leucite

(li’sit), n.— [So called from its white- ne ; ¢ Gr. Aevkdc, white, + -ite2.] A mineral originally found in the recent voleanic rocks of southern Italy, especially at Vesuvius, di eminated through the lavas in crystals, usually trapezohedrons, or in irregular ma es. It has also been observed similarly a ociated in some other regions, as the Eifel in Rhenish Pru ia, the Leucite Hills of Wyoming, etc.; but it is in general of very limited occurrence. It is a silicate of aluminium and pota ium, and has a white or grayish color. It was very early called white garnet, from its similarity to garnet in crystalline form; and it is also called amphigene. Leucite has excited much interest because of the phenomenon of double refraction which its crystals exhibit, this being at variance with the usually accepted isometric form. On account of these “optical anomalies,” and because also of certain variations in external form, it has been referred to the tetragonal (or orthorhombic) system. Recent investigations have shown, however, that at a temperature of 500° C. it becomes isotropic, and hence it is inferred that when formed it was normally isometric, and that the observed variations in form and optical character have resulted from subsequent molecular changes.—Leucite rocks, a series of rocks closely allied to basalt, but containing leucite in the place of feldspar. These rocks are for the most part, so far as known, of very modern origin. They are particularly well developed in southern Italy and the Hifel. See _leucitophyre, phonolite, and tephrite.