Meteoritic Clues Point Chromium Toward Earth's Core

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Science  18 Mar 2011:
Vol. 331, Issue 6023, pp. 1397-1398
DOI: 10.1126/science.1203353

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Chromium, named for the Greek word for color, is a wonderful element that adds green to emeralds, red to rubies, brilliance to plated metals, and corrosion-proof quality to stainless steels. It is Earth's 10th most abundant element and is distributed almost equally between the metallic core and the rest of the planet. It normally occurs as a trivalent ion (able to form bonds with three other atoms), but there is also an oxidized hexavalent form that is hazardous to life. By studying the distribution of chromium's isotopic forms on Earth and in the solar system, researchers can gain insight into the processes that formed planets and produced the differentiation of Earth's core and mantle. On page 1417 of this issue, Moynier et al. (1) report that Earth's chromium isotopic composition is distinct from most primitive meteorites, lending support to one explanation for core and mantle formation. They also report that chromium's isotopic composition varies among different groups of meteorites, suggesting that the early solar nebula had at least two components with different isotopic compositions.