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 of plagioclases for a 3rd-order Birch-Murnaghan EoS can be described by the following pair of equations: K-T0 = 54.1(3) + 0.39(1)XAnXAn < 50 K-T0 = 59.5(3.1) + 0.23(4)XAnXAn > 50 with K'(0) = 5.8 for plagioclase with X-An<20 and K'(0) = 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'(0) = 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.