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Frohbergite (, title by proper name: Max Hans Frohberg), also iron telluride is a rare hydrothermal mineral from the sulfide class, in composition — iron telluride with the ideal formula FeTe2 (contains 82.05% tellurium and 17.95% iron).
Frohbergite occurs in tellurium-rich veins of hydrothermal deposits, sometimes as a thin rim on the periphery of chalcopyrite or as inclusions in native gold, petzite or chalcopyrite. It belongs to the marcasite group and forms a mineral line with mattagamite. Frohbergite most often occurs as fine-grained aggregates. Bezsmertnaya M. S., Logikova L. A., Soboleva L. N. Determination of tellurides under a microscope. — Moscow: Nauka, 1969. — 175 p. (in Russian)
Frohbergite was found in polished sections as a rim up to 15 μ wide around chalcopyrite at the contact with altaite (PbTe), native gold and melonite. Associated minerals: tellurobismuthite, petzite (Ag,Au)2Te, pyrite, marcasite, chalcopyrite, covellite etc. O. M. Shubnikova. New mineral species and varieties discovered in 1945-1949. Proceedings of the Institute of Geological Sciences. Issue 144, mineralogical and geochemical series (No. 16). — Moscow: USSR Academy of Sciences Publishing House, 1953. — 155 p. (in Russian) For a long time, frohbergite was observed only in polished sections as a fine-grained aggregate, so its macroscopic features remained unstudied.
The mineral was named in honor of the Canadian geologist and mineralogist of German origin, Dr. Max Hans Frohberg (1901–1970, Toronto, Canada).
Based on its chemical formula and crystal structure, the mineral was initially assigned to the marcasite group, where it forms a continuous mineral line with mattagamite. Moreover, frohbergite and mattagamite respectively represent the extreme members of a series of Solid solution FeTe2 → cobalt2. Nekrasov I. Ya. Geochemistry, mineralogy and genesis of gold deposits. — Moscow: Nauka, 1991. — 302 p.
According to another classification proposed in the 1970s by the Bulgarian mineralogist Ivan Kostov, plumbotellurides of gold, copper, iron and silver (Au—Сu—Fe—Ag) were assigned to the bilibinskite group, as the most common mineral. Frohbergite was not assigned to this group directly, but its closeness to it was noted, it was established that plumbotellurides, rich in metal content, partially fill the gap in composition between pure tellurides (kostovite AuCuTe4, sylvanite AuAgTe4, frohbergite FeTe2, rickardite Cu5Те3, altaite PbTe) and minerals in the gold-copper system (Au—Сu). Spiridonov E., Bezsmertnaya M., Tatiana Chvileva, Bezsmertny V. Bilibinskite, Au3Cu2PbTe2, a new mineral gold-telluride deposits. Intern. Geol. Rev. 1979. Vol. 21. P. 1411—1415.
Most often, frohbergite occurs as fine-grained aggregates, the mineral is opaque, the color on the polished section is whitish with a bluish or pinkish tint, the luster is metallic, the mineral is strongly anisotropic: from red-orange to ink-blue. In a polished section, the hue of frohbergite is reddish-pink, which distinguishes it from the similar, but yellowish-pink melonite (NiTe2). The anisotropy is indistinct. Bireflection ranges from orange-red to the color of blue ink. Fluoroscopy of a microscopic sample showed its identity with the artificially produced compound FeTe2. In terms of immediate visual impression, the color of frohbergite is very similar to the main variety of germanite from the Tsumeb deposit (Namibia), differing from it in lightness (germanite R is 25%, and frohbergite is 55%) and lower saturation of chromatic tone.
Several iron tellurides were synthesized experimentally for crystallographic and chemical comparison. At temperatures below 519 °C, only two mineraloid compounds were stable: β-FeTe0,9 and, properly, FeTe2 (frohbergite).
Hydrochloric acid, potassium cyanide, iron trichloride, caustic potash and corrosive sublimate do not act on polished frohbergite. Diluted nitric acid (1:1) causes rapid boiling until a black spot is formed.
In its lilac hue and high relief, frohbergite resembles some minerals of the linnaeite-gersdorffite group. In isolated samples, frohbergite was sometimes confused with pyrrhotite. Frohbergite differs from the latter in its increased reflectivity and significantly lower saturation of the color tone, in which the bluish component noticeably predominates. With crossed nicols, the anisotropy of pyrrhotite is more pronounced, but the color effect is absent.
Frohbergite differs little from gersdorffite in optical properties, especially since the main distinguishing feature — optical anisotropy of frohbergite - cannot always be detected due to the extremely fine-grained structure of its aggregates. In addition, gersdorffite often has anomalous anisotropy.
It is obvious that frohbergite is present in the ores of the Zodskoye deposit, however, due to the very small size of the segregations and the extreme similarity of the optical characteristics of frohbergite and gersdorffite, it was not possible to more definitely clarify the diagnosis of the mineral in the 1960s. In the USSR in the 1970s, frohbergite was found in the Ozernoye (Murmansk Oblast, North Karelia, including in Finland) and Altyn-Tash (in the Chelyabinsk region) gold deposits, as well as in the Zhana-Tyube gold mines (Kazakhstan). Frohbergite (Фробергит) in the webmineral.ru database: minerals and deposits of Russia (in Russian) There are more than a dozen deposits around the world where frohbergite has been reliably identified.
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