Peacor, Donald R. Department of Geology, University of Michigan, East Lansing, Michigan.
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A member of the feldspathoid group of minerals. Members of this group are characterized by the following related features: (1) All are aluminosilicates with one or more of the large alkali ions (for example, sodium, potassium) or alkaline-earth ions (for example, calcium, barium). (2) The proportion of aluminum relative to silicon, both of which are tetrahedrally coordinated by oxygen, is high. (3) Although the crystal structures of many members are different, they are all classed as tektosilicates; that is, they consist of a three-dimensional framework of aluminum and silicon tetrahedrons, each unit of which shares all four vertices with other tetrahedrons, giving rise to a ratio of oxygen to aluminum plus silicon of 2:1. Ions of other elements occupy interframework sites. (4) They occur principally in igneous rocks, but only in silica-poor rocks, and do not coexist with quartz (SiO2). Feldspathoids react with silica to yield feldspars, which also are alkali–alkaline-earth aluminosilicates. Feldspathoids commonly occur with feldspars.
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