9:00 AM - 9:15 AM
[SMP33-01] High-temperature Raman spectroscopic study of CO2-containing melanophlogite
Keywords:CO2-containing melanophlogite, In-situ high-temperature Raman spectroscopy, Low-frequency Raman spectroscopy, clathrasil, degassing process
Introduction: Melanophlogite is one of natural silica minerals with a clathrate structure. The structure has two kinds of cages which can host small guest molecules such as CH4, CO2, N2 and H2S. These are M12 and M14 cages correspond to pentagondodecahedral [512] and tetrakaidecahedral [51262] units made of 5- and 6-membered SiO4 rings, respectively. Previous crystal structural studies suggested that CO2 is mostly locating in larger M14 cage. A few years ago, we found a very intense and broad peak below 100 cm-1 in CO2-containing melanophlogite. To clarify the origin of this peak in relation with CO2 guest molecule, heat treatment experiments and in-situ high-temperature Raman spectroscopy measurements were conducted up to 1100 oC.
Experimental: Micro-Raman spectra were obtained using a home-built Raman system made of single-monochromator, CCD detector and Ondax SureBlock filters for low frequency measurement. The heat treatment was conducted using a muffle furnace, and the in-situ study was conducted with a wire-heater cell. Natural shperical single crystals from Fortunillo, Italy were crushed and small fragments were used for the experiments.
Results and discussion (origin of low-freq. peak): For the heat treatment experiments, nearly CO2-free melanophlogite was obtained at 950 oC for 6 h, judging from intensities of CO2 vibrational peaks (Fermi diad). Simultaneously, the low-frequency peak nearly disappeared. For shorter time durations or lower treatment temperatures, CO2 vibrational peaks and low-frequency peak were still visible. We found that CO2 vibrational peaks of the heat treated samples were split.
For the in-situ study, integrated total intensity of CO2 vibrational peaks started to drop at around 450 oC, and simultaneously the low-frequency peak intensity decreased. These peaks completely disappeared at 1100 oC, while melanophlogite itself remained intact. Thus, it was concluded that the low-frequency peak is originated from CO2 molecules, not from melanophlogite structure. Librational and translational modes of CO2 molecules in the cages of melanophlogite would be responsible for the low-frequency peak. Thus, degassing behavior can be studied by monitoring both the low frequency band and the vibrational peaks.
(vibrational peak splitting): At about 450 oC, we noted that the vibrational CO2 peak started to split (or more precisely, new peak appeared at slightly higher frequency and grew with temperature while the original peak became weaker). This splitting is partly quenchable, as demonstrated in the heat treated sample mentioned above. Vibrational peak splitting has been reported for CH4 and H2S in melanophlogites, and has interpreted as occupancy of those molecules in both M14 and M12 cages. Thus, peak splitting observed in our study can be interpreted similarly. No splitting of CO2 has been reported before, except one report at very low temperature. From those observations, we speculated that at above 450 oC, CO2 molecule is able to migrate from original M14 cage to next M14 or M12 cages. Because M14 cages make an infinite chain by sharing 6-membered rings, migration of CO2 from M14 to M14 cages through the 6-membered ring will contribute to long-range diffusion (i.e., degassing). On the other hand, migration of CO2 from M14 to next M12 through 5-membered ring will contribute to disordered distribution of CO2 in M12 and M14 cages, which caused vibrational peak splittings. Thus, high-temperature Raman spectroscopic study could provide us more insight of diffusion behavior of CO2 and possibly other gas molecules in melanophlogite structure.
Experimental: Micro-Raman spectra were obtained using a home-built Raman system made of single-monochromator, CCD detector and Ondax SureBlock filters for low frequency measurement. The heat treatment was conducted using a muffle furnace, and the in-situ study was conducted with a wire-heater cell. Natural shperical single crystals from Fortunillo, Italy were crushed and small fragments were used for the experiments.
Results and discussion (origin of low-freq. peak): For the heat treatment experiments, nearly CO2-free melanophlogite was obtained at 950 oC for 6 h, judging from intensities of CO2 vibrational peaks (Fermi diad). Simultaneously, the low-frequency peak nearly disappeared. For shorter time durations or lower treatment temperatures, CO2 vibrational peaks and low-frequency peak were still visible. We found that CO2 vibrational peaks of the heat treated samples were split.
For the in-situ study, integrated total intensity of CO2 vibrational peaks started to drop at around 450 oC, and simultaneously the low-frequency peak intensity decreased. These peaks completely disappeared at 1100 oC, while melanophlogite itself remained intact. Thus, it was concluded that the low-frequency peak is originated from CO2 molecules, not from melanophlogite structure. Librational and translational modes of CO2 molecules in the cages of melanophlogite would be responsible for the low-frequency peak. Thus, degassing behavior can be studied by monitoring both the low frequency band and the vibrational peaks.
(vibrational peak splitting): At about 450 oC, we noted that the vibrational CO2 peak started to split (or more precisely, new peak appeared at slightly higher frequency and grew with temperature while the original peak became weaker). This splitting is partly quenchable, as demonstrated in the heat treated sample mentioned above. Vibrational peak splitting has been reported for CH4 and H2S in melanophlogites, and has interpreted as occupancy of those molecules in both M14 and M12 cages. Thus, peak splitting observed in our study can be interpreted similarly. No splitting of CO2 has been reported before, except one report at very low temperature. From those observations, we speculated that at above 450 oC, CO2 molecule is able to migrate from original M14 cage to next M14 or M12 cages. Because M14 cages make an infinite chain by sharing 6-membered rings, migration of CO2 from M14 to M14 cages through the 6-membered ring will contribute to long-range diffusion (i.e., degassing). On the other hand, migration of CO2 from M14 to next M12 through 5-membered ring will contribute to disordered distribution of CO2 in M12 and M14 cages, which caused vibrational peak splittings. Thus, high-temperature Raman spectroscopic study could provide us more insight of diffusion behavior of CO2 and possibly other gas molecules in melanophlogite structure.