V. M. Malavergne

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 A) 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 A and an atomic arrangement similar to that of nepouite ([6]Ni3Si2O5(OH)4) or Ni-talc ([6]Ni3Si4O10(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 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.

In situ mapping of high-pressure fluids using hydrothermal diamond anvil cells

We present new results combining high pressures and temperatures attainable in a diamond anvil cell with in situ synchrotron radiation induced micro-X-ray fluorescence measurements. Hydrothermal diamond anvil cells experiments have been performed by measuring the partitioning of Pb between aqueous fluids (pure water or NaCl-enriched water) and hydrous silicate melts of haplogranite composition using synchrotron X-ray fluorescence. The in situ measurements were performed in the range 0.3–1.2 GPa and 730–850 °C both in the aqueous fluid and in the silicate melts being in equilibrium. Pb is strongly partitioned into high-pressure–temperature hydrous melts when Cl is present in either the hydrous melt or the aqueous fluid. Moreover, our comparisons of in situ results with post-mortem results show that significant changes take place during rapid quenching especially when samples are small (few hundred of microns in diameter). Water exsolution is induced by the quench in the silicate melt showing the high mobility of Pb which immediately partitions into the water vapor phase during the quench. The current in situ approach offers thus a pertinent complementary method to the classical experimental petrology investigations.

X-ray transmission properties of intelligent anvils in diamond anvil cells

High-pressure and/or high-temperature analysis of geo- and material science samples routinely employs diamond anvil cells (DACs) as a research instrument. In particular, DACs allow for various in situ characterizations (e.g. Raman and Fourier transform infra red spectroscopies, X-ray diffraction (XRD), X-ray spectroscopy including fluorescence (XRF) and absorption (XAS)) at elevated pressure and temperature. The measurement of pressure (P) and/or temperature (T) in the sample chamber is crucial, but not always accurate, more specifically in the case of low-pressure applications (a few GPa). The development of modified diamonds (intelligent anvils ‘i-anvils’) adapted to a new generation of DACs (intelligent diamond anvil cells: iDAC) can contribute to solve this problem, as the diamond itself serves as the PT sensor, being prepared, for example, by high-energy ion implantation [H. Bureau, M. Burchard, S. Kubsky et al., This volume (2006).] on a micrometric scale. Several most interesting measurement methods used with DACs are based on X-ray techniques (e.g. XRF, XRD, XAS). We present the first results of X-ray transmission measurements with iDACs, performed at the hard-X-ray microfocus beamline ID22 at the ESRF (European Synchrotron Radiation Facility), Grenoble, France. Sensor response to intense irradiation as a function of X-ray energy (E ph∈10;18.1 keV) was investigated. The values of the sensor were found to be independent of the irradiation in the investigated energy range and thus validate the use of these sensors for precise and reliable measurements on a wide range of applications with high-energy synchrotron radiation. No influence of the sensor on the X-ray transmission properties of the anvil has been found.