Felspathoid Group
These minerals are very closely allied to the felspars in chemical composition, but differ from them in crystalline 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.
Nepheline - is a silicate of potash, soda, and alumina ( (Na, K)20, Al203, 2 Si02). It crystallizes in transparent and colorless six-sided prisms, of the hexagonal system. H = 5.5-6; sp. gr. 2.6. The mineral is soluble in hydrochloric acid, gelatinous hydrated silica separating out. It is an important constituent of certain lavas.
Leucite - is composed as follows: K20, AI203, 4 Si02, with some of the potash replaced by soda. It crystallizes in twenty-four-sided figures (trapezohedrons), which belong to the tetragonal system, but can be distinguished from the isometric only by very careful measurement. H = 5.5-5.6; sp. gr. = 2.44-2.56. It is slowly attacked by hydrochloric acid.
Leucite cannot be called a common mineral, but its significance will be better seen when we come to take up the study of rocks.
Analcite - This mineral is usually regarded as a decomposition product, and placed among the zeolites but recent investigations make it very probable that in some cases, at least, analcite is a mineral of primary origin. Its composition is:
Na20, Al203, 4 Si02, 2 H20. Crystallizes in the isometric system, and is colorless in transmitted .light. It is soluble in mineral acids, with separation of gelatinous silica. Sp. gr. = 2.15-2.28.