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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.
Localities
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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