Crystal chemistry of hydration in aluminous orthopyroxene

Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth’s lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refi ned the crystal structure of this potentially signifi cant variant of orthopyroxene. The experimentally produced crystals are small needles up to approximately 20 × 20 × 100 μm in size. Electron microprobe chemical analysis indicates about 11.7 wt% Al2O3. FTIR spectra indicate 7500 ppmw H2O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refi nement from single-crystal X-ray diffraction gives a = 18.1876(7) Å; b = 8.7352(7) Å; c = 5.1789 (5) Å, V = 822.79(11) Å3, which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refi ned from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refi ned structure indicates about 5% vacancy in M2 and signifi cant Al occupancy in both M1 and T2, consistent with its composition, (Mg0.95,0.05), (Mg0.79Al0.21), (Al0.25Si0.75) SiT1 O6. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions.