Synthesis, stability, and properties of Al₂SiO₄(OH)₂: A fully hydrated analogue of topaz

The OH end-member of the F-OH topaz solid-solution series, with a composition close to Al₂SiO₄(OH)₂, has been synthesizedat pressures between 55 and 100 kbar and temperatures up to 1000°C from gels and crystalline starting materials. The results of single-crystal X-ray diffraction [space group Pbnm, a = 4.724(3) Å, b = 8.947(7) Å, c = 8.390(8) Å] show close agreement with the structure of F-rich topaz but a nearly pure dilatation of both the ^[6]Al (5%) and the ^[4]Si (1.5%) sites; the distortion parameters of these sites were slightly affected. Optical properties are n_x || a = 1.700(1), n_y || b = 1.701(1), n_z || c = 1.703(1), Δn = 0.0040(5), and 2V_γ = 28(2)°. In the infrared spectrum, the OH-stretching vibration is split into two bands at 3600 and 3520 cm⁻¹. This synthetic phase, referred to in this paper as “topaz-OH”, breaks down to kyanite + H₂O above 750°C at 60 kbar and above 1050°C at 90 kbar of H₂O pressure. Toward lower pressures and temperatures its stability field is limited by assemblages with diaspore, coesite, and phase Pi of the probable composition Al₃Si₂O₇(OH)₃. It is suggested that “topaz-OH” may form in subducting slabs as a transient phase at depths of about 175 km.