A neutron powder diffraction study of cation ordering in high-temperature synthetic amphiboles

The structures of synthetic fluoro-edenite and pargasite end-members have b een refined by Rietveld analysis of neutron powder diffraction data to R-wp values of 1.4 % and 1.9 %, respectively The quality of the Rietveld refine ments is much higher than has been obtained for synthetic amphiboles by X-r ay powder diffraction. The distribution of Al and Si over T(1) and T(2) has been determined from mean T(1)-O bond lengths. Pargasite crystallized at 1 kbar, 932 degrees C has 1.71 +/- 0.11 Al apfu at T(1) and 0.28 + 0.11 Al a pfu at T(2), implying at least 15 % long-range Al-Si disorder. Within exper imental error fluoro-edenite crystallized at 2 kbar, 1006 degrees C is long -range-ordered, with all Al-[4] at T(1). The size of the error on the long- range Al-Si order parameter (Psi) scales approximately with 2/Al-[4]. This means that an amphibole with one Al-[4] apfu will have an error on Psi that is approximately twice that for an amphibole with two Al-[4] apfu. Consequ ently, AI-Si disorder can be quantified more accurately for pargasites than hornblendes. The AI-Si distribution of synthetic pargasite is similar to t hat of natural high-temperature pargasites. The results of this study indic ate that long-range AI-Si order-disorder in pargasites rather than hornblen des is worth investigating as a geothermometer because of the inherently la rger errors on Psi for hornblendes. Mg-Al ordering on octahedral sites in p argasite has been determined from mean octahedral bond lengths. Al is at M( 2) and M(3) sites, with little or none at M(1): M(2) = 1.5Mg + 0.5Al and M( 3) = 0.5Mg + 0.5Al. These occupancies accord very well with IR and H-1 MAS NMR spectra of synthetic end-member pargasite and are different from those of high-temperature natural pargasites which have M3+ distributed statistic ally over M(2) and M(3), with none at M(1): M(2)= 1.33M(2+) + 0.67M(3+) and M(3)= 0.67M(2+) + 0.33M(3+). The difference in M(2) and M(3) site occupanc ies for natural and synthetic pargasites may reflect fundamentally differen t growth mechanisms. However, both natural and synthetic pargasites show qu alitatively similar ordering behaviour for octahedral cations, pointing to a shared crystal chemistry.