FELDSPATHOID

any of a group of alkali aluminosilicate minerals similar to the feldspars in chemical composition but either having a lower silica-alkali ratio or containing chloride, sulfide, sulfate, or carbonate. The two most abundant feldspathoids are nepheline and leucite. Other significant varieties include kalsilite, sodalite, nosean, and haynite. The physical and chemical properties of the feldspathoids lie between those of the feldspar and zeolite groups. The feldspathoid structure consists of oxygen tetrahedrons with silicon and aluminum atoms at their centres. Each oxygen atom is shared by the SiO4 and AlO4 tetrahedrons, forming a three-dimensional network or framework structure typical of tectosilicates. Alkali ions as well as carbonate, sulfate, and chloride ions occupy the holes (i.e., cavities) between the oxygen atoms. Feldspathoids are found chiefly in igneous and metamorphic rocks. They commonly occur in place of feldspars in alkali-rich, silica-poor rocks. Some of them have commercial importance; they are used as raw materials in the production of alum, glass, and ceramics. any of a group of alkali aluminosilicate minerals similar to the feldspars in chemical composition but either having a lower silica-alkali ratio or containing chloride, sulfide, sulfate, or carbonate. They are considered to be the specific minerals of igneous rocks usually termed alkalic, which is the designation applied to igneous rocks whose alkali content (i.e., amount of sodium and/or potassium ) exceeds the amount required by the available silica to form one or more feldspars plus or minus mica. Minerals of the feldspathoid group whose silica contents are less than those of their feldspar analogues include nephelin, leucite, sodalite, and cancrinite. Additional reading William L. Brown (ed.), Feldspars and Feldspathoids: Structures, Properties, and Occurrences (1984). R.V. Dietrich