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Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

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SIDERITE


SIDERITE (Siderit—Sidérite——идерит) (Beudant, 1832). The name reflects its iron (Gk. “sideros”) content.

Composition & Properties. Carbonate – Fe[CO3], trygonal system. Hardness 3.5-4.5. Density 4. Glass luster to pearl and metallic one. Cleavage in three directions. Fragile. It forms the row of constant miscibility with magnesite and rhodochrosite. Depending on the content of admixtures, they classify such varieties: MnCO3 up to 5% – manganosiderite, up to 15% – oligonite; MgCO3 up to 30% – sideroplesite, up to 30-50% – pistomesite, up to 50-70% – mesotite; Caé up to 12% – siderot, cobalt up to 9% and zinc up to 7.7% – zincoligonite. Siderite is widespread. It is found in the shape of isometric crystals, often bowed, twisted to spherocrystals – spherosiderite, and also in the shape of ball-like oolites, stalactites, kidneys, concretions and nodules up to 1m. in diameter, as well as grained masses. Color: white, gray, yellowish-gray, pale green; oxidized crystals are brownish with reddish tone, sometimes with polychromatic blueing. Transparent to translucent.

Deposits. It is formed in the processes of metamorphism, metasomatism, in hydrothermal veins and sedimentary rocks. In the zone of oxidation it is usually replaced with oxides. In Russia, in the South Urals, siderite is found in significant quantities at the Bakal’skoye deposit and at the Akhtenskiy mine. In the center of Russia, in the Kurskaya Magnetic Anomaly, siderite is found at the Mikhailovskiy mine, near Zheleznogorsk. On the Sakhalin Is. and in the north of Kamchatka, it replaces fossil wood in sedimentary layers. In the Ukraine, in Crimea, there is the Kerchenskoye deposit of siderite. Crystals of siderite available for faceting were found in Great Britain, at the Tavistock Dist., Devon; and on the Cornwall Penin. they discovered an unusual green siderite – chalybit. In France, crystals of siderite up to 15 cm. in alpine veins were found at the La Taillat mine, Isère Dépt. In Germany, transparent crystals of siderite up to 6 cm. were discovered near Neudorf, in the Harz Mts, Lower Saxony; and in ore veins near Freiberg, Saxony. In the east of Austria, deposits of siderite are situated near Erzberg, and near Huttenberg, Carinthia. In Hungary, at the Erdöbénye deposit, near Sorenz, and at some others in the Tokay Mts., they found kidney-shaped aggregates of spherosiderite of the concentric-shell structure. In Portugal, ornamental siderite and its light brown tabular crystals were extracted at the Panasqueira mine. In Denmark, at the Greenland massive, brown siderite up to 10 cm., together with cryolite, is found at the Ivigtut deposit. In Brazil, deposits of siderite are: in Minas Gerais – Novo Limo and Morro Velho with translucent crystals up to 15 cm. in Bolivia – Cerra de Coroloque, Potosí. In Canada, at the Mont St Hilaire, Québec Prov., they found crystals of siderite more than 30 cm. in cross-section. They made faceted stones up to 2.6 ct. from them.

Synonyms. Chalybit, from Gk. “chalips” – steel, because of the Iron content | Oligonite, from Gk. “oligos” – small, because of its less density | Brown spar | Ferrous spar | Oligonite spar | Steel stone.

Cut Gems. For collection purposes, transparent crystals of siderite are slightly faceted. Usually they are not more than 5 ct. The most beautiful among them are those made from Portuguese rough material. Ornamental varieties are cut in cabochons and used in stone-carving.

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