Transition elements in water-bearing silicate glasses/melts. part II. Ni in water-bearing glasses

Abstract

The local coordination environment around Ni(II) in a series of sodium trisilicate (NS3) and albitic (ALB) glasses has been evaluated by using high-resolution XANES and anharmonic EXAFS spectroscopies. The glasses contain ∼1000 to 4000 ppm of Ni and from 0 to 8.2 wt.% water. They were synthesized at pressures between 2.2 and 5 kbars and temperatures between 1050 and 1350 K. The bulk glasses were characterized by using X-ray diffraction, transmission electron microscopy, Raman, and ultraviolet-Vis-NIR spectroscopies. Both hydrous NS3 and ALB glasses show dominant amounts of Ni(II) in relatively regular 6-coordinated environments, in contrast with their anhydrous counterparts, where 5-coordinated Ni dominates. There are also significant differences in the average medium-range environment (2–3.5 Å) around Ni between the anhydrous and hydrous glasses. In the ALB glasses, the presence of water in amounts >2 wt.% induces the formation of nanocrystallites, with an average diameter of ∼40 Å and an atomic arrangement similar to that of nepouite ( [6]Ni 3Si 2O 5(OH) 4) or Ni-talc ( [6]Ni 3Si 4O 10(OH) 2) or a related hydrous Ni-silicate. The presence of Ni-bearing nanocrystallites is thought to be due to the relatively slow quench rate of the high-temperature–high-pressure synthesis apparatus used. These nanophases are difficult to detect by using conventional characterization methods and can cause misleading interpretations of the glass structure if not detected. In contrast, there is no evidence for Ni-rich, nanocrystalline domains in NS3 glasses containing high water contents (up to 8.2 wt.%); instead, two to three Si second neighbors are observed around Ni in all NS3 glasses (and in ALB glasses with water contents <4 wt.%). The bonding of Ni to oxygens in the tetrahedral framework is inconsistent with the presence of large amounts of Ni(H 2O) 6 2+ complexes in these glasses. However, Ni(II) may form Ni(OH) n O 6− n (4+ n)− (n ∼ 6) complexes in hydrous glasses. Our results for Ni combined with results from other studies of 3-d divalent transition metal cations in hydrous silicate glasses suggest that water in silicate melts helps these cations form their preferred coordination environments [6-coordinated for Mn(II), Fe(II), and Ni(II)]. Ni(II) may occur in natural hydrous silicate melts dominantly in 6-coordinated environments, rather than dominantly in 4-coordinated environments, as in anhydrous melts and supercritical aqueous fluids, explaining the compatible behavior of Ni in magmas. However, in situ experiments are required to test this suggestion.