An experimental study of the pressure-temperature stability of MgMgAl-pumpellyite in the system MgO-Al 2 O 3 -SiO 2 -H 2 O

The stability field of MgMgAl-pumpellyite. Mg 5 Al 5 Si 6 O 21 (OH) 7, was determined in the system MgO-Al 2 O 3 -SiO 2 -H 2 O in reversal experiments at pressures between 34 and 100 kbar and temperatures in the range of 597 to 1050 °C. Brackets were obtained on five breakdown reactions (in order of increasing pressure): This phase becomes stable only at pressures of more than 34 kbar and temperatures up to 820 °C. Thus, MgMgAl-pumpellyite may be an H 2 O-containing phase at depths greater than 100 km in the coldest parts of subduction zones.