Abstract

Amphiboles were synthesized at 750 degrees C, 1 kbar (H 2 O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final R Bragg indices in the range 4-9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition co-efficients over M1,3 and M2 where K (super M2+) = (M (super 2+) /Mg) (sub M1.3) /(M (super 2+) /Mg) M2 , and M (super 2+) = Ni (super 2+) or Co (super 2+) , K dNi = 2.98+ or -0.37 and K dCo = 1.34 + or - 0.31. Both K d values are greater than 1.0, whereas (super [6]) r(Ni (super 2+) ) < (super [6]) r(Mg)< (super [6]) r(Co (super 2+) ); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni-Mg or Mg-Co over the M1,3 sites and by ordering of Na and [] (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OH-stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands 3I Ao +2I Bo +I Co and I Bo +2I Co +3I Do are in accord with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

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