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Russian Gemstones Encyclopedia

Vladimir Bukanov. Russian Gemstones Encyclopedia

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PIEMONTITE


PIEMONTITE (Piemontit—Piémontite—ѕьемонтит) (Kenngott, A. 1853), after its discovery location, at the Pravorna mine, St Marcel, Piemonte, Italy.

Composition & Properties. Silicate – Ca2(Mn3+,Fe)(Al,Mn3+)2[O|OH|SiO4|Si2O7], subclasses ring silicates, monoclinic system. Hardness 6.5. Density 3.4-3.5. Glass luster. Perfect cleavage in one direction. Fragile. Piemontite is represented with columnar crystals, radial aggregates and solid grained masses, rarely found in large volume. Translucent to non-transparent. Color: straw-yellow, reddish-brown, rose to red, black. Pleochroism from yellow to amethyst-violet and red. Beside cherry-red piemontite, its varieties are known: rich with MgO picroepidote of red-black color and manganese-containing withamite – red to straw-yellow.

Deposits. Piemontite is found in metamorphic and metasomatic rocks, also more rarely in pegmatites at deposits of manganese. In Russia, it was discovered in the Middle Urals, near the Kytlym settlement, in the shape of raspberry-red columnar crystals up to 3 cm. long. In the South Urals, piemontite is represented in the content of jaspers of the Staro-Muynakovskoye deposit, and at the Urazovskoye deposit it forms piemontite ore. In the Northern Caucasus, at the Urup deposit, lenses-shaped bodies of purple-red quartz are enriched with micro inclusions of piemontite. This ornamental material is used for cabochons. In Austria, in alpine veins among serpentines of the Öchsner massif, near Zillertal, crystals of wine-red and pink piemontite up to 15x5 cm. in size was found. Piemontite as massive material for cabochons is extracted as additional material in Italy, at the Parabona mine, St Marcel, Piedmont; in Great Britain – at the Glen Cow deposit, Scotland; in Sweden – at the Långban mine, near Paisberg; in France – on the Croix Is., Brittany Prov. In Egypt, near ˘ Gabal Dokhan, piemontite withamite is a rock-forming mineral of antique porphyry. In Japan, on the Shikoku Is., piemontite was discovered in glaucophane schists; and in the south of New Zealand – in quartzite schists near Otago. In the U.S.A., findings of piemontite are known at the Franklin deposit, New Jersey, and also in Arizona, California, Missouri and Pennsylvania. There is also an ornamental stone of special interest – piemontite rhyolites.

Transparent crystals are faceted, massive material and rocks with piemontite are cut in cabochons.

Synonyms. Manganepidote | Manganese epidote | Piedmontite | Withamite, after the Scottish mineralogist G. Witham.

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