Nephiline
Nepheline, also called
nephelite (from Greek: νεφέλη, "cloud"),
is a feldspathoid: a
silica-undersaturated aluminosilicate,
Na3KAl4Si4O16, that occurs in intrusive
and volcanic rocks with low silica, and
in their associated pegmatites.
Nepheline crystals are rare
and belong to the hexagonal system,
usually having the form of a short,
six-sided prism terminated by the basal
plane. The unsymmetrical etched figures
produced artificially on the prism faces
indicate, however, that the crystals are
hemimorphic and tetartohedral, the only
element of symmetry being a polar hexad
axis. It is found in compact, granular
aggregates, and can be white, yellow,
gray, green, or even reddish (in the
eleolite variety). The hardness is 5.5 -
6, and the specific gravity 2.56 - 2.66.
It is often translucent with a greasy
luster.
The low index of refraction and the
feeble double refraction in nepheline
are nearly the same as in quartz; but
since in nepheline the sign of the
double refraction is negative, while in
quartz it is positive, the two minerals
are readily distinguished under the
microscope. An important determinative
character of nepheline is the ease with
which it is decomposed by hydrochloric
acid, with separation of gelatinous
silica (which may be readily stained by
coloring matters) and cubes of salt. For
this reason, a clear crystal of
nepheline becomes cloudy when immersed
in acid.
Although sodium and potassium are always
present in naturally occurring nepheline
in approximately the atomic ratio (3:1),
artificially prepared crystals have the
composition NaAlSiO4; the corresponding
potassium compound, KAISiO4, which is
the mineral kaliophilite, has also been
prepared artificially. It has therefore
been suggested that the orthosilicate
formula, (Na,K)AlSiO4, represents the
true composition of nepheline.
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