Orthosilicates – Nephelite Composition, Crystallization & Structure

Composition. Sodium-aluminum silicate, approximately NaAlSi0₄. There is always a few per cent of potash present, sometimes also lime, replacing the soda.

Crystallization. Hexagonal. Rarely in small prismatic crystals with basal plane; sometimes shows pyramidal planes.

Structure. Almost invariably massive, compact, and in embedded grains. Massive variety often called eleolite.

Physical Properties. Distinct cleavage parallel to prism. H. = 5.5-6. G. = 2.55-2.65. Vitreous luster in the clear crystals to greasy luster in the massive variety. Colorless, white or yellowish. In the massive variety gray, greenish and reddish. Transparent to opaque.

Tests. Fusible at 4 to a colorless glass. B. B. gives strong yellow flame of sodium. Readily soluble in hydrochloric acid and on evaporation yields a silica jelly.

Alteration. Easily alters into various other minerals, such as the zeololites, natrolite, analcites, hydronephelite, thomsonite; also sodalite, muscovite, kaolin, ete. 

Occurrence. Nephelite is rarely found except in igneous rocks. It occurs in some recent lavas as glassy crystals, such as are found in the lavas of Vesuvius. The opaque, massive or coarsely crystalline variety is found in the older rocks and is called elreolite. Phonolite, nephelite-syenite and nephelite-basalt are important roeks in which nephelite is an essential constituent. It is only to be found in roeks whose magmas contained an excess of soda over the amount required to form feldspar. It is therefore seldom found in roeks that contain free quartz. Extensive masses of nephelite roeks, nephelite-syenites, are found in Norway. In crystals in the lavas of Vesuvius. Massive and crystallized nephelite is found at Litchfield, Maine, associated with cancrinite. Found near Magnet Cove, Arkansas.

Name. Nephelite is derived from a Greek word meaning a cloud, because when immersed in acid the mineral becomes cloudy. Elmolite is derived from the Greek word for oil, in allusion to its greasy luster.

Cancrinite, H₆Na₆.Ca(NaC0₃)₂,Al₈(Sio₄)₉, is a rare mineral similar to nephelite in occurrence and associations.