In-situ crystal structure determination of a minor phase in a multiphase system at megabar pressure - Dr. Li Zhang
Shanghai, Feburary 16, 2016 — Multiple phases often coexist in a polycrystalline sample as a result of phase equilibrium in high-pressure petrological studies. Seifertite SiO2 likely exists as a minor phase near the Earth’s core-mantle boundary (CMB). Structure determination of a minor phase in a sample contained in a diamond anvil cell (DAC) has never been impossible using conventional powder or single crystal diffraction techniques. Utilizing the multigrain crystallography and newly developed procedures, new work led by HPSTAR scientists, Li Zhang et al., has overcome the difficulties and obtained the first in-situ single-crystal structure of seifertite being a minor phase in a polycrystalline sample at 129 GPa (Am. Mineral. 101, 231–234, 2016).
Seifertite is a dense polymorph of SiO2 and experiments found it stable above 117 GPa and 2500 K, conditions representative of the core-mantle boundary. By simulating the pressure and temperature conditions near the core–mantle boundary, coarse-grained seifertite SiO2 was synthesized as a minor phase in a polycrystalline sample coexisting with the major phase (Mg,Fe)SiO3 post-perovskite (pPv) phase at 129 GPa and 2500 K in a DAC. Data sets from six grains of seifertite having arbitrary orientation have been combined and merged for structure determination and refinement. Observed systematic absences of reflections from the six individual grains allowed only one space group: Pbcn. The determined structure matches the structure of a post-stishovite phase of SiO2from Shergotty meteorite determined at ambient pressure by X-ray powder diffraction.
“This study demonstrates that the multigrain approach can be applied to tough data collections for studying phase transitions, crystal chemistry, and chemical reactions in a petrological multiphase system under P-T conditions representative of the deep Earth”, said Dr. Li Zhang, a staff scientist at HPSTAR. This work was performed in collaboration with HPCAT at the Advanced Photon Source (APS). Other authors include Dmitry Popov and Yue Meng from HPCAT, Junyue Wang, Cheng Ji, Bing Li and Ho-kwang Mao from HPSTAR.
Caption: Coarse-grained XRD pattern of seifertite being a minor phase at 129 GPa and after T quench.