Feldspathoids form a family of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and usually are not found in rocks containing primary quartz. A notable exception where feldspathoids and quartz-bearing rocks are found together is the Red Hill Syenite (See localities below).
Feldspathoids are common constituents of basalt.
They also may be found in diorite and gabbro.
Nepheline is the commonest feldspathoid, cancrinite and leucite are amongst the others.


At Red Hill, Moultonborough, Carroll County, New Hampshire, USA, the syenitic complex is an oval-shaped multiple intrusion. The country rock has been brecciated and contact-metamorphosed near the margin. Six major rock units—five of syenitic composition and one of granite—compose the complex. The oldest unit, the outer coarse syenite, consists mainly of perthite and ferrohastingsite, and is one of three concentrically arranged units. This rock type changes abruptly inward to a zone of nepheline-sodalite syenite. The central portion of the complex is comprised of younger, non-foliated syenite, which has been intruded by two small, plug-like bodies of quartz syenite, and one small intrusion of granite. The five syenitic rock units show clustered properties of being slightly silica-deficient and of high alkali and iron content. Their compositions are representative of a low-temperature residual magma which could yield both feldspathoidal- and quartz-bearing rocks (Geological Society of America Bulletin 83.12.3747–3760).

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