Nepheline

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.

The mineral is one especially liable to alteration, and in the laboratory various substitution products of nepheline have been prepared. In nature it is frequently altered to zeolites (especially natrolite), sodalite, kaolin, or compact muscovite. Gieseckite and liebenerite are pseudomorphs.

Two varieties of nepheline are distinguished, differing in their external appearance and in their mode of occurrence, being analogous in these respects to sanidine and common orthoclase respectively. Glassy nepheline has the form of small, colorless, transparent crystals and grains with a vitreous luster. It is characteristic of the later volcanic rocks rich in alkalis, such as phonolite, nepheline-basalt, leucite basalt, etc., and also of certain d**e-rocks, such as tinguaite. The best crystals occur with mica, sanidine, garnet, etc., in the crystal-lined cavities of the ejected blocks of Monte Somma, Vesuvius. The other variety, known as elaeolite, occurs as large, rough crystals, or more often as irregular masses, which have a greasy luster and are opaque, or at most translucent, with a reddish, greenish, brownish or grey color. It forms an essential constituent of certain alkaline plutonic rocks of the nepheline syenite series, which are typically developed in southern Norway.

The color and greasy luster of elaeolite (a name given by M. H. Klaproth 1809, from Greek words for oil [ἔλαίον] and stone [λίθος]; German: Fettstein) are due to the presence of numerous microscopic enclosures of other minerals, possibly augite or hornblende. These enclosures sometimes give rise to a chatoyant effect like that of cats-eye and cymophane; and elaeolite when of a good green or red color and showing a distinct band of light is sometimes cut as a gem-stone with a convex surface.

Wikipedia source:  https://en.wikipedia.org/wiki/Nepheline

Of course we would more likely have something like this here in NC.
http://www.mindat.org/photo-319925.html

Scott, Remember that Apatite crystal you found at the Haw River Quarry?
Nepheline and Apatite seem to be closely related, but I have to read more about that.

ya know, this reminds me of a rock I found one time out in Chatham County that I thought was interesting enough to haul
home and take a closer look at. (Imagine that!) It had two different layers next to each other and I remember wondering if one was nepheline, or some feldspathoid.
I saw this rock recently while putting in some tulip bulbs, so I know right where it is, but found some pics I took before planting it.
First angle shows one thin, 5-10mm, thick layer of.........?
The second angle shows the layer that probably has nothing to do with nepheline at all.
Our plutonic rock is very alkaline around here,tho, and this looks intrusive?
I guess I'll have to dig it up and break a piece off of it now,