Deprotonation of Fe-dominant amphiboles: Single-crystal HT-FTIR spectroscopic studies of synthetic potassic-ferro-richterite

High-temperature Fourier transform infrared (HT-FTIR) spectroscopy was used to characterize the deprotonation process of synthetic potassic-ferro-richterite of composition A(K0.90Na0.07)B(Ca0.54Na1.46) c F e 4.22 2 + F e 0.78 3 + T S i 8 O 22 W O H 1.70 O 0.30 2 - ^ mcleft( mFe-4.22^2 + mFe-0.78^3 + ight)^ mT mS mi-8 mO-22^ mWleft( mO mH-1.70 mO-0.30^2 - ight). Unpolarized single-crystal spectra were collected both in situ and on quenched samples, and heating experiments were conducted in air, at a rate of 10 °C/min. The room-T spectrum measured before annealing shows a main band at 3678 cm-1 and a minor band at 3622 cm-1; these are assigned to local configurations involving Fe2+ at M(1)M(1)M(3) and facing a filled and an empty alkali-site, respectively. At 400 °C, a new band grows at 3656 cm-1; this is the most intense feature in the pattern at 450 °C. At T ≥ 500 °C, all peaks decrease drastically in intensity, and finally disappear at T > 600 °C. The total absorbance measured in situ increases significantly in the 25 < T < 450 °C range, although the spectra collected on quenched samples show no OH loss in the same T range. This feature is consistent with an increase of the absorption coefficient (ϵ) with T, the reason for which is still unclear. However, this feature has significant implications for the quantitative use of FTIR data in HT experiments. Examination of the relevant OH-stretching bands shows that iron oxidation occurs preferentially at the M(1,3) sites associated with occupied A sites. The deprotonation temperature indicated by FTIR for single-crystals is around 100 °C higher that that obtained by HT-X-ray diffraction (XRD) on single crystal by Oberti et al. (2016), whereas that obtained by HT-XRD on powders is intermediate. This unexpected observation can be explained by considering that: (1) the iron oxidation process, which is coupled to deprotonation and is probed by XRD, occurs preferentially at the crystal surface where it is triggered by the availability of atmospheric oxygen; (2) the proton diffusion, probed by FTIR, is slower that the electron diffusion probed by XRD; thus, the temperature shift may be explained by a much longer escape path for H in single-crystals than in powders. These results suggest that possible effects due to crystals size should be carefully considered in HT experiments done on Fe-rich silicates.