Crookesite is a selenide mineral composed of copper and selenium with variable thallium and silver. ==Characteristics== Its chemical formula is reported either as Cu7(Tl,Ag)Se4 or (Cu,Tl,Ag)2Se. It is formed by precipitation from hydrothermal fluids, and contains by mass: 16.3% Tl, 47.3% Cu, 2.9% Ag, and 33.6% Se. Crookesite is an opaque, bluish g...
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http://en.wikipedia.org/wiki/Crookesite
a rare mineral, selenide of copper, thallium, and silver, (Cu, Tl, Ag)Se, occurring in steel-gray, compact masses.
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https://www.infoplease.com/dictionary/crookesite
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