P-V Equations of State and the relative stabilities of serpentine varieties

Serpentines are hydrous phyllosilicates which form by hydration of Mg - Fe minerals. The reasons for the occurrence of the structural varieties lizardite and chrysotile, with respect to the variety antigorite, stable at high pressure, are not yet fully elucidated, and their relative stability fields are not quantitatively defined. In order to increase the database of thermodynamic properties of serpentines, the P - V Equations of State (EoS) of lizardite and chrysotile were determined at ambient temperature up to 10 GPa, by in situ synchrotron X-ray diffraction in a diamond-anvil cell. Neither amorphization nor hysteresis was observed during compression and decompression, and no phase transition was resolved in lizardite. In chrysotile, a reversible change in compression mechanism, possibly due to an unresolved phase transition, occurs above 5 GPa. Both varieties exhibit strong anisotropic compression, with the c axis three times more compressible than the others. Fits to ambient temperature Birch - Murnaghan EoS gave for lizardite V-0=180.92( 3) angstrom(3), K-0 = 71.0(19) GPa and K'(0)= 3.2(6), and for chrysotile up to 5 GPa, V-0 = 730.57(31) angstrom(3) and K-0 = 62.8( 24) GPa (K'(0) fixed to 4). Compared to the structural variety antigorite is stable at high pressure ( HP) (Hilairet et al. 2006), the c axis is more compressible in these varieties, whereas the a and b axes are less compressible. These differences are attributed to the less anisotropic distribution of stiff covalent bonds in the corrugated structure of antigorite. The three varieties have almost identical bulk compressibility curves. Thus the compressibility has negligible influence on the relative stability fields of the serpentine varieties. They are dominated by first-order thermodynamic properties, which stabilizes antigorite at high temperature with respect to lizardite, and by out-of-equilibrium phenomena for metastable chrysotile ( Evans 2004).