a. A group of comparatively rare rock-forming minerals consisting of
aluminosilicates of sodium, potassium, or calcium and having too little
silica to form feldspar. Feldspathoids are chemically related to the
feldspars, but differ from them in crystal form and physical properties;
they take the places of feldspars in igneous rocks that are undersaturated
with respect to silica or that contain more alkalies and aluminum than can
be accommodated in the feldspars. Feldspathoids may be found in the same
rock with feldspars but never with quartz or in the presence of free
magmatic silica. See also: foid; lenad. AGI
b. A mineral of the feldspathoid group, including leucite, nepheline,
sodalite, nosean, hauyne, lazurite, cancrinite, and melilite.
Syn: feldspathide
Source:
Dictionary of Mining, Mineral, and Related Terms
