nepheline

(-līt')
n.
A mineral of sodium-aluminum or potassium-aluminum silicate, occurring worldwide in igneous rocks and used in the manufacture of ceramics and enamels.

[From Greek nephelē, cloud (because its fragments become cloudy when placed in nitric acid).]

nephelinic neph'e·lin'ic (-lĭn'ĭk) adj.

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A mineral of variable composition: in its purest state, NaAlSiO₄; often nearly Na₃K(AlSiO₄)₄; but generally (Na, K, □, Ca, Mg, Fe²⁺, Mn,Ti)₈(Al, Si, Fe³⁺)₁₆O₃₂, where □ represents vacant crystallographic sites, and Ca, Mg, Fe²⁺, Mn, Ti, and Fe³⁺ are usually present in only minor or trace amounts. The most important variations in nepheline composition are due to crystalline solution of KAlSiO₄ (the mineral kalsilite), and substitution of □ for K. See also Silicate minerals.

The salient physical properties of nepheline are: a Mohs scale hardness of 5.5–6.0; a specific gravity between 2.56 and 2.67; a typically dark gray, light gray, or white color, but it can also be colorless (nepheline is colorless in petrographic thin section); and a vitreous or greasy luster. Nepheline occurs as simple hexagonal prisms or, more commonly, as isolated shapeless grains or irregular polycrystalline masses.

Nepheline is the most abundant feldspathoid mineral; it occurs in a wide variety of SiO₂-deficient (quartz-free) and alkali-rich volcanic, plutonic, and metamorphic rocks. In volcanic rocks, nepheline occurs chiefly as a primary mineral in phonolites, kenytes, and melilite basalts, and it is the characteristic mineral of nephelinites.

Both “pure” (processed) nepheline and nepheline syenite are used as raw materials for the manufacture of glass, various ceramic materials, alumina, pottery, and tile. See also Feldspathoid; Igneous rocks; Nephelinite.

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(Na,K) (Al,Si)
Hexagonal -- hexagonal pyramidal

Environment

Low-silica plutonic rocks and igneous rocks.

Crystal description

Flat hexagonal prisms; the volcanic crystals are very small and clear, with few modifications. Larger, coarse six-sided crystals with corroded surfaces have been found in pegmatitic nepheline syenite dikes. Usually as white grains in rock.

Physical properties

Colorless, white, gray, reddish, smoky. Luster greasy; hardness 5Ɖ-6, specific gravity 2.5-2.65; fracture subconchoidal; cleavage good prismatic. Brittle; transparent to translucent; often fluorescent orange-red in portions of the crystal.

Composition

Sodium, potassium, aluminum silicate (21.8% Na ₂ O, very little K ₂ O, 35.9% Al ₂ O ₃ , 42.3% SiO ₃ ).

Tests

Splinter rounds to a clear glass droplet, with a brilliant yellow sodium flame. A powder is easily soluble in hydrochloric acid, evaporating to make a silica gel.

Distinguishing characteristics

The easy fusibility, usual associates, and unusual luster distinguish it from all minerals except the even more fusible cryolite, which it resembles. It is softer than feldspar and quartz. The somewhat similar scapolite melts to a white glass with small bubbles. Old exposed surfaces of nepheline-bearing rocks always show pits where the nepheline has been dissolved out; feldspar seams cutting through such outcrops stand out in relief.

Occurrence

Crystals are found in the cavities in the limestone block thrown out by the volcano (Monte Somma) that preceded Vesuvius. Nepheline forms grains in plutonic rocks in Karelia (now Karelskaya), Russia, and at Bancroft, Ontario. Small glassy crystals may line cavities in uncommon low-silica volcanic rocks. Large coarse crystals are found in pegmatitic segregations in the Bancroft area where crystals to 6 in. (15 cm) or more across have been noted.

In recent years, nepheline has become an important glass and ceramic raw material. Nepheline-bearing rocks will not contain quartz and melt well. If enough silica were present, feldspar would have formed in place of the nepheline.

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