Why can magnesium be extracted from seawater even though some rocks contain more magnesium?

While some rocks, such as dolomite and magnesite, contain higher concentrations of magnesium than seawater, seawater is utilized as a primary source because it represents an almost inexhaustible supply of the element.1

The decision to extract magnesium from seawater is driven by the accessibility and scale of the resource rather than the concentration level alone. While seawater contains only about 0.13 percent magnesium, it is readily available for large-scale industrial processing, such as the Dow process, where it is mixed with reactive dolomite to precipitate magnesium hydroxide.21

In contrast, mining magnesium from solid rock ores requires conventional mining and concentration methods, which involve different economic and logistical considerations. Ultimately, the choice between using seawater or mineral deposits depends on the specific extraction and refining technology employed, such as electrolysis or thermal reduction, and the relative costs of labor and energy in a given region.34