Metastable high-pressure transformations of orthoferrosilite Fs₈₂

High-pressure single-crystal X-ray diffraction experiments with natural ferrosilite Fs₈₂ (Fe²⁺_0.82Mg_0.16Al_0.01Ca_0.01)(Si_0.99Al_0.01)O₃ orthopyroxene (opx) reveal that at ambient temperature the sample does not transform to the clinopyroxene (cpx) structure, as reported earlier for a synthetic Fs₁₀₀ end-member (Hugh-Jones et al., 1996), but instead undergoes a series of two polymorphic transitions, first above 10.1(1)GPa, to the monoclinic P2₁/c phase β-opx (distinctly different from both P2₁/c and C2/c cpx), also observed in natural enstatite (Zhang et al., 2012), and then, above 12.3(1)GPa to a high-pressure orthorhombic Pbca phase γ-opx, predicted for MgSiO₃ by atomistic simulations (Jahn, 2008). The structures of phases α, β and γ have been determined from the single-crystal data at pressures of 2.3(1), 11.1(1), and 14.6(1)GPa, respectively. The two new high-pressure transitions, very similar in their character to the P2₁/c-C2/c transformation of cpx, make opx approximately as dense as cpx above 12.3(1)GPa and significantly change the elastic anisotropy of the crystal, with the [100] direction becoming almost twice as stiff as in the ambient α-opx phase. Both transformations involve mainly tetrahedral rotation, are reversible and are not expected to leave microstructural evidence that could be used as a geobarometric proxy. The high Fe²⁺ content in Fs₈₂ shifts the α-β transition to slightly lower pressure, compared to MgSiO₃, and has a very dramatic effect on reducing the (meta) stability range of the β-phase.