Optical phonons, OH vibrations, and structural modifications of phlogopite at high temperatures: An in-situ infrared spectroscopic study

Abstract

The thermal behavior of optical phonons and OH vibrations of phlogopite (a trioctahedral mica) was examined at temperatures up to 1000 K using in situ infrared spectroscopy. The results showed that with increasing temperature, O-K bands in phlogopite exhibited a relatively strong variation in frequency in a manner similar to those in muscovite. The work revealed that different types of OH bands (fundamentals and combinations) have very different thermal behavior or temperature dependence, and their absorption coefficients are commonly not constant on heating. OH combination bands that are associated with summation processes of multi-phonon interactions commonly show a decrease in their intensities on heating, but in contrast combination bands due to difference processes generally exhibit an increase. This means that temperature dependencies of their absorption coefficients need to be considered when using the Beer-Lambert law to determine or estimate OH contents or hydrogen concentrations at high temperatures. The results showed a structural anomaly associated with a discontinuity in the temperature derivative of the wavenumber of Al-O and Si-O vibrations and O-H stretching near 600 K. However, framework-related phonon modes in the FIR and MIR regions do not suggest a break of the original monoclinic structural symmetry in the investigated temperature region. The complex changes are attributed to temperature-induced alteration of local configuration involving TO 4 tetrahedra and a possible change of the orientation of OH dipoles, in addition to a previously reported distortion of MO 6 octahedra. Increasing temperature to 1000 K also causes partial dehydroxylation, as evidenced by the disappearance of the OH band near 3623 cm -1 and the decrease in OH band height and area of other OH bands. The study did not record the formation of H 2 O inside phlogopite as a result of partial dehydroxylation. The work offers new data and findings that have important implications in understanding the complex structural modifications and the behavior of phonon modes and the thermal stability of hydroxyls on approaching the dehydroxylation, as well as the way hydrogen is released from micas at high temperatures. Our data also show that phologpite becomes less transparent with increasing temperature suggesting a change of radiative properties and ability to transmit heat, which could be of interest for modeling thermal-transmission in crustal rocks.

Nature of hydrogen defects in clinopyroxenes from room temperature up to 1000 °C: Implication for the preservation of hydrogen in the upper mantle and impact on electrical conductivity

“…Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld Bagdassarov 1993; Withers et al 1999;Zhang et al 2007Zhang et al , 2016Tokiwai and Nakashima 2010;Della Ventura et al 2017). However, only few studies addressed temperature dependence of absorption coefficient of O-H vibration of hydrogen defects in NAMs (Yang et al 2010(Yang et al , 2015.…”

Intensive evaluation of radiation stability of phlogopite single crystals under high doses of γ-ray irradiation

“…Optically transparent phlogopite -KMg 3 (AlSi 3 O 10 )(OH) 2single-crystal lms were brought from the University of Cambridge, UK. Its accurate composition was analyzed and cited as sample (277) in the literature, 25 containing marginal additional elements such as Na or Fe.…”

“…These peaks could be assigned as follows. The peaks at 601 and 675 cm À1 correspond to OH bending and O-Al-O vibration, 25 respectively. The peaks at 741 and 803 cm À1 correspond to Al-O vibration and Si-O stretching, 25 respectively.…”

“…The peaks at 601 and 675 cm À1 correspond to OH bending and O-Al-O vibration, 25 respectively. The peaks at 741 and 803 cm À1 correspond to Al-O vibration and Si-O stretching, 25 respectively. In addition, the peak at 911 cm À1 corresponds to Si-O-Al or Si-O vibration, 44 and the peak at 3710 cm À1 corresponds to Mg 3 -OH vibration.…”

“…In some cases, the OH group can be replaced by F, Cl, etc. 25 Although phlogopite single crystals have not been recommended for the disposal of HLRW, their layered structure is similar to the matrix of clay, which is recommended for this purpose. Simultaneously, the content of impurities and H 2 O lying in this material is marginal.…”