feldspathoid
Any of a group of silicate minerals resembling feldspars but containing less silica. Examples are nepheline (NaAlSiO4 with a little potassium) and leucite (KAlSi2O6). Feldspathoids occur in igneous rocks that have relatively high proportions of sodium and potassium. Such rocks may also contain alkali feldspar, but they do not generally contain quartz because any free silica would have combined with the feldspathoid to produce more feldspar instead.
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