Bjorn O. Mysen

Raman spectroscopy of silicate melts at magmatic temperatures: Na2O1bSiO2, K2O1bSiO2 and Li2O1bSiO2 binary compositions in the temperature range 25–1475°C☆

High-quality Raman spectra of silicate glasses, supercooled liquids and liquids have been obtained in situ to temperatures of 1475°C. The success of the spectroscopic technique is fundamentally dependent on the ability to avoid spectra degradation caused by black-body radiation from the furnace. This can be accomplished by focussing the diameter of the exciting laser beam to ∼ 1 mm, and to control the depth of focus in the sample to 6–40-μm depth. The samples can be viewed visually through a microscope throughout the process, which ensures that the distance between the sources of blackbody radiation (furnace wall and bubbles in glass and liquid) and focus is optimized. With this technique, more than 100 spectra of glasses, supercooled melts and melts in the systems Li2O1bSiO2, Na21bSiO2 and K2O1bSiO2 have been recorded at 25–1475°C in the frequency range most sensitive to the overall anionic structure of amorphous silicates (800–1300 cm−1). The coexisting structural units generally are SiO32− (Q2), Si2O52− (O3) and SiO2 (Q4) in all glasses and melts in the temperature range investigated. The abundance of the structural units appear sensitive to temperature. The temperature dependence is qualitatively consistent with a shift to the right of the reaction: Si2O52−(2Q3)⇌SiO32−(Q2)+SiO2(Q4 with increasing temperature. From intensity variations with temperature of relevant Si1bO stretch bands, the free energy for this reaction probably is sensitive to both bulk melt NBO/Si and to the electronic properties of the alkali metal.

Experimental determination of element partitioning and calculation of phase relations in the MgO‐FeO‐SiO2 system at high pressure and high temperature

Mg-Fe partitioning between coexisting phases, magnesiowustite (Mw) and olivine (α), Mw and β-phase, Mw and spinel (γ), and Mw and perovskite (Pv), has been determined experimentally with piston-cylinder apparatus, the multianvil device and the diamond anvil cell technique at pressures between 2 and 28 GPa and temperatures between 1473 and 1773 K. The solution parameters of each solid solution were obtained by fitting the experimental data simultaneously using the Margules formulation. The optimized solution parameters (in J/mol) are WMwMg-Fe = 16100, WMwFe-Mg = 26300 − 5.56T; WαMg-Fe = 4500 + 130P, WαFe-Mg = 6500 + 130P; WβMg-Fe = 1000, WβFe-Mg = 2000; WγMg-Fe = 900 − 1.10T WγFe-Mg = 3900; and WPvMg-Fe = 4130 −1.37T + 110P, WPvFe-Mg = −4050 − 2.45T + 150 P where P is in GPa and T in K. These parameters are consistent with solution calorimetry and phase equilibrium data. Phase relations in the MgO-FeO-SiO2 system were calculated by using the Margules solution model combined with internally consistent thermodynamic data of the pure phases. The computation provides independent constraints for phase relations in the system, in addition to those directly determined in P-T-Xi space.

Role of aluminium in the silicate network: In situ, high-temperature study of glasses and melts on the join SiO2-NaAlO2

Abstract The structure of glasses and melts in the NaAlSiO2-SiO2 system was determined in situ as a function of temperature by Raman spectroscopy, in the temperature range of 300 K–1800 K. The high-frequency envelope of the Raman spectra of Al-free SiO2 glass is deconvoluted in three different bands (1050, 1150, and 1200 cm−1) with the 1150 and 1200 cm−1 bands attributed to Si-O0 stretch vibrations in Q4 species in the fully polymerized network. There is a decrease in their frequencies with increasing Al/(Al + Si) of the glasses and melts in the Al/(A1 + Si) range investigated (0–0.5). The spectra are consistent with Al3+Si4+ substitution in two coexisting, three-dimensionally interconnected structural units. These units have different average intertetrahedral angles. Aluminum partitions preferentially into the unit with the smaller angle at low Al/(Al + Si) (less than ~0.3), and into the unit with the larger angle at higher concentrations. There is no evidence for major structural changes (i.e., redistribution of Al) in the glasses and melts as a function of increasing temperature.

Trace element partitioning and melt structure: An experimental study at 1 atm pressure

Diopside-melt and forsterite-melt rare earth (REE) and Ni partition coefficients have been determined as a function of bulk compositions of the melt. Available Raman spectroscopic data have been used to determine the structures of the melts coexisting with diopside and forsterite. The compositional dependence of the partition coefficients is then related to the structural changes of the melt. The melts in all experiments have a ratio of nonbridging oxygens to tetrahedral cations (NBOT) between 1 and 0. The quenched melts consist of structural units that have, on the average, 2 (chain), 1 (sheet) and 0 (three-dimensional network) nonbridging oxygens per tetrahedral cation. The proportions of these structural units in the melts, as well as the overall NBOT, change as a function of the bulk composition of the melt. It has been found that Ce, Sm, Tm and Ni crystal-liquid partition coefficients (Kcrystal−liqi = CcrystaliCliqi) decrease linearly with increasing NBOT. The values of the individual REE crystal-liquid trace element partition coefficients have different functional relations to NBOT, so that the degree of light REE enrichment of the melts would depend on their NBOT. The solution mechanisms of minor oxides such as CO2, H2O, TiO2, P2O5 and Fe2O3 in silicate melts are known. These data have been recast as changes of NBOT of the melts with regard to the type of oxide and its concentration in the melt. From such data the dependence of crystal-liquid partition coefficients on concentration and type of minor oxide in melt solution has been calculated.

The structural state of iron in oxidized vs. reduced glasses at 1 atm: A57Fe Mössbauer study

AbstractA general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na2O-Al2O3-SiO2-Fe-O, CaO-Al2O3-SiO2-Fe-O, and MgO-Al2O3-SiO2-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with57Fe Mössbauer spectroscopy.For oxidized glasses with Fe3+/∑Fe>0.50, the isomer shift for Fe3+ is in the range ∼0.22–0.33 mm/s and ∼0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe3−. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ∼0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe3+ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems.The values of the isomer shifts for Fe2+ ions in glasses irrespective of Fe3+/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe2+ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe2+. The assignment of the isomer-shift values of Fe2+ to octahedral coordination is in agreement with the results of other spectral studies.For reduced glasses (Fe3+/∑Fe≈<0.50), the value of the isomer shift for Fe3+ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe3+/∑Fe in the range of

Structural similarity of glasses and melts relevant to petrological processes

f_{O_2 }

An optical cell for Raman spectroscopic studies of supercritical fluids and its application to the study of water to 500°C and 2000 bar

between 10−3 and 10−6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing