A NANCY ROSS-STYLE DESCRIPTION OF HIGH-PRESSURE HYDROUS STISHOVITE ((Si,H4)O2).

Nancy Ross has been, and is, a major influence in the interpretation of high pressure (hp) phase transitions, including second-order and quasi-second-order transitions induced by pressure. For instance, Ross, Ko and Prewitt (1989) famously described how a mineral, geikelite (MgTiO₃ ilmenite) transforms to perovskite structure at high pressure, then quenches to a LiNbO₃ structure during decompression at room temperature. Many parallel examples have since been found, most recently a highly aluminous Bridgmanite ((Mg,Al,Si)O₃) that quenches into a lithium niobite form (Ishii et al, 2017). The relationship between the LiNbO₃ and perovskite structures was first described by Megaw (1973).

In this presentation, a composition and pressure-induced sequence between an 11-coordinated oxygen packing and 12-coordinated hcp oxygen packing is described in SiO₂-H₂O. In stishovite (hp SiO₂), which has the rutile structure with the 11-cordinated oxygen packing (O'Keefe and Hyde, 1996), the addition of one to several percent of an H₂O component to the structure at 9 GPa and temperatures from 300-650 degrees C causes a significant expansion of the a lattice parameter while the c lattice parameter remains relatively constant (Spektor et al, 2011). The enthalpies of solution in calorimetry combined with the partial molar volume of H₂O in stishovite points to a significant increase of the water solubility with pressure (same ref.).

Subsequent in-situ work in the laser-heated diamond-anvil cell (Nisr et al, 2020) showed that the addition of H₂O to stishovite under pressure does indeed become more favorable, and up to 8 weight percent H₂O is allowed. The addition of water also favors a transition to the CaCl₂ structure with structural distortions far greater than those seen in dry SiO2, approaching hcp packing of oxygens, and the stabilization of hydrous CaCl₂ down to 30 GPa. At higher pressures (60 GPa), hcp (Si,H₄)O₂ is attained with a presumed disordered NiAs structure, unquenchable.

The high solubility of water in stishovite above 30 GPa leads to interesting behavior in mantle systems. Because of the addition of a new component (H₂O) to stishovite, it becomes possible for hydrous stishovite to coexist stably with bridgmanite, leading to the possible presence of stishovite in the lower mantle of the Earth taking up any H₂O component that is present (Huawei Chen PhD. thesis, 2019).