Talc under tension and compression: spinodal instability

[1] Talc occurs as a product of alteration of ultramafic rocks and may be a significant constituent of oceanic lithosphere and the arc mantle wedge. We explore the physical properties and structure of talc over a wide pressure range with density functional theory using the variable cell shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. For LDA, the theoretical equation of state agrees with room temperature experimental measurements of the volume to within 2 % when estimated effects of lattice vibrations are accounted for. We predict a spinodal mechanical instability (vanishing bulk modulus) at a volume 4 % larger than the experimental volume at ambient conditions. Results for compression and tension are well 2represented by a fourth-order Birch-Murnaghan finite strain expansion with K0