE

extrusive igneous rock, coloured ash gray to nearly black, that contains leucite and augite as large, single crystals (phenocrysts) in a fine-grained matrix (groundmass) of leucite, augite, sanidine, apatite, sphene, magnetite, and melilite; in this regard it is similar to nephelinite (q.v.), which contains nepheline in place of leucite. Leucitites are rare rocks and are known mostly from Tertiary or Recent strata; hence, they are generally younger than 66,400,000 years. Perhaps the best known occurrence is near Rome, where leucite lavas are thinly spread from Mt. Vesuvius, 200 kilometres (125 miles) south of the city, to Lago (lake) di Bolsena, 80 kilometres (50 miles) north. Other occurrences include the Mufumbiro region, Uganda; the West Kimberley region, Australia; and the eastern slope of the Rocky Mountains, U.S. Like the nephelinites, leucite-rich basalts are divided according to their mineralogical composition: leucitite contains no olivine or plagioclase; leucite-basalt contains olivine but no plagioclase; leucite-tephrite contains plagioclase but no olivine; and leucite-basanite contains both plagioclase and olivine. In all other respects these rocks are similar. With an increase in the nepheline content, leucite-rich basalts pass over into the nepheline-rich varieties, as at Hamberg, near Bhne, Ger.