Phase transition and mixing behaviour of the cummingtonite-grunerite solidsolution

Natural amphiboles with composition close to the binary join cummingtonite- grunerite and crystals of the same samples annealed at 700 degreesC for 55. 5 h, in order to obtain different degrees of non-convergent cation order, h ave been characterised by means of X-ray single-crystal diffraction and IR spectroscopy. Long-range order parameters describing the non-convergent ord er of Mg/Fe among the different octahedral sites have been calculated from the site occupancies of the investigated samples. Values of the O6-O5-O6 an gles and of the [M4-O] mean bond distances depend on the C2/m --> P2(1)/m p hase transition for a given degree of order. In the IR spectra. only two ph onon lines dominated by the bending of the tetrahedral chains are sensitive to the displacive phase transition and to the different degree of cation o rder: all the other wavenumber shifts are correlated with compositional cha nges only. The local strains arising from the cation substitution, ordering and phase transition have been quantified by means of the autocorrelation function. Very small local heterogeneities are associated with the Mg/Fe su bstitution and disordering in samples at intermediate composition. The disp lacive phase transition seems to occur in order to reduce local distortions and the P2(1)/m samples are as homogeneous as orthorhombic anthophyllites. The orthorhombic structure, however, appears less flexible than the monocl inic in accommodating cations larger than Mg at the octahedral sites.