Crystal Structure of Sinhalite MgAlBO4 under High Pressure

• Published in: Journal of physical chemistry. C Vol. 119; no. 12; pp. 6777 - 6784
• Main Authors: Santamarı́a-Pérez, D, Errandonea, D, Gomis, O, Sans, J. A, Pereira, A. L. J, Manjón, F. J, Popescu, C, Rodrı́guez-Hernández, P, Muñoz, A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26-03-2015
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp512131e

We report on high-pressure angle-dispersive X-ray diffraction data up to 27 GPa for natural MgAlBO4 sinhalite mineral and ab initio total energy calculations. The experimental bulk modulus of sinhalite is B 0 = 171(3) GPa with a first-pressure derivative of B 0′ = 4.2(3). A comparison with other olivine-type compounds shows that the value for B 0 is 27% larger than that of Mg2SiO4 forsterite and 29% smaller than that of Al2BeO4 chrysoberyl. These differences are interpreted, on the basis of our ab initio calculations, in terms of the relative incompressibility of Al–O bonds in AlO6 octahedra (with a calculated bulk modulus of 250(1) GPa) as compared to Mg–O bonds in MgO6 octahedra (with a calculated bulk modulus of 130(1) GPa). The spatial cation distribution in the Pbnm orthorhombic unit cell and different polyhedral compressibilities entails a strong anisotropic compression comparable to that of forsterite. The axial compressibilities are 1.06(2) × 10–3, 2.17(2) × 10–3, and 1.30(3) × 10–3 GPa–1 for a, b, and c axes, respectively. The crystal chemistry of sinhalite under compression is compared to that of other olivine-like compounds. Compressibility trends and possible high-pressure phases are discussed.