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Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

NEPHELINE


NEPHELINE (Nephelin—Néphéline—Ќефелин) (Haüy, R.-J. 1800), from Gk. “nephele” – a cloud, after it’s the cloudy appearance that is developing when the mineral is immersed in strong acid.

Composition & Properties. Silicate – (Na,K)[AlSiO4], subclass framework silicate, hexagonal system. Hardness 5.5-6. Density 2.5-2.6. Glass luster to greasy one. Fragile. Nepheline is usually represented with massive grained aggregates. Its dull, mostly greenish variety elaeolith is found as short-columnar or thick-table crystals. Transparent to non-transparent. Color: white, gray, yellowish, greenish, bluish, dark green, brick-red, meat-red. Some varieties display opalescence. Needle mineral inclusions in cabochons of nepheline generate to the effect of cat’s-eye, and inclusions of lamellar crystals of hematite generate to aventurescence.

Deposits. Nepheline is a typical mineral of the process of alkali metasomatism; it is found in syenites and their pegmatites. In Russia, on the Kola Penin., in Khibiny, at the apatite Aikuaiventchorr deposit, nepheline is a rock-forming mineral of ornamental nepheline syenite: chibinite and lujavrite. Chibinite is composed on 40-45% with microcline, 35-45% with nepheline, aegirine and eudialyte. In eudialyte lujavrite the content of nepheline is les than 30%, microcline – not more than 25%, aegirine – 40-80%, eudialyte – 10-30%. The ornamentality of these rocks depends on large inclusions of raspberry-red eudialyte, and in porphyrite urtite on almost square separate fragments of nepheline, which content reaches 90%. In the Alluayv Mt., they discovered gem-quality crystals of translucent elaeolith. In the Urals, in alkali massifs of the Ilmeny and Vishnevye Mts., in the process of extraction of pegmatites they found large blocks of meat-red nepheline as additional material. In the region of Ilmeny Mts. they extracted also nepheline sunstone with aventurescence. In the north of the Krasnoyarsk Region, in alkali-basic rocks massifs of Guli and Odikhincha, the Khatanga area, findings of nepheline crystals up to 5 cm. long are not rare. In other countries, jewelry-ornamental nepheline is extracted in Greenland –– at the Julianehåb deposit. In the south-east of Norway, in alkali pegmatites of the deposits Sandefjord, Vestfold, and Langesund, Telemark, they found gem-quality reddish-brown and bluish-green nepheline, which displayed the effect of cat’s-eye in cabochons. It is also known at the deposits of Sweden – Alnön Is. and Almunge, Uppsala; in Germany – at the deposits Katzenbukele and Odenwald, Hessen, and also in Kaiserstuhl, Baden-Wurtemberg. In Italy, nepheline was at first discovered in the lavas of Vesuvius. It is found here as large glassy-transparent crystals. In Portugal, its crystals were discovered at the Serra de Monchique locality. In Myanmar and Korea, nepheline is used as an ornamental stone. In the U.S.A., jewelry-ornamental nepheline is known in Maine State, at the Lightfield deposit, Cennebeck Co; and in Arkansas, in the Hot Springs Co. In Canada, its crystals, even over 60 cm., are found at the Dungannon locality, near Bancroft, Ontario Prov.

Ornamental rocks. In Russia, in the Kuznetskiy Alatau, Kemerovo Region, they discovered a new variety of ornamental and coating stone berelite. It is an alkali rock of dark color with light rose insets of nepheline. Among the other nepheline-containing ornamental rocks, we should mention jadeolite from Myanmar – chromium-containing dark green syenite, resembling jadeite. In the Ukraine, there is feldspathic syenite mariupolite in area of Mariupol’ massif; and in Uzbekistan – silicified syenite from the Kalmykarskoye (Olmaliq) deposit. In South Greenland, this type of rocks is represented with kakortokite from the Fiskenaesset deposit. This rock contains bright spots of purple-red eudialyte and golden threads, wisps and rosettes of arfvedsonite. Its stripped varieties are not less ornamental; they are composed with white inter-layers of microcline, red ones with domination of eudialyte and black ones of aegirine and arfvedsonite. In the U.S.A., they use hydronephelénite in fine-grained rose rock as an ornamental stone. It is formed as a product of hydrothermal transformation of nepheline – spreustein and is composed with mixture of cancrinite, sodalite, zeolite and paragonite. Similar rock is known also in Russia at Khibiny, on the Kola Penin.

Synonyms. Elaeolith, from Gk. “elaeon” – oil, because of its greasy luster, old | Orthorhombic feldspar | Fettstein, Germ. | Phonit, from Gk. “phone” – sound | Fat stone.

Ornamental rocks. Eudialyte urtite and urtite after the Luyavr Urt Mt. in Khibiny, Kola Penin., Russia | Eudialytite, after the content | Jadeolite, after the similarity with jadeite | Kakortokite, after the discovery location near the of Kakortok Settlement – the Eskimo name of the settlement of Julianehoba, South Greenland | Sommite, after the discovery location at the Monte Somma deposit, Italy | Spreustein, from Germ. “spreu” – chaff and “stein” – a stone, syn. chaff stone | Nepheline syenite, after the discovery location near Syena, now Assuan, Egypt.

Treatment. Gem-quality nepheline is rarely used as a faceted stone. They make faceted stones not more than 1-2 ct. from it. Usually, it is cut in cabochons. Nepheline-containing rocks are used for manufacturing of balls, flat insets and souvenirs.

Similarity. Nepheline looks like apatite, cancrinite, feldspars and scapolite.

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