In situ high-pressure single-crystal X-ray study of aegirine, NaFe³⁺Si₂O₆, and the role of M1 size in clinopyroxene compressibility

The crystal structure of a synthetic aegirine crystal, NaFe³⁺ Si₂O₆, was studied at room temperature, under hydrostatic conditions, over the pressure range 0-11.55 GPa using single-crystal X-ray diffraction. Unit-cell data were determined at 16 pressures, and intensity data were collected at eight of these pressures. A third-order Birch-Murnaghan equation of state fit to the P-V data from 0-11.55 GPa yielded K₀ = 117(1) GPa, K₃′ 3.2(2), and V₀ = 429.40(g) Å ³. Aegirine, like the other Na-clinopyroxenes that have been examined at high pressure, exhibits strongly anisotropic compression, with unit strain axial ratios ε₁:ε ₂:ε₃ of 1.00:2.38:2.63. Silicate chains in aegirine become more O-rotated with pressure, reducing ∠O3-O3-O3 from 174.1(1)° at ambient pressure to 165.5(5)° at 10.82 GPa. No evidence of a phase transition was observed over the studied pressure range. The relationship between M1 cation radius and bulk modulus is examined for 14 clinopyroxenes, and two distinct trends are identified in a plot of these values, The distinction between these trends can be explained by the presence or absence of antipathetic bonds around M2, a feature first described by McCarthy et al. (2008). Aegirine, with Fe³⁺, has nearly the same bulk modulus, within error, as hedenbergite, with Fe²⁺, despite the difference in M2 bonding topology, M2 (Fe) valence and ambient unit-cell volume. Several explanations for this apparent paradox are considered.