STRUCTURAL TRANSFORMATION IN NI-BEARING NA2SI2O5 GLASS AND MELT

The coordination environment around Ni at minor concentration levels (2 wt. % NiO) in Na2Si2O5 glass and melt (293-1250K) has been investigated by Ni K-edge X-ray Absorption Fine Structure (XAFS) spectroscopy. X-ray Absorption Near Edge Structure (XANES) spectra indicate that both (IV)Ni and (V)Ni occur in the glass, whereas (IV)Ni is dominant in the melt. The average anharmonic Ni-O distances derived from XAFS spectra of the glass at 293 K and the melt above 1150 K are 2.00 (+/- 0.02) and 1.97 (+/- 0.02) angstrom, respectively. The Ni-O distance in the glass is consistent with a mixture of (IV)Ni and (V)Ni, whereas that in the melt is consistent with dominantly (IV)Ni, suggesting a Ni-coordination change during glass melting. This major change in Ni coordination indicates significant reorganization of the melt during quenching and crystallization. These observations for a compositionally-simple melt/glass system may help explain the enrichment of Ni in early-formed igneous minerals. They also suggest the possibility of a pressure-induced coordination change around Ni in the mantle which may influence geochemical modeling of the deep Earth.