Feldspars

Feldspars are tectosilicates (q.v.) composed of SIO4 and AlO4 tetrahedra and various cations (Ba, K, Na, Ca). There are:

(1) Alkaline feldspars forming a continuous series at high temperatures and a discontinuous series at low temperatures, between orthoclase Si3AlO8K and albite Si3AlO8Na. The intermediate species are microcline and sanidine. Perthites are intergrowths of albite in orthoclase or microcline.

(2) Sodium-calcium feldspars, or plagioclases, forming a continuous series between albite and anorthite Si2Al2O8Ca. The intermediate species are oligoclase, andesine, labrador and bytownite.

(3) Heavy feldspars, which are rarer, are represented by celsian Si2Al2O8Ba and the hyalophanes (celsian + 60 at 90% orthose).