Topaz-OH (Al2SiO4(OH)2), phase egg (AlSiO3OH) and delta-AlOOH are hydrous phases in the Al2O3-SiO2-H2O system that have been found to be stable at successively higher pressures up to those of the lower mantle, and thus may be important in the siliceous sediments of subducting slab. Although the crystal structures of these phases have been determined by X-ray diffraction, this technique is insensitive to H positions and Al-Si disorder. We have applied 1H, 29Si and 27Al NMR and micro-Raman spectroscopy to gain additional insights. Our study revealed that the octahedral Al-Si distribution in phase egg is partially disordered, which is expected to have a significant effect on its physical and thermodynamic properties and stability field. In addition, these data also suggest the presence of some structural disorder in topaz-OH. Finally, our data support that hydrogen bonding is strong in phase egg and delta-AlOOH, but weak in topaz-OH.
