Crystal chemistry of phase B and an anhydrous analogue: implications for water storage in the upper mantle

Abstract

WATER locked in mineral phases within the Earth's mantle may be as significant for the development of life as water now found at the planet's surface. The Earth's original hydrosphere probably would not have survived a collision with a Mars-sized object, such as may have formed the Moon¹. The water now on the surface could have been replenished by further impacts of smaller planetesimals; however, at least a portion could have been stored in minerals deep in the mantle being released gradually through volcanic eruptions. This mechanism requires a stable high-pressure phase able to store water under mantle conditions. Of the several hydrous phases studied in the past, the material known as phase B has the highest density and is the only known hydrous form stable at pressures corresponding to depths of 400–500 km (refs 2, 3). Although phase B has been known for over twenty years, its crystal structure and crystal chemistry have remained an unsolved problem. Here we describe the crystal structures of phase B (Mg₁₂Si₄O₁₉(OH) ₂) and a chemically and structurally similar anhydrous form, AnhB (Mg₁₄Si₅O₂₄). These structures contain silicon in both fourfold and sixfold coordination; the silicon octahedra share all twelve edges with magnesium octahedra in a unique thirteen-cation cluster. Description of the structures of phases AnhB and B requires 18 and 40 atoms, respectively, demonstrating that high-pressure phases can have very complicated crystal structures. The multitude of octahedral sites in these phases could lead to rather complicated fractionation behaviour during solid–solid or solid–liquid transitions in the presence of other octahedrally coordinated ions such as Al, Fe, Ti and Mn.