caa a22 4500 463217160 CHVBK 20180405153200.0 cr unu---uuuuu 170326e20071101xx s 000 0 eng 10.1007/s00269-007-0181-7 doi (NATIONALLICENCE)springer-10.1007/s00269-007-0181-7 Dehydration experiments on natural omphacites: qualitative and quantitative characterization by various spectroscopic methods [Elektronische Daten] [M. Koch-Müller, I. Abs-Wurmbach, D. Rhede, V. Kahlenberg, S. Matsyuk] A series of natural omphacites from a wide range of P, T occurrences were investigated by electron microprobe (EMP), infrared (IR)-, Mössbauer (MS)- and optical spectroscopy in the UV/VIS spectral range (UV/VIS), secondary ion mass spectrometry (SIMS) and single crystal structure refinement by X-ray diffraction (XRD) to study the influence of hydrogen loss on valence state and site occupancies of iron. In accordance with literature data we found Fe2+ at M1 as well as at M2, and in a first approach assigned Fe3+ to M1, as indicated by MS and XRD results. Hydrogen content of three of our omphacite samples were measured by SIMS. In combination with IR spectroscopy we determined an absorption coefficient: ε i,tot=65,000±3,000lmolH2O −1cm−2. Using this new ε i,tot value, we obtained water concentrations ranging from 60 to 700ppm H2O (by weight). Hydrogen loss was simulated by stepwise heating the most water rich samples in air up to 800°C. After heat treatment the samples were analyzed again by IR, MS, UV/VIS, and XRD. Depending on the type of the OH defect, the grade of dehydration with increasing temperature is significantly different. In samples relatively poor in Fe3+ (<0.1 Fe3+ pfu), hydrogen associated with vacancies at M2 (OH bands around 3,450cm−1) starts to leave the structure at about 550°C and is completely gone at 780°C. Hydrogen associated with Al3+ at the tetrahedral site (OH bands around 3,525cm−1, Koch-Müller et al., Am Mineral, 89:921-931, 2004) remains completely unaffected by heat treatment up to 700°C. But all hydrogen vanished at about 775°C. However, this is different for a more Fe3+-rich sample (0.2 Fe3+ pfu). Its IR spectrum is characterized by a very intense OH band at 3,515cm−1 plus shoulder at 3,450cm−1. We assign this intense high-energy band to vibrations of an OH dipole associated with Fe3+ at M1 and a vacancy either at M1 or M2. OH release during heating is positively correlated with decrease in Fe2+ and combined with increase in Fe3+. That dehydration is correlated with oxidation of Fe2+ is indirectly confirmed by annealing of one sample in a gas mixing furnace at 700°C under reducing conditions keeping almost constant OH− content and giving no indication of Fe2+-oxidation. Obtained data indicate that in samples with a relatively high concentration of Fe2+ at M2 and low-water concentrations, i.e., at a ratio of Fe2+ M2/H>10 dehydration occurs by iron oxidation of Fe2+ exclusively at the M2 site following the reaction: $$ {\left[ {{\text{Fe}}^{{{\text{2 + [ M2]}}}}{\text{OH}}^{ - } } \right]} = {\left[ {{\text{Fe}}^{{{\text{3 + [ M2]}}}} {\text{O}}^{{{\text{2}} - }} } \right]} + {\text{1/2}}\;{\text{H}}_{{\text{2}}} \uparrow . $$ In samples having relatively low concentration of Fe2+ at M2 but high-water concentrations, i.e., ratio of Fe2+ M2/H<5.0 dehydration occurs through oxidation of Fe2+ at M1. Springer-Verlag, 2007 Natural omphacite nationallicence OH incorporation nationallicence Dehydration nationallicence Infrared spectroscopy nationallicence X-ray structure refinement nationallicence Mössbauer spectroscopy nationallicence UV/VIS spectroscopy nationallicence Secondary ion mass spectrometry nationallicence IR absorption coefficient for water nationallicence Koch-Müller M. Department 4, Chemie der Erde, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany aut Abs-Wurmbach I. Institute of Applied Geoscience, Technical University Berlin, 10623, Berlin, Germany aut Rhede D. Department 4, Chemie der Erde, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany aut Kahlenberg V. Institute of Mineralogy and Petrography, University of Innsbruck, 6020, Innsbruck, Austria aut Matsyuk S. Institute of Geochemistry and Ore Formation, Ukrainian Academy of Science, 22 142, Kiev, Ukraine aut Physics and Chemistry of Minerals Springer-Verlag 34/9(2007-11-01), 663-678 0342-1791 34:9<663 2007 34 269 https://doi.org/10.1007/s00269-007-0181-7 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s00269-007-0181-7 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Koch-Müller M. Department 4, Chemie der Erde, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany aut NATIONALLICENCE 700 1- Abs-Wurmbach I. Institute of Applied Geoscience, Technical University Berlin, 10623, Berlin, Germany aut NATIONALLICENCE 700 1- Rhede D. Department 4, Chemie der Erde, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany aut NATIONALLICENCE 700 1- Kahlenberg V. Institute of Mineralogy and Petrography, University of Innsbruck, 6020, Innsbruck, Austria aut NATIONALLICENCE 700 1- Matsyuk S. Institute of Geochemistry and Ore Formation, Ukrainian Academy of Science, 22 142, Kiev, Ukraine aut NATIONALLICENCE 773 0- Physics and Chemistry of Minerals Springer-Verlag 34/9(2007-11-01), 663-678 0342-1791 34:9<663 2007 34 269 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer