Single-crystal X-ray diffraction data were collected to characterise the macroscopic solid solution and cation-ordering behaviour in the system enstatite-ferrosilite. A suite of 18 natural orthopyroxene samples with compositions across the enstatite-ferrosilite join was examined. Annealing experiments were carried out to obtain different degrees of order in crystals with composition 50% Fs. Powders of orthopyroxenes of 6 compositions between MgSiO3 (En100) and FeSiO3 (Fs100) were synthesised and analysed by X-ray powder diffraction.

The order parameter Q was obtained by a minimisation procedure combining single-crystal X-ray diffraction data and chemical analyses. Diffraction data show that there is no significant excess volume of mixing on the enstatite-ferrosilite join. The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit cell parameters with composition. Variations of unit cell parameters as a function of degree of order appear to be negligible. Linear correlation of mean M1-O and M2-O bond lengths with M1 and M2 site populations, respectively, results from a combination of different trends in the individual bond lengths. Mg-Fe ordering and substitution on the octahedral sites are accommodated by lengthening of the tetrahedral chains along the c axis and by rotation of tetrahedra on the same axis.

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