Theoretical investigation of moganite.

The structure and properties of two different modifications ofmoganite 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 ofmoganite has been elucidated, and it is argued thatmoganite is significantlymore compressible than quartz. Observed and calculated NMR spectra are compared, and it is found that the bonding in quartz andmoganite 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 shellmodel interatomic potentials appear to bemore transferable from quartz tomoganite than partial-charge rigid ion equivalents