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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

CORDIERITE, GROUP: CORDIERITE, SEKANINAITE


CORDIERITE group (Cordierit—Cordiérite—Кордиерит) (Lucas, J.A.H. 1813), after A.P. Louis Cordier (1777-1862), Fr. geologist, who first described the mineral. Here also another, ferrous, variety is described – sekaninaite, with which cordierite forms an isomorphic row.

Composition & Properties. Silicate – (Mg,Fe2+)2(Al2Si)[4][Al2Si4O18], subclass rings silicates, orthorhombic system. Hardness 7-7.5. Density 2.5-2.6. Glass luster to greasy one. Fragile. Cordierite is found usually as grained masses, crystals are rare. Transparent to half-transparent. Color: blue to violet-blue, often grayish-blue, more rarely greenish-gray, yellowish, brown, colorless. It is characterized with strong pleochroism – in different directions its color changes from colorless to violet. That’s why it is also called dichroite. This property should be taken into account in the process of faceting. Cordierite cat’s-eye in cabochons, often depend on gas-liquid inclusions, oriented in one direction. When inclusions of fine flakes of hematite, magnetite and lepidocrocite are represented, cordierite displays asterism and aventurescence. The brightness of the second effect depends on the angle of observation, because these inclusions are parallel oriented. These inclusions produce reddish tone in bloodshot iolite, or cerasite.

Deposits. Usually it is formed under metamorphism of magnesium and aluminum-containing rocks. In Russia, ornamental cordierite was discovered in the Urals, in pegmatites of Shaytanka and Bazhenovskiy Bor, where it was represented with massive aggregates up to 10 cm. wide. In the South Altai, there were findings of cordierite isolations up to 12x14 cm. In Yakutia similar isolations of cordierite were discovered in the Anabarskiy shield along the Great Kuonamka River. In Aldan, at the Atubin deposit, resources of cordierite are calculated in the volume of 1 ton, with the output of gem-quality rough material about 1%. In North Pribaikalia, in the Malyy Akitkan Ridge, in the basin of the Kutim River, in metasomatic transformed cordierite gneisses, they find isolations of transparent violet cordierite up to 50 cm. wide. In Tadzhikistan, in the region of the Kukh-i-lal deposit, in the West Pamirs Mts., in miarolitic cavity of oligoclase pegmatites, they found crystals of half-transparent cordierite up to 5 cm. long. In the East Pamirs Mts., gem-quality cordierite was discovered to the northwest from the settlement of Murghob, near the Akbaytal Pass, along the Sassyk Su River, where veins were found with its crystals a massive isolations up to 12 cm. wide. Transparent segments, available for faceting, reached here 1 cm3 . In cabochons they sometimes display the effect of cat’s-eye. To the east from the Akbaytal Pass, on the Zorburuliuk River, they discovered the Cordierite-3 deposit with gem-quality rough material.

In South Norway, noble cordierite has been known since the time of Vikings. At the deposits Arendal and Bamble (Bamle), it displays aventurescence, and the effect of cat’s-eye in cabochons. In the northern part of the country, at the Kragerø deposit, Telemark, cordierite forms a mass of glassy, reaching significant the sizes. At the Tvedestrand deposit, Aust-Adiger, they also extracted cordierite. In Sweden crystals of cordierite up to 20 cm. at the Näverberg deposit, near Falun, Dalarnas, were found. long. In Finland, cordierite could be evaluated as a national stone because of the quantity of its findings and the quality of the rough material. At the deposits Pielavasi and Orijärvi, Keski-Suomi Dist., its transparent crystals reached 5 cm. long. In Germany, near Bodenmais, Bavaria, and crystals of light green cordierite reached 4 cm.; and in Saxony, near Waldheim, this variety received the local name prismatite, or prismatic quartz. In Poland, in the Sudety Mts., in the Ovl Mt., in pegmatites, they found transparent greenish cordierite with weak violet tone in crystals up to 7 cm. long. In Czech Rep., at the Dolní Bory deposit, Moravia, in pegmatites, together with cordierite they discovered its ferrous variety – sekaninaite.

On Madagascar, cordierite was extracted in the region of Antsirabe, to the south from Antananarivo, in the Byte Mt. In the southwest of the island, near Toliara, there is another deposit of cordierite, from which ornamental half-transparent pale blue material is coming. In Africa, cordierite is extracted in Tanzania – near Babati, in Namibia – near Karasburg, in South Africa – at the Blue Dragon mine, Cape Prov. In Australia numerous deposits of cordierite are known in the Northern Territory, including the Harts Range deposit; and in the Queensland State, there is a deposit of ornamental cordierite in the region of Mt. Isa. At placers of Sri Lanka, cordierite is found in crystals up to 2 cm. It is often colorless or pale blue, with inclusions of hematite it displays aventurescence and asterism in the shape of a four-radial star in cabochons. Cordierite is extracted as additional material at placers of Myanmar (Burma). In India, it is represented in significant volume in the Orissa State and in Tamil Nädu, in the region of Chennai (form. Madras). Cordierite from India displays the effect of cat’s-eye in cabochons.

In Greenland, gem-quality cordierite was discovered on the Uijordlerssuak Is. In Canada, in the North-Western Territories, similar cordierite was discovered in the region of Big Slave Lake. Another deposit is situated on the Baffin Land Is., where they received more than 200 of faceted stones from cordierite, extracted from pegmatites. In the Québec Prov., transparent cordierite was found at the Asbestos deposit, from which they received faceted stones up to 8.82 ct. In the Manitoba Prov. crystals of cordierite up to 20 cm. of the same quality were discovered at the Stall Lake mine, near Snow Lake; as well as in the British Columbia Prov., on the Prince Rupert Rain Is. In the U.S.A., findings of gem-quality light bluish cordierite are known in Connecticut, at the Haddam Neck deposit, from which they received faceted stones up to 2 ct. In New Hampshire, cordierite in crystals up to 20x15x15 cm., available for cabochon cutting, was extracted from pegmatites at Richmond, Cheshire Co. In Brazil, in Minas Gerais State, to the northwest from Governador Valadares, it was discovered at placers in the form of nodules up to 40 cm. in cross-section. In the Paraíba State, at Umburang deposit, gem-quality greenish-light blue cordierite was discovered near Picui.

Synonyms. Cerasite, from Gk. “cerasos” – cherry, after its color | Magnesio-cordierite | Dichroite, from Gk. – bi-colored | Hard fahlunit, obs. | Iolite, from Gk. “ios” – violet and “litos” – a stone, after its color | Sakura-ishy, Jap. | Spanish lazulite | Lolith (loloith, jolith), obs. | Peliom, from Gk. “pelioma” – bruise, after the similarity of color | Polychroite, from Gk. “polychromos” | Pseudosapphir | False sapphire | Lux sapphire (luxsaphir, Germ.) | Occidental sapphire | Water sapphire | Steinheilite, after the name of former governorship in Finland | Violet stone | Trichroite, after its strong pleochroism.

Treatment. Transparent segments of cordierite crystals are rarely big, that’s why faceted stones usually are up to 10 ct. In the process of faceting, to receive the highest thickness of color, a master should put a table facet of a stone under the right angle to the edges of a prism. In the Smiths. Inst., Wash., there are faceted cordierites from Sri Lanka – 15.6 and 10.2 ct., and from the U.S.A. – up to 9 mm in diameter, from the Haddam deposit, Connecticut. From Finnish deposits of cordierite they manufactured suits of adornments for the Danish Princess Margaret and Diana, the Princess of Wales. The biggest processed cordierite is a unique piece of a gem-quality crystal, 177 gm. (885 ct.) in mass, which is exhibited in the British Museum, London. Cordierite was used for imitation of sapphire, its light blue stones were called water sapphire, and dark blue ones – lynx sapphire.

Legends. According some legends, famous sea-voyagers – the Vikings – used cordierite in navigation. Because of its strong pleochroism, it gave the possibility to fix the Sun position even in cloudy days. In Russia, cordierite was called varangian stone (Varangian meant the Vikings).

Synthesis. Synthetic cordierite of gray color is received for the jewellery purposes. For improvement of quality been subjected to processing by implantation of cobalt’s ions, and after annealing becomes dark blue because of spinel formation in an external layer. Beryllic. indialite, synthetic analogue of cordierite – Mg2Be3[Al2Si6O18]. Its glasses possess wide color scale. In their properties they are close to cordierite. Because of their high hardness, good optical characteristics and low expansion factor, they are used as rough material for faceting.

Similarity. Cordierite looks like benitoite, kyanite, sapphire, sekaninaite and tanzanite.

Sekaninaite (Sekaninait—Sékaninaïte—Секанинаит) (Stanûk, J. & Miškovský, J. 1975), after Josef Sekanina (b.1901), Czech mineralogist who found the mineral at Dolní Bory, Moravia, Czech Rep. in 1928. Silicate – (Fe2+,Mg)2(Al2Si)[4][Al2Si4O18], orthorhombic system. Hardness 7-7.5. Density 2.77. Glass luster. It is a rare variety of ferrocordierit, which forms large crystals up to 70 cm. long, of thick blue color with violet tone. They are non-transparent to weakly translucent at the edges, they possess multiple cracks and display clear separation. Meanwhile, they made faceted stones and cabochons from this material. In Russia, sekaninaite was discovered in Yakutia, South Verkhoyanskiy Territory, in the middle reaches of the Kurum River, a tributary of the Tara River. Findings sekaninaite as collection material are known also in Germany at the Grüne mine, near Schutzbach, Rhineland-Palatinate; in Japan – in Mie Pref.; and in the U.S.A., Oxford Co., Maine.

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