Crystal structure of a new high-pressure polymorph of topaz-OH

A new high-pressure form of topaz-OH (denoted here topaz-OH II) was obtained at 14 GPa and 1400 °C. The X-ray diffraction pattern of this phase can be indexed by an orthorhombic cell with a = 4.72318(5), b = 8.91480(9), and c = 2.77276(3) Å. The lengths of a and b are similar, but c is approximately a third of that for a previously reported topaz-OH (denoted topaz-OH I). The structural formula of topaz-OH II can be written as (Al 0.68 Si 0.32 )(O 0.66 (OH) 0.34 ) 2 , suggesting significant cation disorder in the structure. The crystal structure of topaz-OH II is solved using powder synchrotron X-ray diffraction data, and refined with constraints provided by a separate multi-nuclear NMR study. The structure has similarities with topaz-OH I and diaspore (α-AlOOH) structures, having partially occupied double edge-shared octahedral chains, and 2 × 1 tunnels with partially occupied tetrahedral and octahedral sites.