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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-p

The serpentine mineral antigorite is known to be stable up to 6 GPa (200 km) at 620°C [1]. Under more extreme conditions, it breaks down to a dehydrated assemblage (Atg = Ol + Tlc (En) + H2O). Earthquakes at intermediate mantle depth may be triggered by dehydration embrittlement caused by antigorite breakdown. In spite of the importance of antigorite at subduction zones, little is known of its properties at high pressures. Knowledge of the elasticity of antigorite is important for the interpretation of seismological observations at subduction zones and for the detection of antigorite in this and other geological settings. This knowledge may be achieved, in principle, either experimentally or computationally. However, both routes are troublesome. Experimentally, one is faced with the high density of defects usually affecting single-crystal sized specimens, and with the difficulties in dealing with such a large unit cell and a DAC-shadowed reduced dataset. Computationally, the antigorite system is quite large and thus very costly. Moreover, a good guess of the crystal structure is mandatory to enhance the chances to successfully achieve any reliable result. We report here the results of an ab initio quantum mechanical investigation of the antigorite structures proposed in the recent literature. The structures investigated are the m = 17 and the m = 16 polysomes, XRD refined by Capitani & Mellini [2, 3] and a second m = 17 polysome proposed by Dódony & co-workers [4]. Basically, the first two structure models differ from the last by the presence of tetrahedral 8-reversals and the absence of octahedral offsets. The total free energy for the three models after structural relaxation are -6.983189, -6.986766, -6.977770 eV/atom, respectively. From these results it turns out that the models with tetrahedral 8-reversals and without octahedral offsets are energetically favoured with respect to the model excluding tetrahedral 8-reversals and including octahedral offsets. For c