Zinc makes up about 75 ppm (0.0075%) of Earth's crust, making it the 24th most abundant element. Typical background concentrations of zinc do not exceed 1 μg/m3 in the atmosphere; 300 mg/kg in soil; 100 mg/kg in vegetation; 20 μg/L in freshwater and 5 μg/L in seawater. The element is normally found in association with other base metals such as copper and lead in ores. Zinc is a chalcophile, meaning the element is more likely to be found in minerals together with sulfur and other heavy chalcogens, rather than with the light chalcogen oxygen or with non-chalcogen electronegative elements such as the halogens. Sulfides formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. Sphalerite, which is a form of zinc sulfide, is the most heavily mined zinc-containing ore because its concentrate contains 60–62% zinc.