In situ X-ray diffraction investigation of lawsonite and zoisite at high pressures and temperatures

The P-V-T behavior of synthetic lawsonite [CaAl2(Si2O7)(OH)(2). H2O] and sy nthetic zoisite [Ca2Al3(SiO4)(Si2O7)O(OH) has been determined under high pr essures and high temperatures up to 7 GPa and 1000 degrees C using a MAX 80 cubic anvil high-pressure apparatus. Several experimental runs were carrie d out for both minerals, first compressing the samples at room temperature and then heating them until a breakdown reaction occurred. Generally, the s amples were mixed with vaseline to ensure hydrostatic pressure transmitting conditions, NaCl served as an internal standard for pressure calibration. The pressure data at room temperature were fitted to a Birch-Murnaghan equa tion of state (K'(T0) assumed to be 4), yielding isothermal bulk moduli K-T 0 = 106.7 +/- 1.3 GPa for lawsonite and 125.1 +/- 2.1 GPa for zoisite, resp ectively. These values agree well with recent results. The high-temperature high-pressure data were fitted to the high-temperature Birch-Murnaghan equ ation of state. In contrast to other investigations, partial derivative K(T )partial derivative T)(P) is slightly negative for lawsonite. The sign is n ot affected by considering the thermal expansion data at ambient pressure. Similar calculations for zoisite yielded a positive partial derivative K(T) partial derivative T)(P) without using high-temperature data at atmospheric pressure whereas a negative sign resulted from calculations taking into ac count the 1 bar thermal expansivity Calculations of several mineral equilib ria involving zoisite and lawsonite with the Berman data set supplemented b y the P-V-T data presented here correspond well to experimentally determine d pressure stability limits of these phases.