LAWSONITE - UPPER PRESSURE STABILITY AND FORMATION OF HIGHER DENSITY HYDROUS PHASES

The high-pressure phase relationships in a H2O-saturated synthetic CaO -Al2O3-SiO2-H2O (CASH) system were studied by multi-anvil experiments. The most extreme pressure condition under which pure lawsonite [CaAl2 Si2O7(OH)(2) . H2O] exists is 120 kbar at 960 degrees C. This maximum stability is located at the intersection of the lawsonite breakdown re action with the topaz-OH + stishovite = phase ''egg'' reaction. At low er pressures and higher temperatures lawsonite decomposes to grossular + topaz-OH + stishovite + H2O, whereas at lower pressures and lower t emperatures lawsonite first reacts to grossular + phase egg + topaz-OH + H2O and at still lower temperatures to grossular + phase egg + dias pore + H2O. The two latter reactions have positive dP/dT slopes, with lawsonite on the low-pressure side, and thus delimit the occurrence of lawsonite toward higher pressures, The occurrence of topaz-OH (10.7 w t% H2O) is limited through a reaction to phase egg (7.5 wt% H2O) + dia spore; the phase boundary extends from 110 kbar and 720 degrees C to 1 30 kbar and 920 degrees C. Phase egg is inferred to have a composition of AlSiO3(OH) and a monoclinic unit cell similar to that proposed by Eggleton et al. (1978). The high-pressure breakdown of lawsonite in CA SH does not result in an anhydrous assemblage. Lawsonite is known to o ccur experimentally in basaltic and andesitic compositions to at least 77 kbar; however, it is unknown whether topaz-OH and phase egg appear in natural multicomponent systems, in particular those saturated in k yanite and H2O.