Abstract text (incl. figure legends and references)

Mg­₂SiO₄ is a ceramic with geological and engineering relevance. It is the most abundant mineral in the Earth's upper mantle, meaning its properties dominate mantle behaviour. Additionally, it is a refractory material with applications in microwave dielectric devices and promising bio-compatible properties.

It has been shown that the grain boundary energy is inversely correlated to the total area of that interface in a system(Li et al., 2009). The Grain Boundary Character Distribution (GBCD) shows the proportion of different types of interfaces in a system and is key to understanding melt extraction, texture formation, creep regimes, and electrical and optical properties of materials. It has also been shown that the relative grain boundary energy varies with grain size(Bojarski et al., 2013). In this study we aim to investigate these relationships in the Mg­₂SiO₄ system, with studies of specific interfaces to determine their complexions.

Synthetic Mg­₂SiO₄ samples with grain sizes of 0.4, 3.4, 7.8 and 5.4 μm were synthesised by vacuum sintering(Koizumi et al., 2010) and subsequently characterized with Electron Backscatter Diffraction (EBSD) and Atomic Force Microscopy (AFM). Over 230,000 grain boundary segments were extracted from EBSD maps encompassing over 70,000 grains to extract the Grain Boundary Character Distribution (GBCD) of each sample. Thermal etching was used to reveal grain boundary grooves which were mapped using AFM to extract their depth profile. From this data the relative grain boundary energy for different grain sizes was determined using a simplified Young's Equation. Finally, thin sections were prepared using mechanical and ion polishing for HRTEM imaging of individual interfaces.

Analysis of thermal grooves shows a decrease in average relative grain boundary energy with increasing grain size from 0.48 arbitrary units for 0.4 μm grain size to 0.41 for an average grain size of 7.8 μm. Furthermore we observe a difference in the relative grain boundary energy between normal and abnormally grown grains. We will discuss these results in relation to the GBCD and structure.

Bojarski, S. A., Knighting, J., Ma, S. L., Lenthe, W., Harmer, M. P., & Rohrer, G. S. (2013). The relationship between grain boundary energy, grain boundary complexion transitions, and grain size in Ca-doped Yttria. Materials Science Forum, 753, 87–92. https://doi.org/10.4028/www.scientific.net/MSF.753.87

Koizumi, S., Hiraga, T., Tachibana, C., Tasaka, M., Miyazaki, T., Kobayashi, T., Takamasa, A., Ohashi, N., & Sano, S. (2010). Synthesis of highly dense and fine-grained aggregates of mantle composites by vacuum sintering of nano-sized mineral powders. Physics and Chemistry of Minerals, 37(8), 505–518. https://doi.org/10.1007/s00269-009-0350-y

Li, J., Dillon, S. J., & Rohrer, G. S. (2009). Relative grain boundary area and energy distributions in nickel. Acta Materialia, 57(14), 4304–4311. https://doi.org/10.1016/j.actamat.2009.06.004