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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

PLAGIOCLASE, GROUP: ALBITE, OLIGOCLASE, LABRADORITE, ANDESINE, BYTOWNITE, ANORTHITE


Plagioclases group (Plagiocklase—Plagioclases—Плагиоклазы) (Breithaupt, A. 1847), from Gk. “plágios” – oblique and “klásis” – breaking, alluding to the small obliquity of the angle between its {001} and {010} cleavages. Triclinic system. Extreme members of the isomorphic row from albite to anorthite. Hardness 6-6.5. Density 2.6 (albite) - 2.75 (anorthite). Perfect cleavage in two directions, angle is about 86°. Glass luster, pearl one on cleavage planes. A characteristic feature of plagioclases is the tendency to twinning of individual crystals; sometimes it leads to polysynthetic twins. Twinning and decay of hard solutions are the reasons of such optical effects as opalescence and iridescence. They are often observed in almost all the feldspars. Well-formed crystals of plagioclases usually possess platy or platy-columnar shape. In pegmatite veins they are represented with giant-crystalline and fine-grained aggregates. Non-transparent to translucent, sometimes transparent. Color: from colorless to bright colored varieties depending on admixtures and inclusions.

Albite (Albit—Albite—Альбит) (Gahn, J.G. Berzelius, J.J. 1815), from Lat. “albus” – white, alluding to its color. Albite – NaAlSi3O8 or (Na,Ca)[(Si,Al)4O8], is a sodium feldspar, it is found as lamellar crystals of white, yellow, green, light bluish color or colorless ones. Green because of the admixture of chromium, albite is called pseudojadeite after its color. There is an opinion, that the nature of light blue color of albite, as well as in amazonite, is connected with the admixture of lead. In pegmatites, lamellar crystals – cleavelandite form giant-crystalline aggregates and solid, sugar-like masses. Sugar-like albite contains sometimes sheaf-like aggregates of astrophyllite or aegirine – so called astrophyllite suns, or aegirine suns. In alpine veins of phases of green schists findings of transparent albite crystals, available for faceting, are not rare. Polysynthetic twinning of albite produces light bluish iridescence in crystals because of interference of light at their edges. Albite forms an isomorphic row with alkali feldspars, in which submicroscopic spindle-like isolations of albite – so called perthites, or “microperthites” appear under the decay of hard solutions. Accordingly, in albite such isolations are formed with alkali feldspars. In transparent or half-transparent albite silver play of color or opalescence is observed because of interference of light on microperthites, that’s why this stone is also called albite moonstone. Because the orthoclase phase is often dominating in products of decay of feldspars called moonstone, they are usually classified as orthoclase. The oblong shape of perthites in albite can produce the effect of cat’s-eye. The role of perthite inclusions in albite is the most obvious in greenish-light blue amazonite, where they form an ornamental pattern.

Deposits. Albite is widespread at deposits of pegmatites and in hydrothermal veins. In Russia, on the Kola Penin., at the Rovgora deposit, Western Keivy, they found translucent crystals of albite 7 cm. in size. In the Middle Urals, albite available for faceting, in crystals 5x4x2 cm. in size was found in quartz-albite vein in the Voron’ya Mt., near Nizhniy Tagil. Albite was also found in South. Pribaikalia, in the region of the Slyudyanka deposit. In Tadzhikistan, it was discovered in pegmatites of the East Pamirs, in the region of Rangkul’ Lake. Inclusions of hematite on cleavage planes of lamellar albite produce aventurescence in it. Such albite sunstone was discovered in the north of Norway, at the Seilann deposit. In Switzerland, transparent crystals of albite, 4 cm. in size, are not rare in alpine vein Tinzen locality, Graubünden canton; and to 18,5 cm. occur in Gibelmatte, Matti Valley, Binntal. In Kenya, at the deposits Kio Hill and Usulu transparent albite in light blue and light green crystals, 12x10x3 cm. in size, looks like aquamarine. The color of these crystals depends on inclusions of tiny leaflets of vermiculite. Such transparent green-light blue and blue albite is known in Zimbabwe, at the St Anna deposit, near Miami. On Madagascar Is., at the Anjanabonoina deposit they extract noble albite, including that one with the effect of moonstone. At deposits of Myanmar, according the last information, in ornamental jade-albite, or maw-sit-sit jade, not albite but zeolite is dominating.

In Canada, in the Ontario Prov., at the Silver Crater mine, to the west from Bancroft, they extract crystals of albite peristerite up to 30 cm. long, with reddish iridescence, and in the Québec Prov. – albite peristerite of white, cream and brown-rose hue. In the U.S.A., in Virginia, at the Rutherford mine, Amelia Co., they found available for faceting, colorless, pale blue and green tabular crystals of albite up to 10 cm. long, and at the Media deposit, they extracted albite sunstone. In Georgia and Pennsylvania, there are deposits of albite moonstone. In Wyoming, there is nephrite, or Wyoming jade with inclusions of fine-grained flaky albite, which is especially ornamental. In Brazil, at the Itatiaia mine, the region of Conselheiro Peña, they found large crystals of transparent albite, and also they are known in deposits Virgem da Lapa, Morro Velho, Minas Gerais.

Synonyms. Adinole | Albiclase | Cleavelandite, after the Amer. mineralogist P. Cleveland | Cryptoclase | White feldspar | Canadian moonstone | Plagioclase moonstone | Pericline, twinned | Amazonite plagioclase | White schorl | Sodaclase | Platy spar | Siliceous spar | Pigeon stone | White stone.

Cut Gems. Transparent crystals of albite are faceted, in other cases, it is cut in cabochons. In the Royal Ontario Museum, there is a cabochon of albite with the effect of cat’s-eye from Myanmar, 12.25 ct., and in the Museum Calgary, Canada, a similar cabochon is 11.13 ct.

Oligoclase (Oligoklas—Oligoclase—Олигоклаз) (Breithaupt, A.1826), from Gk. “oligos” – small and “clas” – to break, because it was thought to have a less perfect cleavage than albite. Composition – (Na,Ca)[(Si,Al)4O8]. It is represented with translucent, rarely transparent, lamellar crystals. Color: usually colorless, also white, yellowish, light bluish. Fine twinned lamellar clusters of albite and oligoclase display light bluish iridescence, which depends on interference of light; they are called peristerite or lazur-oligoclase. The space of such iridescent separate fragments in rock can reach 3 dm.2. Because of this effect, peristerite is classified as a jewelry-ornamental stone. The color of iridescence of oligoclase depends on the thickness of twinned plates. Under the less thickness, the color is blue-light blue. This variety of oligoclase, resembling labradorite, is called lynx-eye in Western gemology. In Russia, it is known as belomorite. Sometimes peristerite is falsely called moonstone, oligoclase fish-eye or fischauge, because of their resemblance. At the same time, there is really transparent to translucent oligoclase moonstone. Another variety is oligoclase sunstone of yellow or brownish-red color, which is characterized with plate inclusions along cleavage planes. Such inclusions can be brown flakes of mica, plates of goethite or hematite, and rarely even native gold. Their regular disposition in the mineral produces sparkly luster and glimmer – aventurescence.

Deposits. Oligoclase is a typical mineral of pegmatites, some magmatic rocks and schists. In Russia, transparent oligoclases with the effect of moonstone are known in mica pegmatites of Karelia and the Kola Penin.. On the bank of the Onega Bay in the White Sea, in the group of pegmatites, near Belomorsk, at the Slyudyanoy Bor deposit, they extracted iris oligoclases, which were called after the discovery location belomorites. The second group of similar deposits is situated in the Loukhskiy Dist. – Khetalambino, Nikonova Varaka and others; the third group is near the railway station Polyarnyi Krug – the Sinyaya Pala deposit and others. In pegmatites of Slyudyanoy Bor, the space of iridescent surface of light gray crystals of oligoclase reached 17x8 cm.2 Only the half of these crystals displays bright iridescence. Beside large separate blocks of belomorite, oligoclase is found as inclusions in microcline. Such ornamental stone with the content of belomorite up to 50%, with the average size of separate blocks to 5 cm., is characterized with high level of ornamental properties; it has its own name – mosaic pegmatite. On the Kola Penin., pegmatite veins with iris plagioclase are found in the southwest – at the deposits Rikolatva and Sliudyanskoye, and in the east – at the Strelninskoye deposit. In the Middle Urals, pegmatites with peristerite are known at the deposits Murzinka and Izumrudnye Kopi; at the South Urals – in pegmatites of the Ilmeny Mts., in the region of Miass. In Pribaikalia, they were found at the Slyudyanka deposit. Pegmatites with oligoclase sunstone are in Transbaikalia – along the Selenga River and in the Udinskiy Ridge. In Kyrgyzstan, in ornamental syenites of the Ottuk deposit, near Rybach’ye, by Ysyk-Köl Lake, they extracted oligoclase-andesine with iridescence. In Tadzhikistan, in the region of the Kukh-i-Lal deposit, West Pamirs Mts., in miarolitic cavities of oligoclase pegmatites they found crystals of half-transparent greenish-white oligoclase, from 5 to 25 cm. wide. In Norway, oligoclase with aventurescence, because of inclusions of hematite plates along two cleavage directions, was discovered at the deposits Bamble, Bjerdammen, Havredal and Tvedestrand, Aust-Agder. In Kenya, at the Machakos deposit, they find gem-quality colorless to pale blue oligoclase and oligoclase sunstone. In South Africa, similar ornamental material in Letaba, Northern Prov., and Northern Cape Prov., was found. In the U.S.A., ornamental oligoclase, including sunstone and moonstone, was extracted at a deposit near Mitchell, North Carolina, where they made cabochons from it, up to 5 ct.; and also at the Mineral Hill deposit, Delaware Co., Pennsylvania. Gem-quality peristerite was extracted also from pegmatites of the New York State. In Oregon, peristerite is situated in lavas as crystals from rose to green hue, up to 2.5 cm. long. In Canada, significant quantities of pale red and light bluish peristerite come to the market from deposits near Villeneuve and West Portland, Québec Prov.; and also from the region of Haibl and Montagle, Ontario Prov. There are also deposits of oligoclase sunstone in that region.

All the ornamental varieties of oligoclase are cut mainly in cabochons, and also in the shape of balls and beads.

Synonyms. Oligoclase – Hecatholite, obs. | Kenyan feldspar | Natronspodumene | Oregon sunstone.

Peristerite, from Gk. “peristera” – a dove, because of its iridescence. – Belomorite, after the discovery location near the White Sea | Fish-eye | Wolf ’s-eye | Girasol oligoclase | Ceylon opal.

Legends. Oligoclase is a talisman of nursery teachers, doctors, animal trainers and artists. Astrologers suppose it to be a lucky stone of those, who were born under the sign of Leo. In the U.S.A., sunstone is an official symbol of Oregon.

Andesine (Andesin—Andésine—Андезин) (Abich, W.H. 1841), after its discovery location in lavas from Marmato, the Andes Mts., Bolivia. Coposition – (Na,Ca)[(Si,Al)4O8]. In India, half-transparent crystals of andesine moonstone are known as well as massive nephrite-green grained aggregates – andesine jade. Among rocks composed mainly with andesine, we should mention andesites and anorthosites – from Old Fr. name of plagioclase – “anorthose”. They are used usually as coating stones. As an ornamental stone, the most interesting from them is a variety composed with translucent iris andesine. In Canada, it received its own name – galaxite. For small work of art and cabochons they use large crystals of andesine from these rocks.

Labradorite (Labradorit—Labradorite—Лабрадор) (Werner, A.G. 1780), after its discovery locality on the St Paul Is., Labrador Penin., Canada. Coposition – (Ca,Na)[(Si,Al)4O8]. As a faceting stone, labradorite is well-known in the U.S.A. because of findings of its transparent crystals in the western and eastern parts of the country. Suitable for processing iris labradorite is found usually in the form of porphyry-like isolations in rocks of basic content – labradorites and basalt lavas. Their size reaches 10 cm. in cross-section, and in some cases even 80 cm. In ornamental labradorite the quantity of iridescent grains of labradorite, 7-10 cm. in size, reaches 100 pices/m2. Iridescence of labradorite (from Gk. “iris” – a rainbow), called in Western gemology labradorescence, is the result of rainbow interference color of complete spectrum, from red to violet. It appears at the verges of fine-plated twins of variable content. Separate blocks of labradorite, iridescent in dark tones, are called also buffalo-eye, bullock’s-eye or ox-eye, and with green iridescencelabradorite lynx-eye.

Deposits. In Russia, in the south-west of Transbaikalia, there is the Burguiskoye deposit of ornamental labradorite, where “eyes” of labradorite in the rock, up to 15 mm. in size, display light blue-blue and green-blue iridescence. In the Ukraine, in the Zhitomir Region, numerous deposits of ornamental and coating labradorites and anarthosite gabbro are known: Golovinskoye, Kamennobrodskoye, Guta Dobrynskaya, Siny Kamen’ and others, which have been being developed since 1835. The bigest “eyes” in labradoritites at the Paramonovskoye deposit reached 27x27 cm. and thickness not less than 9 cm.

In the south-east of Finland, near Ylämaa, in gabbro-anorthosites there are big “eyes” of labradorite. Because of iridescence in a large spectrum, they were called spectrolite. Fine-dispersed inclusions of ilmenite along the cleavage planes in this spectrolite produce the effect of cat’s-eye. On Madagascar and in Myanmar (Burma), they find transparent crystals of labradorite with light bluish opalescence and silky chatoyancy, which depend on inclusions of dark colored needles of rutile. Such labradorite is called black moonstone or Madagascar moonstone. On Madagascar, they find also labradorite sunstone with inclusions of native copper, and if there are inclusions of magnetite, hematite or ilmenite it displays aventurescence. There, in labradorite, as well as in ilmenite, they observed inclusions of native silver in the shape of fine needles. In Australia, in New South Wales, near Bous River, they extract pale yellow transparent labradorite.

Classical deposits of labradorite are situated in Canada, mainly in the Newfoundland Prov. – on the islands of St Paul and Tebor. The size of isolations of labradorite reached there 1 m. long. Because of inclusions of hematite, they display aventurescence. Besides, labradorite of good quality is known in Greenland. In the U.S.A., labradorites is known in Utah, at the Lake Clear deposit. In Nevada, they found transparent crystals of labradorite up to 2.5 cm. wide, and in New Hampshire, they made faceted stones up to 4 ct. from pale yellow labradorite. In Oregon, at the Ponderosa mine, Lake Co, noble labradorite is found in basalt lavas in the form of crystals, up to 8 cm. wide. The most part of them is of gem-quality. Among them 90% are straw-yellow, 9% are rose and about 1% are colored red and green. Yellow hue of labradorite depends on the admixture of iron (Fe3+), rose one – on micro-inclusions of native copper, other tones depend on the level of decay of hard solution of copper in labradorite. Because of its high jewelry characteristics, it is called in trade – gold labradorite, heliolite, sunstone or Oregon sunstone. Similar labradorite with red hue like in spinel is discovered on Hawaii, on the Maui Is. There are green varieties there, too. They are similar to light green tourmaline, yellow as beryl and polychromatic red-green with iridescent color – parrot wing. Labradorite has the same solidity and activities of refraction as beryl. Transparent green variety of labradorite from Maui has the trade name Hawaiian topaz. In Mexico, in the Madok Dist., Baja California, they discovered transparent labradorite, from which they received faceted stones up to 40 ct., and in the Chihuahua State – up to 65 ct.

Synonyms. Carnatite, after the discovery location in the Karnataka State, India | Changeant, Fr. from iridescence | Labradorite feldspar | Greenland labradorite, with bright blue play of color | Indian labradorite, yellow-milky with peacock feather play of color | Oyama labradorite, from the Oyama deposit, in Japan | Mauilite (maulite), after the discovery location on the Maui Is., Hawaii, the U.S.A. | Labradorite moonstone | Plagioclase moonstone | Feldspar opaline | Pseudoalbite | Labradorite spar | ~ stone: ingermanland ~, Labradorite ~, rainbow ~, sparkling ~, tausini ~, from Pers. “tausi” – a peacock, after its iridescence.

Quality improvement. Pale colored iris labradorite after the heating under 500-700°C for 3-5 hours gets brighter because bright rose, yellow and brownish tones are strengthened. Very crackled samples of translucent labradorite are improved with filling the cracks with blue paint. Such material from India was shown at the exhibition in Munich in faceted stones and in cabochons.

Treatment. The history of labradorite in jewelry begins in 1775 with the delivery of the first lot of labradorite to Europe. In the 19th cent., work, of art from this stone were decorated with brilliants, they were in fashion in London and Paris. However, the source of this fashion takes place in Russia. It is not by occasion, that the author of “Natural History” J.L.L. Buffoon (1707-1788) called labradorite – Russian stone. In 1781, during the construction of a road in St Petersburg, they found several boulders of labradorite. From beautiful, iridescent segments gem-quality insets were made. Soon they came in fashion; the price of such inset for a ring was rather high. One of those boulders, 80x45 cm. in size, was sold for 1000 rubles, from the other two at the Peterhof lapidary factory two vases for the Ekateriniskiy Palace in Tsarskoye Selo were manufactured.

Transparent crystals of labradorite are faceted, stones up to 20 ct. were received from them, and the biggest ones reached 100 ct. In the Smiths. Inst., Wash., a collection of faceted yellow labradorite from deposits of the U.S.A. is exhibited: from Utah – 11.1 ct., from Nevada – 5.8 ct., from Oregon – 23.8 and 39 ct. There is also a stone from Mexico – 23.43 ct. Translucent labradorites with optical effects are cut in cabochons.

Bytownite (Bytownit—Bytownite—Битовнит) (Thomson, T. 1935), after its discovery location in Bytown, the original name of Ottawa, Ontario, Canada. Coposition – (Ca,Na)[(Si,Al)4O8]. As material for faceting, they use pale yellow and reddish-brown crystals of bytownite from deposits of the U.S.A. in Arizona, New Mexico and in Oregon, Lake Co. In Mexico, yellow gem-quality grains bytownite up to 8 cm. occur at Nuevo Casas Grandes, Chihuahua State. Varieties of bytownite with aventurescence are known in Mexico, and on Madagascar – those with the effect of cat’s-eyeBytovnite cat’s-eye. As a rarity we could mention glassy bytownite – mascelynite, so called after the Engl. mineralogist N.S. Maskelin. It is also called meteoritic glass, because it is formed in meteorite craters as a result of melting of feldspar rocks after the meteorite strike. Among the rocks composed with bytownite, the most interesting as ornamental stones are in Russia: anorthosites with iris bytownite in the north of the Krasnoyarsk Region. A massif of such rocks is situated in the upper reaches of the Kotuikan River, 20 km. upper its mouth. Iris bytownite is linked mainly with contacts of pegmatite veins in the massif. The thickness of these veins is not more than 5 m., the size of the blocks of iris bytownite is up to 10x10 cm. Color of bytownite is dark gray, with iridescence in violet-blue tones in one of the cleavage planes, and greenish-golden in the other plane. Sometimes, they observe the ring pattern of iridescence of the type of an ox-eye. Nowadays, the part of the massif with described anartosites is in the complex of the Khatanga reserve. From rocks composed with bytownite the most interesting as an ornamental stone is variolitic gabbro. It contains rounded isolations of aggregates of bytownite mounted in hornblende. Such texture makes this rock very decorative, that’s why it is called – orbicular diorite, orbicular corsite, or globular gabbro, as well as Napoleonite and Napoleon porphyry. From the similar Kurzov diorite, extracted in the Ukrainka (Kurtsy), Crimea Ukraine, a vase was made; it is exhibited in the Gatchina Palace by St Petersburg now. Similar diorite or corsite with radial crystals of bytownite was extracted on the Corsica Is., France. Vases from this rock decorate the Malmeson Palace by Paris.

Anorthite (Anorthit—Anorthite—Анортит) (Rose, G. 1823), from Gk. “an” – not (without) and “orthos” – right, alluding to its oblique crystals. Composition – (Ca,Na)[(Si,Al)4O8]. There is its iridescent variety – black moonstone, resembling labradorite; and also translucent ornamental varieties of rose and blue color with inclusions of sillimanite. Rose or peach-red anorthite has the trade name diploite, from Gk. “dipleos” – double; and gray-green – lepolite, from Gk. “lepos” – shell and “litos” – a stone. Anorthite in the form of crystals up to 4 cm. in length is met on Erebus Mt. in Antarctic. In Russia, as a potential ornamental stone, we should mention giant-grained metasomatic gabbro rock – kataranskite, or violet gabbro from the region of the Mys (cape) Kataranskiy in the Kanadalaksha Bay of the White Sea. Large accumulations of anorthite are almost completely replaced in it with aggregate of secondary minerals. According the color, they are called violet. Gray-violet hue depends on fine-dispersed inclusions of manganese-containing mica – fengite. In the U.S.A. similar rock is – latrobite, described at the Bolton deposit, Massachusetts. In Russia among ornamentals stones containing anorthite, we should mention leopard gabbro and star gabbro of the Chineiskiy massif, the Amur Region. In first of them, at the white anorthite background, there are beautiful radial-rayed forms of dark green pyroxene or black titanomagnetite up to 2 cm. wide. In leopard gabbro these isolations look like spots up to 12 cm. wide. Another ornamental rock of this region is iris anorthosite from the Saibalakhskoye deposit of the Kuranakhskiy gabbro massif in the north-west of the Amur Region.

Synonyms.Calciclase | Christianite, in honor of ancient capital of Norway – Christiania | Lynx-eye | Latrobite, after the surname C.J. Latrobe | Mornite,after the discovery location near Morne, Ireland | Radauite (radanite), after the discovery location at the Harz Mts., near Radautal, Germany | Rosite, after Swedish mineralogist G. Rose (1798-1873) | Sandvikit, obs. | Silicite, obs. | Spectrolite.

Cut Gems. All the varieties of plagioclases are classified as non-expensive stones because of their low hardness. They are used for beads, pendants and collar buttons. Usually they are grinded and polished in cabochons; transparent crystals and varieties with light effects are faceted sometimes.

Similar Gemstones & Imitations. Plagioclases look like amblygonite, scapolite and alkali feldspars, but they differ from these stones with hardness, solidity and optical properties. Recently, as an imitation of noble labradorite, they use its artificially manufactured improved variety – rainbow labradorite with polychromatic iridescence. It is received with the method of evaporation of fine film from precious metals on cut in cabochon or faceted labradorite.

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