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Reference.org
Clarke number
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<p>Clarke number or clarke is the relative abundance of a <a href="/facts/Chemical_element/0jducgWD">chemical element</a>, typically <a href="/facts/Abundance_of_elements_in_Earth%2527s_crust/UDSVHH56">in Earth's crust</a>. The technical definition of "Earth's crust" varies among authors, and the actual numbers also vary significantly. </p>