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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

AMBLYGONITE, GROUP: AMBLYGONITE, MONTEBRASITE


AMBLYGONITE group (Amblygonit—Amblygonite—јмблигонит) (Breithaupt, A. 1817 in Hoffman, C.A.S., 1817) from Gk. “amblys” – blunt and “gonios” – angle, in reference to its obtuse (nearly 90°) cleavage angle.

Composition & Properties. Phosphate – (Li,Na)Al[(F,OH)|PO4], triclinic system. Hardness 5.5-6. Density 3.0-3.1. Pearl luster, glass one. Cleavage in two directions. Fragile. Amblygonite forms an isomorphic row with montebrasite, which is described here, too. Amblygonite is found as giant, poorly-formed columnar crystals or as complete masses. Half-transparent to translucent. Color: white with different tones from yellowish to brownish, sometimes with spotty pattern of rose and light blue color. Transparent crystals of noble amblygonite are golden-yellow in color and very rare. Amblygonite, often together with montebrasite, is found in granite pegmatites.

Deposits. In Russia, in the north of the Sakha (Yakutia), they discovered the Kestior deposit of amblygonite, in the Batagay-Alyta area. Its findings are also known in the Krasnoyarsk Region, on the Enashimo River, near the Severo-Yeniseyskiy. In East Kazakhstan, they find amblygonite at deposits of East Kalba, in the region of Ust’-Kamenogorsk; and in North Tadzhikistan – at the Samzhon deposit, the Turkestanskiy Ridge. In Namibia, at the Karabib deposit, they discovered a light lilac variety of gem-quality amblygonite; and in Kenya – deep-brown. In Myanmar, transparent amblygonite was extracted at the Sakangyi deposit, Katha Dist. In the U.S.A., amblygonite is found in Maine State at deposit Mt. Rubellite, Oxford Co., and also of Newry Co. and Greenwood Co., where its transparent crystals reached 18 cm. It had the local name hebronite there. In South Dakota, in the Custer Co., at the mines Hugo and Black Hills, aggregates of amblygonite reached 200 tons. There they found the biggest crystal of amblygonite, 102.06 kg., and 762x244x183 cm. in size. In New Hampshire, at the Palermo deposit, a non-transparent crystal of amblygonite was 366x61 cm. in size. In Canada, in the North-Western Territories, there is the Yellowknife Boglier deposit of amblygonite. In Brazil, in the São Paolo State, at the deposits Mogi das Cruzes, yellow crystals of amblygonite up to 15 cm. length were extracted and also at the Boa Vista deposit; and in Minas Gerais State – at the Linopolis deposit.

Synonyms. Hebronite, after the discovery location near Hebron, Maine, the U.S.A. | Amblygonite prismatic spar.

Treatment. Faceted amblygonite is often mixed with montebrasite. The dominating weight of stones is not more than 15 ct., but some stones reached 80 ct. In Canada, in the Royal Ontario Museum, there is a stone 15.6 ct., and in the Museum Calgary, Alberta Prov., Canada – 47 ct. In the Smiths. Inst., Wash., the U.S.A. – 62.5 ct., from Brazil; and 19.7 ct., from Myanmar.

Similarity. Amblygonite can be taken for apatite, brazilianite, citrine, heliodor, hiddenite and scapolite.

Montebrasite (Montebrasit—Montebrasite—ћонтебразит) (Des Cloizeaux, A. 1871), after the discovery location in Montebras, Creuse Dept., France. Phosphate – (Li,Na)Al[(OH,F)|PO4], triclinic system. It is close to amblygonite in the composition. Hardness 5.5-6. Density 3. Glass luster to pearl one. Perfect cleavage in one direction. Transparent to translucent. It is found mainly in massive grained aggregates of white, greenish to dark green color. Like amblygonite, it is linked with granite pegmatites and is found at the same deposits.

In Russia, on the Kola Penin., a finding of montebrasite was registered in pegmatites of Voron’i Tundry. It was discovered as complete masses in the East Sayan Mts., at the Vishnyakovskoye deposit, near the junction of the Tagul and Biriusa Rivers. In the north of Sweden, at the Varuträsk deposit, to the west from Shellefteo, montebrasite was discovered in significant quantities. In Finland, it is known to the south from Turku, at the Kimito Is.; in Spain – in the Extremadura region. At deposits of Madagascar they extracted its green crystals of gem-quality. In Namibia, they found gem-quality rough material of pink-violet hue at the Karibib deposit. In Western Australia, transparent crystals were found at the Ubina deposit. In Brazil, pegmatites with montebrasite were extracted at the Mogidad Cruges deposit in the states São Paolo; and Minas Gerais – at the Linopolis deposit with crystals up to 4 cm. There, at the Vale Rio Doce deposit, they discovered a sodium containing variety of montebrasite in the shape of crystals up to 15 cm. in cross-section. In the U.S.A., similar pegmatites are known in South Dakota, at the Bob Ingersol mine in the region of Keystone, as well as in the states of Connecticut and Maine.

Gem-quality montebrasite, as well as amblygonite, is faceted or cut in cabochons. The most part of faceted stones, sold as amblygonite, are really montebrasites in the composition. The maximum size of faceted stones is up to 100 ct.; they were made from Brazilian rough material.

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