STRUCTURE, ORDERING AND CATION INTERACTIONS IN CA-FREE P2(1) C CLINOPYROXENES/

A suite of 14 synthetic Ca-free P2(1)/c low-clinopyroxenes of composit ions across the enstatite-ferrosilite (En-Fs) join have been studied b y X-ray powder diffraction and Mossbauer spectroscopy. The crystal str ucture of one sample with composition X-Fs=0.39 was determined by sing le-crystal X-ray diffraction. The powder diffraction data show that th ere 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=3 1.261(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-l inear 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 co rrelated to the difference in site occupancies between M1 and M2 as de termined by Mossbauer spectroscopy and arises directly from the expans ion of M2-O1 and M2-O3 bonds upon initial substitution of Fe for Mg in enstatite. The hyperfine parameters from Mossbauer spectra are consis tent with variations in the average local environment as recorded by t he X-ray data. Asymmetric line broadening of the Mossbauer spectra pro vides evidence for next-nearest neighbour effects, and is consistent w ith no significant clustering of Fe or Mg within the samples.