Nepheline (), also called nephelite, is a rock-forming mineral in the feldspathoid groupa silicon dioxide-undersaturated aluminosilicate (sodium3potassiumaluminium4silicon4oxygen16) that occurs in intrusive rock and with low silica, and in their associated . It is used in glass and ceramic manufacturing and other industries, and has been investigated as an ore of aluminium.
Description and properties
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.
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The crystals appear to have more symmetry than they actually possess,
but unsymmetrical etched figures produced artificially on the prism faces indicate that the crystals are and , the only element of symmetry being a polar hexad axis.
Nepheline is found in compact, granular aggregates, and can be white, yellow, gray, green, or reddish. Its
hardness on the Mohs scale is 5.5 to 6, and its
specific gravity 2.60–2.65. It is often translucent with a greasy luster.
The low refractive index and the feeble birefringence in nepheline are nearly the same as in quartz; but since the sign of the double refraction is negative in nepheline, while it is positive in quartz, the two minerals are readily distinguished under the microscope.
The mineral is prone to alteration to (especially natrolite), sodalite, kaolin, or compact muscovite.
Structure and composition
The
aluminosilicate backbone of nepheline has a fairly open structure of interlocked six-member rings. This resembles the structure of
tridymite, with aluminum substituting for every other silicon atom. This structure produces one nearly hexagonal interstitial site and three irregular interstitial sites per unit cell. In ideal nepheline, the hexagonal sites are occupied by
potassium ions and the irregular sites by smaller sodium ions, yielding an atomic ratio of sodium to potassium of 3:1. This corresponds to an ideal weight percentage of K
2O of 8.1% The range of compositions seen in natural nepheline is 3% to 12% K
2O. Small amounts of calcium may be present as well.
At elevated temperature, nepheline forms a complete solid solution series with kalsilite, KAlSiO4. At temperatures below about , there is a wide miscibility gap between nepheline and kalsilite, similar to the miscibility gap between microcline and albite. A composition falling in this gap will experience exsolution as it cools, where nepheline and kalsilite separate into separate microscopic layers (lamellae).
Occurrence
Nephelinite is a rock-forming mineral found in silica-poor
. These include nepheline syenite,
foidite, and
phonolite. It is often found along with
leucite, sodalite, potassium feldspars, and sodium-rich
plagioclase,
, or
, but almost never in association with
quartz. Notable outcrops of nepheline-bearing rocks are found on the
Kola Peninsula; in Norway and South Africa; and at Litchfield, Maine; Magnet Cove, Arkansas; and Beemerville, New Jersey, in the United States.
found near Bancroft, Ontario contain large deposits of high-purity nepheline.
Elaeolite (a name given by M. H. Klaproth 1809, from Greek language words for oil ἔλαιον and stone λίθος; )
Uses
Because of its high
alumina content,
iron-free nepheline is valued for use in glass manufacturing in place of feldspar. Most of the nepheline used for this purpose comes from
Ontario. Nepheline produced as a byproduct of
apatite mining in the Kola Peninsula has found uses in ceramics, leather, rubber, textiles, wood, and the oil industry. It can be used as a filler in paints, plastics, foam rubber, and sorbent.
Nepheline of Kiya Shaltyr deposit (Kemerovo Region, Russia) is also used as a raw material for aluminium manufacturing.
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