Structure and Physical Properties of Hydrogrossular Mineral Series
The mineral hydrogrossular series (Ca₃Al₂(SiO4)₃₋ₓ(OH) ₄ₓ; 0 ≤ x ≤ 3) are important water bearing minerals found in the upper and lower part of the Earth’s mantle. They are vital to the planet’s hydrosphere under different hydrothermal conditions. The composition and structure of this mineral series are important in geoscience and share many commonalities with cement and clay materials. Other than the end members of the series x = 0 (grossular) and x = 3 (katoite) which have a cubic garnet structure, the structure of the series is totally unknown. We used large-scale ab initio modeling to investigate the structures and properties for hydrogrossular series for x = 0, 0.5, 1, 1.5, 2, 2.5, 3. Results indicate that for x > 0 and x < 3, the structures are tetragonal. This shows that there is structural change related to the lowering of overall symmetry associated with the composition of SiO4 tetrahedra and AlO6 octahedra. Total Bond order also explains the reason behind the change in the compressibility of the series. The electronic structure, mechanical and optical properties of the hydrogrossular series are calculated and the results for grossular and katoite are in good agreement with the available experimental data. The x–dependence of these physical properties for the series supports the notion of the aforementioned structural transition from cubic to tetragonal. Key words: Hydrogrossular series, structural competition, electronic structure, mechanical properties, and optical properties.
Table of Contents
Introduction -- Theoretical background -- Simulation packages and methods used -- Structural modeling of the hydrogrossular series -- Electronic and mechanical properties of the hydrogrossular -- Conclusion -- Appendix