EQUATIONS OF STATE OF PLAGIOCLASE FELDSPARS

The volume variation with pressure of seven intermediate plagioclase feldspars has been determined by high-pressure single-crystal X-ray diffraction. The bulk moduli (in GPa) of plagioclases for a 3rd-order Birch-Murnaghan EoS can be described by the pair of equations:

K_T0=54.1(3) + 0.39(1)X(An) for X(An) < 50 and K_T0=59.5(3.1) + 0.23(1)X(An) for X(An) > 50

with K'=5.8 for plagioclase with X(An) < 20 and K'=3.2 for X(An) > 35. These parameters can also be used in a Murnaghan EoS to describe the volume variation of plagioclase feldspars up to pressures of 3 GPa. For a Murnaghan EoS with K'=4, the values of the bulk moduli can be described by a single equation, K_T0=57.7(6) + 0.24(1)X(An), with a small loss in the accuracy of the predicted volumes up to pressures of 3 GPa.