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)LixFexMg2Si2O6 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 hM1-Oi bond length across all analysed samples, including the endmembers protoenstatite (PEN), orthoenstatite (OEN) and LiFe3+Mg2Si2O6. It seems that theM2 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 hM2-Oi, with the PPX trend converging toward the parameters of the LiFe3+Mg2Si2O6 endmember, whereas the OPX trend, including OEN, diverges largely from these parameters.