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 EXAFS spectroscopy. The mean Ni-O distance, d(Ni-O) = 2.08 Å, suggests a preferential location of Ni in the M1 site. The observed distance is smaller than the Mg-O distance in the M1 site of α-Mg2SiO4 [mean d(M1-O) = 2.10 Å] and is similar to the average M1-O distance in olivines close to the San Carlos composition [mean d(M1-O) = 2.09 Å]. The most important relaxation effect resulting from the Ni for Mg substitution is a smaller mean M1-O distance. The medium-range distribution of Ni (up to 4.5 Å from the central Ni atom) in the San Carlos olivine structure was estimated by modeling EXAFS spectra, considering various M1 and M2 cation distributions around M1 (Ni) sites. These models suggest medium-range ordering of Ni and Fe in adjacent M1 and M2 sites of the olivine structure and nonideal behavior of Ni in San Carlos olivine.

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