FIRST PRINCIPLE COMPRESSIBILITY OF ANTIGORITE m = 17 UP TO 30 GPa

The zero pressure structure of the m = 17 antigorite polysome [Mg48Si34O85(OH)62] has been recently investigated by first principle methods (Capitani et al. 2009). Here we present the results of the investigation of the antigorite structure up to 30 GPa making use of the same computational method. Basically, the 291-atom primitive cell was allowed to relax at different volumes using density functional theory and the plane wave pseudopotential method implemented in the VASP code (Kresse & Furthmuller,1996). Both, local density (LDA) and generalized (GGA) gradient approximations to exchange and correlation were used. Theoretical results are in good agreement with recent single crystal diamond anvil cell experiments (Nestola et al., 2009). As for most sheet silicates, LDA and GGA bracket the experimental data, so that LDA would provide a better match if phonon excitation is taken into account (Mookerjee and Stixrude, 2008). The third order Birch-Murnaghan (BM3) equation of state (EoS) yields a better agreement with experiments if data above ~ 17 GPa are excluded from fitting: V0 = 2806.8 Å3, K0 = 71.0, and K’ = 5.3 for LDA (static calculations), to be compared with V0 = 2914.1 Å3, K0 = 62.9, and K’ = 6.1 for the experiments (dataset up to ~ 6 GPa). The reason is an abrupt change in compressional behavior above ~ 17 GPa. A similar discontinuity – in that case related to antigorite softening – was also found in the experiments, even if at much lower pressure (6 GPa). In the present case, the change in compressibility is related to a change in axial anisotropy and unit cell contraction mechanism. While at low pressure the relevant axial anisotropy is Kb > Ka > Kc – alike experiments – with increasing pressure the a parameter becomes more compressible than c. At the same time, the contraction of the unit cell that is mainly accomplished by interlayer thinning and halfwave flattening at moderate pressure, with increasing pressure switches to an abrupt increase of the in-plane tetrahedral rotation angle (ditrigonalization) and wavelength shortening and curling, which in addition occur differently in the short and long halfwaves.