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.