Description:
Nepheline is a major rock forming mineral that is not
often sold in rock shops due to a lack of good crystals
or attractive specimens. It is a major component of
several igneous rocks called nepheline syenite, nepheline
monzonite and nephelinite. The basic difference between
these is in the amount and types of feldspars present.
In nepheline syenite potassium feldspars or K-spars
are the predominant feldspar. In the nepheline monzonite
rocks both k-spars and plagioclase feldspars are present
in near equal proportions. And finally in the nephelinites
there is little of any of the feldspars present and
the rock is mostly nepheline.
The formula of nepheline in some sources will list it
as NaAlSiO4. There are very few natural nephelines that
have this "pure" chemistry although it produces
a stable structure and it is manufactured for use in
ceramics and glass production. Potassium is always present
in some amounts and often the chemical analysis of nepheline
will approach Na3K(AlSiO4)4. This result reflects the
fact that the alkali sites for the sodiums and potassiums
have an interesting difference in the amount of space
within the nepheline structure. There is actually one
site out of four that is larger than the other three
sites. This larger site is a more comfortable fit for
the larger potassium ion. Nepheline is a member of the
feldspathoid group of minerals. Minerals whose chemistries
are close to that of the alkali feldspars but are poor
in silica (SiO2) content, are called feldspathoids.
As a result or more correctly as a function of the fact,
they are found in silica poor rocks containing other
silica poor minerals and no quartz. If quartz were present
when the melt was crystallizing, it would react with
any feldspathoids and form a feldspar. Localities that
have feldspathoids are few. Nepheline is reactive to
acids although it does not bubble like many of the carbonates.
If powdered it will dissolve in hydrochloric acid and
if clear specimens are dipped in acid they will become
cloudy or frosted. This could be helpful in distinguishing
nepheline from some similar looking feldspars, scapolite
and cryolite. The greasy luster of nepheline also is
diagnostic. Massive nepheline with a greasy luster is
given the variety name "eleolite" which is
derived from the greek word for oil. Nepheline is derived
from the greek word for cloud in allusion to its cloudy
or translucent crystals and masses. |
