T238 Advances in Mineral Sciences from Atomistic to Macro Perspectives I: Recognition of Dr. Nancy L. Ross, 2024 Roebling Medalist of the Mineralogical Society of America

Wednesday, 25 September 2024
8:00 AM - 12:00 PM
206B

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

Paper number: T238-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

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).

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-405805

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.