Crystal structure and doping in synthetic enstatite: an analysis of Li/Fe³⁺-doped single-crystal samples

A series of Li⁺/Fe³⁺-doped enstatite crystals of composition Mg_(2-2x)Li_xFe_xSi₂O₆ were synthesized and structurally characterized. Under the selected experimental conditions, we grew three crystals of Pbca orthopyroxene (OPX: x = 0.270-0.313) and two crystals of Pbcn protopyroxene (PPX: x = 0.156-0.164) using the flux-growth technique. The observed variation in the polyhedral volume and distortion of the M2 octahedron as a function of Li/Fe³⁺ doping suggests the presence of an upper limit, at least for the OPX samples. The same linear relation was observed between the polyhedral volume and ⟨M1-O⟩ bond length across all analysed samples, including the endmembers protoenstatite (PEN), orthoenstatite (OEN) and LiFe³⁺Si₂O₆. It seems that the M2 octahedron plays a crucial role in stabilizing the pyroxene topology in either the PEN or the OEN form, because the PPX and OPX samples show two distinct linear relations between the M2O₆ polyhedral volume and ⟨M2-O⟩, with the PPX trend converging toward the parameters of the LiFe³⁺Si₂O₆ endmember, whereas the OPX trend, including OEN, diverges largely from these parameters.