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Magnesium occurs in minerals such as dolomite, magnesite, and carnallite, as well as in seawater. It is obtained by electrolysis of molten magnesium chloride or by thermal reduction. The light, silvery-white metal develops a protective surface film and burns with an intense white flame. It reacts slowly with cold water but more vigorously with steam, and dissolves in dilute acids with hydrogen release. Magnesium also forms Grignard reagents, which are useful in carbon–carbon bond synthesis. Its compounds have varied applications: magnesium oxide is used as a refractory and antacid, magnesium hydroxide as a laxative and antacid, and Epsom salt as a cathartic. In the body, magnesium supports chlorophyll structure and ATP-dependent enzymes; deficiency and excess can have significant clinical effects.