Spectroscopic evidence for five-coordinated Ni in CaNiSi₂O₆ glass

NiK-edge EXAFS and XANES and optical absorption spectroscopy experiments have been performed on a CaNiSi₂O₆ glass. The data are consistent with the presence of ^[5]Ni in a trigonal bipyramidal symmetry site, a previously unknown surrounding. Minor amounts of ^[4]Ni are also present. This environment explains why the Ni-O distances derived from EXAFS in the glass are shorter than in the crystalline equivalent and provides a rationale for the high olivine/liquid partition coefficients of Ni that govern its preferential location in mafic and ultramafic rocks.