The feldspathoids are a group of
tectosilicate mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...
s which resemble
feldspar
Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...
s but have a different structure and much lower
silica content. They occur in rare and unusual types of
igneous rocks, and are usually not found in rocks containing primary quartz. A notable exception where feldspathoids and quartz-bearing rocks are found together is the
Red Hill Syenite.
Foid, a contraction of the term ''feldspathoid'', is applied to any igneous rock containing up to 60% modal feldspathoid minerals. For example, a
syenite with significant
nepheline present can be termed a ''nepheline-bearing syenite'' or ''nepheline syenite'', with the term ''nepheline'' replaceable by any foid mineral. Such terminology is used in the Streckeisen (
QAPF) classification of igneous rocks.
Feldspathoid minerals
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*Sodalite Group
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References
Tectosilicates
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