Structural transformation in Ni‐bearing Na2Si2O5 glass and melt

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Abstract
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 IVNi and VNi occur in the glass, whereas IVNi 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) Å, respectively. The Ni‐O distance in the glass is consistent with a mixture of IVNi and VNi, whereas that in the melt is consistent with dominantly IVNi, 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.