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NEPHELINE

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Originally appearing in Volume V19, Page 384 of the 1911 Encyclopedia Britannica.
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NEPHELINE , a See also:

rock-forming See also:mineral consisting of See also:sodium, See also:potassium and See also:aluminium silicate, Na6K2A18Si9O34. Its crystals belong to the hexagonal See also:system, and usually have the See also:form of a See also:short six-sided See also:prism terminated by the basal See also:plane. The unsymmetrical etched figures produced artificially on the prism faces indicate, however, that the crystals are See also:hemimorphic and See also:tetartohedral, the only See also:element of symmetry being a polar hexad See also:axis. The hardness is 5. The specific gravity (2.6), the See also:low See also:index of See also:refraction and the feeble See also:double refraction are nearly the same as in See also:quartz; but since in nepheline the sign of the double refraction is negative, whilst in quartz it is See also:positive, the two minerals are readily distinguished under the See also:microscope. An important determinative See also:character of nepheline is the ease with which it is decomposed by hydrochloric See also:acid, with separation of gelatinous See also:silica, (which may be readily stained by colouring matters) and cubes of See also:salt. A clear crystal of nepheline when immersed in acid becomes for this See also:reason cloudy; hence the name nepheline, proposed by R. J. See also:Hauy in 18or, from Gr. ve¢fXn, a See also:cloud. Although in naturally occurring nepheline sodium and potassium are always See also:present in approximately the atomic ratio 3 : I, artificially prepared crystals have the See also:composition NaAlSiO4; the corresponding potassium See also:compound, KAISiO4, which is the mineral kaliophilite, has also been prepared artificially. It has therefore been suggested that the orthosilicate See also:formula, (NaK)AlSiO4, represents the true composition of nepheline. The mineral is one specially liable to alteration, and in the laboratory various substitution products of nepheline have been prepared.

In nature it is frequently altered to See also:

zeolites (especially See also:natrolite), See also:sodalite, See also:kaolin, or compact See also:muscovite. Gieseckite and liebenerite are pseudomorphs. Two varieties of nepheline are distinguished, differing in their See also:external See also:appearance and in their mode of occurrence, being analogous in these respects to sanidine or glassy See also:orthoclase and See also:common orthoclase respectively. " Glassy nepheline " has the form of small, colourless, transparent crystals and grains with a vitreous lustre. It is characteristic of the later volcanic rocks See also:rich in alkalis, such as See also:phonolite, nepheline-See also:basalt, leucitebasalt, &c., and also of certain See also:dike-rocks, such as tinguaite. The best crystals are those which occur with See also:mica, sanidine, See also:garnet, &c., in the crystal-lined cavities of the ejected blocks of See also:Monte Somma, See also:Vesuvius. The other variety, known as elaeolite, occurs as large, rough crystals, or more often as irregular masses, which have a greasy lustre and are opaque, or at most translucent, with a reddish, greenish, brownish or See also:grey See also:colour. It forms an essential constituent of certain alkalineplutonic rocks of the nepheline-See also:syenite See also:series, which are typically See also:developed in See also:southern See also:Norway. The colour and greasy lustre of elaeolite (a name given by M. H. See also:Klaproth in 1809, from Gr. gXaiov, oil, and XtOos, See also:stone; Ger. Fettstein) are due to the presence of numerous microscopic enclosures of other minerals, possibly See also:augite or See also:hornblende.

These enclosures sometimes give rise to a chatoyant effect like that of See also:

cat's-See also:eye and cymophane; and elaeolite when of a See also:good See also:green or red colour and showing a distinct See also:band of See also:light is some-times cut as a See also:gem-stone with a See also:convex See also:surface. Closely allied to nepheline, and occurring with it in some nepheline-syenites, is the See also:species cancrinite, which has the composition H6Na6Ca(NaCO3)2 Als(SiO4)9. It is frequently of a See also:bright yellow colour, and has sometimes been cut as a gem-stone. (L. J. S.) NEPHELINE-SYENITE, or ELAEOLITE-SYENITE, a holocrystalline plutonic rock which consists largely of nepheline and See also:alkali See also:felspar. The rocks are mostly See also:pale coloured, grey or See also:pink, and in See also:general appearance they are not unlike granites, but dark green varieties are also known. They do not contain quartz, as that mineral and nepheline are mutually exclusive. From See also:ordinary syenites they are distinguished not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis or in rare earths. Orthoclase and See also:albite are the See also:principal feispars; usually they are intergrown to form perthite. In some rocks the potash felspar, in others the soda felspar predominates. Soda-See also:lime feispars such as See also:oligoclase and See also:andesine are rare or entirely absent.

Fresh clear See also:

microcline is very characteristic of some types of nepheline-syenite. Sodalite, colourless and transparent in the slides, but frequently pale See also:blue in the See also:hand specimens, is the principal felspathoid mineral in addition to nepheline. As a See also:rule these two crystallize before felspar, but they may occur in perthitic intergrowth with it. The commonest ferro-magnesian mineral is pale green augite, which may be surrounded by rims Of dark-green, pleochroic soda-augite (aegirine). The latter forms See also:long See also:flat prisms or bundles of radiating needles. A dark reddish-See also:brown See also:biotite is very common in some of these rocks and a See also:white mica, probably not muscovite but See also:lepidolite, is occasionally present. The hornblende may be brown, brownish-green, blue or blue-See also:black, belonging as a rule to the varieties which contain soda; it is often intergrown with the See also:pyroxene or enclosed in it. The dark-brown triclinic hornblende aenigmatite occurs also in these rocks. See also:Olivine is rare, but may be found in some basic forms of nepheline-syenite. The commonest accessories are See also:sphene, See also:zircon, See also:iron ores and See also:apatite. Cancrinite occurs in several nepheline-syenites; in others there is fluor-spar or melanite garnet. A See also:great number of interesting and rare minerals have been recorded from nepheline-syenites and the See also:pegmatite See also:veins which intersect them.

Among these we may mention eudialyte, eukolite, mosandrite, rinkite, johnstrupite, lavenite, hiortdahlite, perofskite and lamprophyllite. Many of these contain See also:

fluorine and the rare earths. Nepheline-syenites are rare rocks; there is only one occurrence in Great See also:Britain and one in See also:France and See also:Portugal. They are known also in Bohemia and in several places in Norway, See also:Sweden and See also:Finland. In See also:America these rocks have been found in See also:Texas, See also:Arkansas and Massachussetts, also in See also:Ontario, See also:British See also:Columbia and See also:Brazil. See also:South See also:Africa, See also:Madagascar, See also:India, See also:Tasmania, See also:Timor and See also:Turkestan are other localities for the rocks of this series. They exhibit also a remarkable individuality as each occurrence has its own See also:special features; moreover a variety of types characterizes each occurrence, as these rocks are very variable. For these reasons, together with the numerous rare minerals they contain, they have attracted a great See also:deal of See also:attention from petrographers. Many types of nepheline-syenite have received designations derived from the localities in which they were discovered. The laurdalites (from Laurdal in Norway) are grey or pinkish, and in many ways closely resemble the laurvikites of southern Norway, with which they occur. They contain anorthoclase felspars of See also:lozenge-shaped forms, biotite or greenish augite, much apatite and sometimes olivine. Some of these rocks are porphyritic.

The foyaites include the greater number of known nepheline-syenites and are called after Foya in the Serra de See also:

Monchique (southern Portugal), from which they were first described. They are grey, green or reddish, and mostly of massive structure with preponderating potash felspar, some nepheline, and a variable (often small) amount of femic minerals. Pyroxene-, hornblende- and biotitefoyaites have been recognized according to their mineral composition. Examples of the first-named occur in southern Norway with the laurdalites; they contain aegirine and black mica. At Alno See also:Island in the Gulf of See also:Bothnia (Sweden) similar rocks are found bearing enclosures or altered See also:limestone with See also:wollastonite and See also:scapolite. In Siebenburgen (See also:Hungary) there is a well-known rock of this See also:group, very rich in microcline, blue sodalite and cancrinite. It contains also orthoclase, nepheline, biotite, aegirine, acmite, &c. To this type the name ditroite has been given from the See also:place where it occurs (Ditro). Pyroxene-foyaite has been described also from Pouzac in the See also:Pyrenees (S. France). Mica-foyaite is not very common, but is known at Miask in the Ural Mountains (miaskite), where it is coarse-grained, and contains black mica, sodalite-and cancrinite. The hornblende-foyaites are usually brown or `blue, and .ntensely dichroic, but may contain also biotite or augite.

Rocks of this class occur in Brazil (Serra de Tingua) containing sodalite and often much augite, in the western See also:

Sahara and Cape Verde Islands; also at Zwarte Koppies in the See also:Transvaal, Madagascar, Sao Paulo (in Brazil), Paisano Pass (See also:West Texas) and See also:Montreal, See also:Canada. The rock of See also:Salem, See also:Mass., U.S.A., is a mica-foyaite rich in albite and aegirine: it accompanies See also:granite and essexite. Litchfieldite is another well-marked type of nepheline-syenite, in which albite is the dominant felspar. It is named after See also:Litchfield, See also:Maine, U.S.A., where it occurs in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar nepheline-syenite is known from See also:Hastings Co., Ontario, and contains hardly any orthoclase, but only albite felspar. Nepheline is very abundant and there is also cancrinite, sodalite, scapolite, See also:calcite, biotite and hornblende. The lujaurites are distinguished from the rocks above described by their dark colour, which is due to the abundance of minerals such as augite, aegirine, arfvedsonite and other kinds of See also:amphibole. Typical examples are known near Lujaur on the White See also:Sea, where they occur with umptekites and other very See also:peculiar rocks. Other localities for this group are at Julianehaab in See also:Greenland (with sodalite-syenite) ; at their margins they contain pseudomorphs after See also:leucite. The lujaurites frequently have a parallel-banding or gneissose structure. Sodalite-syenites in which sodalite very largely or completely takes the place of nepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsonite and eudialyte.

Cancrinite-syenite, with a large percentage of cancrinite, has been described from Dalekarlia (Sweden) and from Finland. We may also mention urtite from Lujaur Urt on the White Sea, which consists very largely of nepheline, with aegirine and apatite, but no felspar. Jacupirangite (from Jacupiranga in Brazil) is a blackish rock composed of titaniferous augite, See also:

magnetite, See also:ilmenite, perofskite and nepheline, with secondary biotite. The chemical peculiarities of the nepheline-syenites are well marked, as will be seen from the following analyses. They are exceedingly rich in alkalis and in alumina (hence the abundance of felspathoids and alkali felspars) with silica varying from 5o to 56 %, while lime, See also:magnesia and iron are never present in great quantity, though somewhat more variable than the other components. As a group, also, these rocks have a low specific gravity. Si0s. AI,03_ FeO. Fe:O3. CaO. MgO. See also:K20.

Na2O. Laurdalite 34.55 19'07 3.12 2'41 3.15 1.98 4.84 7.67 Ditroite . 56.30 24.14 1.99 0.69 0.13 6.79 9.28 Litchfieldite 60.39 22.57 2.26 0'42 0.32 0.13 4.77 8.44 Lujaurite . 54.14 20.61 2.08 3.28 1.85 0'83 5.25 9'87 (J. S.

End of Article: NEPHELINE

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