Ewing, Rodney C. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico.
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Minerals that contain uranium (U) or thorium (Th) as an essential component of their chemical composition. Examples are uraninite (UO2) or thorite (ThSiO4). There are radioactive minerals in which uranium and thorium substitute for ions of similar size and charge. There are approximately 200 minerals in which uranium or thorium are essential elements, although many of these phases are rare and poorly described. These minerals are important, as they are found in ores mined for uranium and thorium, most commonly uraninite and its fine-grained variety, pitchblende, for uranium. Thorite and thorogummite are the principal ore minerals of thorium. Minerals in which uranium and thorium occur in trace amounts, such as zircon (ZrSiO4), are important because of their use in geologic age dating. The isotope uranium-238 (238U) decays to lead-206 (206Pb); 235U decays to 207Pb; 232Th decays to 208Pb; thus, the ratios of the isotopes of uranium, thorium, and lead can be used to determine the ages of minerals that contain these elements. See also: Dating methods; Lead isotopes (geochemistry); Thorite
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