Nepheline
Nepheline is one of the members of the Feldspathoid group of minerals that includes Cancrinite, Haüyne, Lazurite, Leucite, Nepheline and Sodalite. The chemical formula of Nepheline is (Na, K)AlSiO4 and its molecular weight is 146.08 gm. Minerals with chemistries that are close to those of the alkalai Feldspars, but are poor in silica (SiO2) content, are called Feldspathoids. Nepheline is an extremely rare gem because clean, facetable crystals are very rare and usually very small. The hardness is 5.5 – 6, and the specific gravity 2.55 – 2.66. It is often translucent with a greasy lustre. Occasionally, opaque cabochons can be found in colors of red, green, brown or grey, some with minute inclusions that cause a cats eye effect. Colored varieties of Nepheline are called Elaeolite. Eli elite is named from the Latin word for oil because of its greasy lustre. The main source of facet grade crystals is Davis Hill, Mont Saint-Hilaire, Bancroft, Quebec, Canada.
| Category: | Tectosilicate, feldspathoid |
| Chemical Formula: | (Na,K)AlSiO4 |
| Crystallography: | Hexagonal – Pyramidal |
| Molecular Weight: | 146.08 gm |
| Crystal Habit: | Crystals are stout six- or twelve-sided prisms, terminated, typically poorly formed, to 10 cm. Also granular, compact, massive. |
| Twinning: | On [1010], [3365], and [1122] |
| Cleavage: | [1010] Poor, [0001] Poor |
| Fracture: | Subconchoidal |
| Tenacity: | Brittle |
| Hardness (Mohs): | 5.5 – 6.0 |
| Density: | 2.55 – 2.66 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Barely Detectable, GRapi = 95.15 (Gamma Ray American Petroleum Institute Units) |
| Color: | Colorless, White, Gray, Yellowish; variously colored by impurities |
| Transparency: | Transparent to nearly Opaque, from inclusions or alteration |
| Luster: | Vitreous to Greasy |
| Refractive Index: | 1.526 – 1.546 Uniaxial ( – ) |
| Birefringence: | 0.0030 – 0.0040 |
| Dispersion: | Low |
| Pleochroism: | Visible; colorless to gray |
