Introduction: accidental synthesis

Pumpellyite is a complex hydrous Ca-silicate with additional (Al, Fe3+) and (Mg, Fe2+) (Coombs et al., 1976). It occurs mainly in very-low-grade metamorphic rocks, preferably of basic compositions, for example in former amygdules of metabasalts. In the classical grid of metamorphic facies (Winkler, 1974), the ‘pumpellyite–prehnite facies’ is located in the temperature range of 300° ± 50 °C and at pressures below about 3kbar. This is in general agreement with experimental data, such as those obtained by Schiffman & Liou (1980), who determined the stability field of the Fe-free pumpellyite end member Ca4MgAl5Si6O21(OH) 7, which they named MgAl-pumpellyite. They found the upper thermal stability of this phase to lie near 350 °C for a water pressure of 3 kbar. However, they also recognized that this temperature limit increases with water pressure, so that the stability field of MgAl-pumpellyite extends to at least 390 °C at 8 kbar.

The experimental petrology group at Bochum has recently conducted high-pressure studies in excess of 30 kbar, with the goal of determining the phase relations of unusually (Ca, Fe, Na)-poor, but Mg-rich metapelites, which had been found, through field and laboratory studies, to be particularly diagnostic for very deep-seated metamorphism of crustal rocks in subduction zones (Chopin & Schreyer, 1983, Chopin, 1984, Schreyer, 1985). Because of its fundamental importance, the system MgO–Al2O3–SiO2–H2O (MASH) was a primary objective.