SITE OCCUPANCIES IN SYNTHETIC MONOCLINIC AMPHIBOLES - RIETVELD STRUCTURE REFINEMENT AND INFRARED-SPECTROSCOPY OF (NICKEL, MAGNESIUM, COBALT)-RICHTERITE

Amphiboles were synthesized at 750 degrees C, 1 kbar (H2O) on the bina ry joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richte rite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final R-Bragg indices in the rang e 4-9%, and by powder infrared spectroscopy in the principal OH-stretc hing region. Site-occupancy refinement of Ni-Mg and Mg-Co distribution s give the partition coefficients over M1,3 and M2 where KM2+=(M2+/Mg) (M1,3)/(M2+/Mg)(M2), and M2+=Ni2+ or Co2+, K-d(Ni)=2.98+/-0.37 and K-d (Co)=1.34+/-0.31. Both K-d values are greater than 1.0, whereas ([6])r (Ni2+) <([6])r(Mg) <[6]r(Co2+); 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 composi tions show fine structure caused by ordering of Ni-Mg or Mg-Co over th e M1,3 sites and by ordering of Na and square (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the pri ncipal OH-stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observe d spectra. The relative observed intensities of the combinations of ba nds 3I(0)(A)+2I(0)(B)+I-0(C) and I-0(B)+2I(0)(C)+3I(0)(D) are in accor d with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (ene rgy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crys tal infrared spectra of amphiboles, this result suggests that differen t local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.