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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

TANTALITE-MN


MANGANOTANTALITE (Mangano-Tantalit—Manganotantalite—ћанганотанталит) (Nordenskiöld, A.E. 1877). Named as the manganic analogue to ferrotantalite.

Composition & Properties. Oxide – Mn(Ta,Nb)2O6, orthorhombic system. Hardness 6-6.5. Density 7.7-7.9. Glass luster. Cleavage in one direction. Fragile. Manganotantalite is represented with short-columnar, isometric or table crystals. Color: from rose and bright red to reddish-brown. Translucent and extremely rarely transparent. Its dark color can be lightened with burning on the air up to 1200°C because of the oxidation of manganese.

Deposits. Manganotantalite is found in rare-metal granite pegmatites. It is stable for weathering and is extracted on placers. In Russia, in the South Urals, dark brownish translucent crystals of manganotantalite with the size up to 7x11x14 mm. were found in pegmatites of the Svetlinskoye deposit; as well as on placers on the Sanarka River, in the region of Plast town. In Tadzhikistan, crystals of manganotantalite were discovered in the Eastern Pamirs Mts., in pegmatites of the region of Rangkul’ Lake. In Congo DR, Kinshasa, red tabular crystals up to 10 cm. occur in placers around Kamitunga. In Mozambique, gem-quality dark red crystals of this stone, together with bismutotantalite and stibiotantalite, were extracted from pegmatites of the Muiane deposit, in the Alto Ligonha Region; and on Madagascar – at the Andilana deposit. On Sri Lanka, gem-quality manganotantalite was extracted as additional material on placers. In China, its crystals up to 2 cm. long were found at the Karasu deposit, in the region of Koktogai, Mongolian Altai. In India, manganotantalite was discovered in tourmaline pegmatites near Gilgit, Jammu and Kashmir State (disputed area). In Brazil, transparent reddish-brown crystals of manganotantalite up to 1 cm. long in pegmatite of Golconda, Minas Gerais, were found; in Rio Grande do Norte – in sperb crystals up to 8x6 cm. at the Equador deposit. They are known under the trade name alvarolite after the Brazilian Admiral A. Alvaro. Crystals of manganotantalite, available for processing, were found also in the U.S.A.: in California – at the Pala deposit; in Connecticut – at the Portland deposit; in Virginia – at the Amelia mine.

Cut Gems. For faceted manganotantalite the effect of fire is typical. Transparent segments of crystals are usually not more than 3 cm., that’s why faceted stones more than 10 ct. is extremely rare. In the Museum of Calgary in, Canada, there is such a gem of reddish-brown color, in 3.05 ct., from Brazil.

Synthesis. There is such synthetic analogue as synthetic lithium tantalite, which is produced for technical and jewelry purposes. Without admixtures, this colorless, transparent material displays the dispersion twice as much as that of diamond. Hardness 5.5-6. Density 7.3. For the same purposes they produce colorless synthetic potassium tantalite with admixture of niobium, which displays the similar properties. At the jewelry market it has been known since 1961. Its hardness is 6.5, density is 6.43. Almost the same in properties and usage is synthetic potassium-tantalum-niobate.

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