Site occupancies in synthetic monoclinic amphiboles; Rietveld structure refinement and infrared spectroscopy of (nickel, magnesium, cobalt)-richterite

Terza Universita di Roma, Dipartimento di Scienze Geologiche, Rome, Italy
CNRS, France
University of British Columbia, Canada
University of Manitoba, Canada
Natural History Museum, United Kingdom

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 _d ^Ni = 2.98+ or -0.37 and K _d ^Co = 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 ^A _o +2I ^B _o +I ^C _o and I ^B _o +2I ^C _o +3I ^D _o 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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