FeldspathoidsMinerals occurring in low-Si terrestrial rocks, feldspathoids are similar to feldspars in structure and properties, but contain about two-thirds as much silica as alkali feldspar. They do not coexist with quartz, because the excess of SiO2 converts feldspathoids into feldspar. The most common feldspathoids are: nepheline, NaAlSiO4, falsilite, KAlSiO4, and leucite, KAlSi2O6. Their compositions can be displayed using a tetrahedron with quartz at the apex (below). The ranges of observed substitutions are shown by the colored fields.

Some or all content above used with permission from J. H. Wittke.

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