High-pressure experiments on the stability of hydrous minerals likely to be present in the Earth's mantle provide constraints on the distribution of water in the mantle, and the form in which it is stored. However, there has been confusion about compositions and structures between hydrous minerals. In regions of elevated mantle temperature, water may be stored not in hydrous minerals but in anhydrous minerals, melts, and fluids. Such water is important to understand the dynamics and evolution of the mantle, volcanism, and metasomatism. In this article, the state and migration of water in the mantle are reviewed.
