Using Multigrain Crystallography to Explore the Microstructural Evolution of the <i>α</i>-Olivine to <i>γ</i>-Ringwoodite Transformation and <i>ε</i>-Mg₂SiO₄ at High Pressure and Temperature

• Main Authors: Brian Chandler, Michelle Devoe, Martin Kunz, Hans-Rudolf Wenk
• Format: Article
• Language: English
• Published: MDPI AG 2021-04-01
• Series: Minerals
• Subjects: multigrain crystallography; phase transformations; plastic deformation olivine; ringwoodite; ε-Mg₂SiO₄
• Online Access: https://www.mdpi.com/2075-163X/11/4/424

The introduction of multigrain crystallography (MGC) applied in a laser-heated diamond anvil cell (LH-DAC) using synchrotron X-rays has provided a new path to investigate the microstructural evolution of materials at extreme conditions, allowing for simultaneous investigations of phase identification, strain state determination, and orientation relations across phase transitions in a single experiment. Here, we applied this method to a sample of San Carlos olivine beginning at ambient conditions and through the <i>α</i> olivine → <i>γ</i>-ringwoodite phase transition. At ambient temperatures, by measuring the evolution of individual Bragg reflections, olivine shows profuse angular streaking consistent with the onset of yielding at a measured stress of ~1.5 GPa, considerably lower than previously reported, which may have implications for mantle evolution. Furthermore, <i>γ</i>-ringwoodite phase was found to nucleate as micron to sub-micron grains imbedded with small amounts of a secondary phase at 15 GPa and 1000 °C. Using MGC, we were able to extract and refine individual crystallites of the secondary unknown phase where it was found to have a structure consistent with the ε-phase previously described in chondritic meteorites.