Structure, ordering and cation interactions in Ca-free P21/c clinopyroxenes

A suite of 14 synthetic Ca-free P2(1)/c low-clinopyroxenes of compositions across the enstatite-ferrosilite (En-Fs) join have been studied by X-ray powder diffraction and Mossbauer spectroscopy. The crystal structure of one sample with composition X-Fs=0.39 was determined by single-crystal X-ray diffraction. The powder diffraction data show that there is no significant (<0.25% at X-Fs=0.5) excess volume of mixing on the enstatite-ferrosilite join. The molar volumes are described by V=31.261(17)+1.677(23) X-Fs cm(3).mol(MSiO3)(-1). The linear behaviour of volume with composition arises from the compensating effects of non-linear changes in the unit-cell parameters with composition. Unweighted fits to the data yielded the relationships: a=9.6100(15)+0.105(2)X-Fs, b=8.8156(28)+0.218(12)X-Fs+0.0481(117)X-Fs(2), c=5.1702(4)+0.0879(21)X-Fs-0.0214(21)X-Fs(2), beta=108.345(8)+0.947(30)X-Fs-0.805(26)X-Fs(2). The strong positive deviation of beta from linearity is directly correlated to the difference in site occupancies between M1 and M2 as determined by Mossbauer spectroscopy and arises directly from the expansion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mossbauer spectra are consistent with variations in the average local environment as recorded by the X-ray data. Asymmetric line broadening of the Mossbauer spectra provides evidence for next-nearest neighbour effects, and is consistent with no significant clustering of Fe or Mg within the samples.