These minerals are very closely allied to the felspars in chemical composition and geological relations, but differ from them in crystal form and physical properties. They have a much more restricted distribution than the felspars, but have, nevertheless, an important bearing upon the classification of certain groups of rocks in which they occur.

1. Nepheline

Nepheline is a silicate of potash, soda, and alumina (Na, K)2 0(A1203, 2 Si02). It crystallizes in transparent and colourless six-sided prisms, of the hexagonal system. 11=5.5-6; sp. gr. = 2.6. It is an important constituent of certain lavas.

2. Leucite

Leucite is composed as follows: K20, A1203, 4 Si02, with some of the potash replaced by soda. It crystallizes in twentyfour-sided figures (trapezohedrons), which belong to the tetragonal system, but can be distinguished from the isometric only by very careful measurement. 11=5.5-5.6; sp. gr. = 2.44-2.56.