Pressure induced coordination change of Al in silicate melts from Al K edge XANES of high pressure NaAlSi₂O₆‐NaAlSi₃O₈ glasses

Aluminum K‐edge XANES spectra of high pressure and high temperature (4.4 GPa and 1575°C) glasses along the NaAlSi 2 O 6 (Jd)‐NaAlSi 3 O 8 (Ab) join are reported using synchrotron radiation, and shown to provide direct experimental evidence for the pressure‐induced coordination change of Al. Five‐ and six‐fold coordinated Al ( 5 Al and 6 Al), characterized by Al K‐edge positions at 1567.8 and 1568.7 eV, respectively, first appear in glass of composition Jd 60 Ab 40 and increase in proportion progressively with increasing Jd content. The end‐member jadeite glass contains about 6% of each of 5 Al and 6 Al. The present direct measurements confirm literature suggestions for the important role of Al in controlling viscosity and diffusion in mantle melts.