Nepheline

Nepheline
General
Category Silicate mineral
Chemical formula (Na,K)AlSiO4
Identification
Molar mass 146.08
Color White, Gray, Brown, Brownish gray, Reddish white
Crystal habit Massive granular to prismatic
Crystal system Hexagonal Space Group: P 63
Twinning On [1010], [3365], and [1122]
Cleavage [1010] Poor
Fracture Subconchoidal
Mohs scale hardness 6
Luster Vitreous - greasy
Streak White
Specific gravity 2.55 - 2.65, average = 2.59
Optical properties Uniaxial (-)
Refractive index nω = 1.529 - 1.546 nε = 1.526 - 1.542
Birefringence δ = 0.003 - 0.004
Other characteristics non-radioactive, non-magnetic, non-fluorescent
References [1][2][3]

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. It is often found in mica schist and gneiss.

Grayish-white nepheline crystals with dark schorlomite from Bou-Agrao Mt., Tamazeght complex, High Atlas Mts, Morocco (size: 6.0 x 4.4 x 3.8 cm)

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 dike-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 and stone; 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.

References


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Look at other dictionaries:

  • Néphéline — Catégorie IX : silicates[1] Général …   Wikipédia en Français

  • néphéline — ● néphéline nom féminin Aluminosilicate de sodium du système hexagonal, appartenant au genre feldspathoïde. (La néphéline se forme dans les roches magmatiques sous saturées en silice [syénite néphélinique, phonolite].) ⇒NÉPHÉLINE, subst. fém.… …   Encyclopédie Universelle

  • Nepheline — Néphéline Néphéline Catégorie IX : silicates Général …   Wikipédia en Français

  • nepheline — [nef′əlīt΄nef′ə lēn΄, nef′əlin] n. [Fr néphéline < Gr nephelē, cloud (see NEBULA) + ine, INE1] a hard, light colored, hexagonal mineral, (Na,K)AlSiO4, used in making glass, china, etc.; sodium potassium aluminum silicate: sometimes called… …   English World dictionary

  • Nepheline — Neph e*line (n[e^]f [ e]*l[i^]n), Nephelite Neph e*lite (n[e^]f [ e]*l[imac]t), n. [Gr. nefe lh cloud: cf. F. n[ e]ph[ e]line. Cf. {Nebula}.] (Min.) A mineral occuring at Vesuvius, in glassy hexagonal crystals; also elsewhere, in grayish or… …   The Collaborative International Dictionary of English

  • nepheline — /nef euh lin/, n. a feldspathoid mineral, essentially sodium aluminum silicate, NaAlSiO4, occurring in alkali rich volcanic rocks. Also, nephelite /nef euh luyt /. [1805 15; < F néphéline, equiv. to néphél ( < Gk nephéle cloud) + ine INE2] * * *… …   Universalium

  • nepheline — noun A feldspathoid mineral of silica poor igneous, plutonic and volcanic rocks. Chemically, Nepheline is a plagioclase feldspar with insufficient silica to satisfy the chemical bonds. Because of the unfilled bonds, Nepheline weathers rapidly and …   Wiktionary

  • nepheline — also nephelite noun Etymology: French néphéline, from Greek nephelē cloud more at nebula Date: circa 1814 a hexagonal mineral that is a usually glassy crystalline silicate of sodium, potassium, and aluminum common in igneous rocks • nephelinic… …   New Collegiate Dictionary

  • nepheline — [ nɛf(ə)lɪn] noun a colourless, greenish, or brownish aluminosilicate mineral occurring in igneous rocks. Origin C19: from Fr. néphéline, from Gk nephelē cloud (because its fragments become cloudy on immersion in nitric acid) + ine2 …   English new terms dictionary

  • nepheline — nefelinas statusas T sritis chemija apibrėžtis Mineralas. formulė KNa₃[AlSiO₄]₄ atitikmenys: angl. nepheline rus. нефелин …   Chemijos terminų aiškinamasis žodynas

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