INTRACRYSTALLINE DISTRIBUTION OF NI IN SAN-CARLOS OLIVINE - AN EXAFS STUDY

The location of minor amounts of Ni (0.44 wt% NiO) in the structure of San Carlos olivine has been determined using Ni K-edge fluorescence E XAFS spectroscopy. The mean Ni-O distance, d(Ni-O) = 2.08 Angstrom, su ggests a preferential location of Ni in the M1 site. The observed dist ance is smaller than the Mg-O distance in the M1 site of alpha-Mg2SiO4 [mean d(M1-O) = 2.10 Angstrom] and is similar to the average M1-O dis tance in olivines close to the San Carlos composition [mean d(M1-O) = 2.09 Angstrom]. The most important relaxation effect resulting from th e Ni for Mg substitution is a smaller mean M1-O distance. The medium-r ange distribution of Ni (up to 4.5 Angstrom from the central Ni atom) in the San Carlos olivine structure was estimated by modeling EXAFS sp ectra, considering various M1 and M2 cation distributions around M1 (N i) sites. These models suggest medium-range ordering of Ni and Fe in a djacent M1 and M2 sites of the olivine structure and nonideal behavior of Ni in San Carlos olivine.