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Theoretical investigation of moganite

¹ Institut für Mineralogie/Abt. Kristallographie, Senckenberganlage 30, D-60054 Frankfurt a.M., Germany
² TCM Group, Cavendish Laboratory, Cambridge CB3 0HE, UK
³ Nanochemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, GPO Box U 1987, Perth 6845, Western Australia
⁴ Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK
⁵ Accelrys, 334 Cambridge Science Park, Cambridge CB4 0WN, UK
⁶ Laboratoire de Minéralogie-Cristallographie de Paris, Université Pierre et Marie Curie, 4 Place Jussieu, F-75252 Paris, France

^* Corresponding author, e-mail: u.hantsch{at}kristall.uni-frankfurt.de

The structure and properties of two different modifications of moganite have been studied using density functional theory, and the results have been compared to quartz. It is shown that the enthalpy difference between quartz and moganite, whose structure can be understood as Brazil twinning of quartz on a unit cell length scale, is negligible. This explains the significant amount of moganite in fine-grained quartz samples, as well as the frequent occurrence of Brazilian twinning in quartz. The compression mechanism of moganite has been elucidated, and it is argued that moganite is significantly more compressible than quartz. Observed and calculated NMR spectra are compared, and it is found that the bonding in quartz and moganite is very similar, consistent with the results of a Mulliken population analysis. The elastic stiffness coefficients of moganite have been predicted, and it is shown that formal-charge shell model interatomic potentials appear to be more transferable from quartz to moganite than partial-charge rigid ion equivalents.

Key-words: moganite, quartz, Brazil twinning, high pressure, density functional theory.