Amorphous Materials: Properties, structure, and durability: Oxidation state of iron in hydrous phono-tephritic melts
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Jan A. Schuessler
,
Roman E. Botcharnikov
Abstract
The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed H2O-CO2 fluids was studied experimentally at 1200 and 1250 °C and pressures from 50 to 500 MPa. The oxygen fugacity (fO₂) varied from NNO-2.9 to NNO+2.6 in log fO₂, relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the Fe2+/ΣFe ratio of milligram-sized samples within ±0.03 (2σ). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The Fe2+/ΣFe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on Fe2+/ΣFe at given fO₂ was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower Fe2+/ΣFe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher K2O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in FeOT (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log fO₂ and Fe3+/Fe2+ for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as H2O or K2O, on the redox state of Fe in silicate melts.
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- Amorphous materials: Properties, structure, and durability: Atomic structure and transport properties of MgO-Al2O3 melts: A molecular dynamics simulation study
- Amorphous Materials: Properties, structure, and durability: Oxidation state of iron in hydrous phono-tephritic melts
- Amorphous materials: Properties, structure, and durability: Quantitative Raman spectroscopy: Speciation of Na-silicate glasses and melts
- Dissolution-reprecipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)
- High-pressure behavior of gypsum: A single-crystal X-ray study
- Presence and zoning of hydrous components in leucite from the Alban Hills volcano, Rome, Italy
- Herderite from Mogok, Myanmar, and comparison with hydroxyl-herderite from Ehrenfriedersdorf, Germany
- Application of Raman spectroscopy to quantify trace water concentrations in glasses and garnets
- Neutron diffraction study of δ-AlOOD at high pressure and its implication for symmetrization of the hydrogen bond
- A simple predictive model for the thermal expansion of AlSi3 feldspars
- New data on PGE alloy minerals from a very old collection (probably 1890s), California
- High-pressure study on lead fluorapatite
- High-pressure Al-rich hexagonal phases—What are their kin?
- Leucite at high pressure: Elastic behavior, phase stability, and petrological implications
- Thermodynamic mixing properties of Rb-K feldspars
- Demicheleite, BiSBr, a new mineral from La Fossa crater, Vulcano, Aeolian Islands, Italy
- A lattice dynamical study of the aragonite and post-aragonite phases of calcium carbonate rock
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- Raman spectroscopy of CaIrO3 postperovskite up to 30 GPa
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