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 Fe
2+ at M1 as well as at M2, and in a first approach assigned Fe
3+ 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 ± 3,000 lmol
H2O ?1 cm
?2. Using this new
ε i,tot value, we obtained water concentrations ranging from 60 to 700 ppm H
2O (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 Fe
3+ (<0.1 Fe
3+ pfu), hydrogen associated with vacancies at M2 (OH bands around 3,450 cm
?1) starts to leave the structure at about 550°C and is completely gone at 780°C. Hydrogen associated with Al
3+ at the tetrahedral site (OH bands around 3,525 cm
?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 Fe
3+-rich sample (0.2 Fe
3+ pfu). Its IR spectrum is characterized by a very intense OH band at 3,515 cm
?1 plus shoulder at 3,450 cm
?1. We assign this intense high-energy band to vibrations of an OH dipole associated with Fe
3+ at M1 and a vacancy either at M1 or M2. OH release during heating is positively correlated with decrease in Fe
2+ and combined with increase in Fe
3+. That dehydration is correlated with oxidation of Fe
2+ 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 Fe
2+-oxidation. Obtained data indicate that in samples with a relatively high concentration of Fe
2+ at M2 and low-water concentrations, i.e., at a ratio of Fe
2+ M2/H > 10 dehydration occurs by iron oxidation of Fe
2+ 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 Fe
2+ at M2 but high-water concentrations, i.e., ratio of Fe
2+ M2/H < 5.0 dehydration occurs through oxidation of Fe
2+ at M1.
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