Rapid intracrystalline exchange of octahedrally-coordinated divalent cations in amphiboles: an in situ high-temperature neutron diffraction study of synthetic potassic richterite ~AK~B(NaCa)~C(Mg_(2.5)Ni_(2.5))Si_8O_(22)(OH)_2

摘要

The distribution of Mg and Ni over the octahedrally-coordinated sites M( 1,2,3) in Ni-substituted potassic richterite solid solution, ~AK~B(NaCa)~C(Mg_(2.5)Ni_(2.5))Si_8O_(22)(OH)_2 , synthesized at 0.1 GPa/750 deg C (25 days), has been studied in situ to 700°C by neutron powder diffraction with Rietveld structure refinement. Using a 2.7 g sample it was possible to make short data collections (3 h) at each temperature at intervals of 50 deg from 50 to700 deg C and locate the onset of cation exchange. Above 700 deg C the amphibole decomposes rapidly and so only an on-heating dataset was collected. Unit-cell parameters increase smoothly upon heating, with no discontinuities evident. Site occupancies of M(1), M(2) and M(3) sites were refined from site-scattering values. The initial X_(Ni) values of M(1), M(2) and M(3) sites, corresponding to the synthesis temperature of 750 deg C, are 0.57(1), 0.34(1) and 0.68(2), respectively. Above 400 deg C, there is initial ordering whereby thermal annealing allows the disordered Mg-Ni distribution of the quenched synthesis product to adjust to be closer to those appropriate for lower temperatures of the heating sequence. This ordering involves the exchange of Mg and Ni between M(1) and M(2), with no significant change in M(3) occupancy. At 700 deg C, X_(Ni) values of the M(1), A/(2) and M(3) sites are 0.64(1), 0.27(1) and 0.69(2), respectively. No disordering was observed, due to the short duration of the high-temperature data collections and decomposition at 750 deg C. The results of this study indicate that divalent cations exchange between octahedral sites in amphiboles in a matter of hours.