Equation of state of phase E

The isothermal equation of state (EoS) of phase E, Mg1.96(7)Fe0.072(5)Si1.0 4(5)H3.7(8)O6, has been determined using high-pressure single-crystal X-ray diffraction. A third-order Birch Murnaghan EoS fit to pressue-volume data collected from room pressure and temperature to 6.7 GPa reveals that phase E has the lowest bulk modulus, K-T = 92.9(7) GPa, and highest pressure deri vative of the bulk modulus, K' = 7.3(2), for any dense hydrous magnesium si licate (DHMS) yet measured. A parameterized third-order Birch-Mumaghan EoS was also fit to the unit-cell parameters which display significant curvatur e with increasing pressure. This analysis shows that the c-axis (K-c = 89.1 (10) GPa) is 6% more compressible than the a-axis (K-a= 94.8(6) GPa), with little of the anisotropy commonly observed in other layered structures. The high K' is indicative of the similarity to layers of the brucite structure . The introduction of interlayer cation polyhedra to the structure serves t o reduce both the anisotropy, by reducing the compressibility perpendicular to the sheets, and the ability to shear, by increasing the coherence betwe en layers.