Occurrence of Leucite in Volcanic Rocks from Tristan da Cunha

DURING recent petrological research on volcanic rocks collected by the Royal Society Tristan da Cunha Expedition 1 , many specimens of varying composition have been found to contain leucite. The mineral occurs interstitially, is weakly birefringent with a high negative relief, and displays poorly denned lamellar and cross-hatched twinning. The refractive index, determined in sodium light by the immersion method, is 1.507 ± 0.003. Although similar to analcime in thin section, the refractive index is much higher (analcime, n = 1.487) even allowing for the possibility of desilication 2 or the substitution of potassium for sodium ions 3 . Identification of the mineral as leucite was confirmed by the Department of Mineralogy, British Museum (Natural History), with four X-ray powder photographs (Nos. 9652, 9845, 9846 and 9847) on material picked from three rock crushes.