期刊
PHYSICS AND CHEMISTRY OF MINERALS
卷 39, 期 6, 页码 455-464出版社
SPRINGER
DOI: 10.1007/s00269-012-0499-7
关键词
Moganite; Chalcedony; Quartz; Silica rocks; Flint; Heat treatment; Raman band assignment; Silanole (SiOH)
资金
- ANR
- Toulouse, FR
Chalcedony is a spatial arrangement of hydroxylated nanometre-sized alpha-quartz (SiO2) crystallites that are often found in association with the silica mineral moganite (SiO2). A supplementary Raman band at 501 cm(-1) in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm(-1) in sedimentary chalcedony, representing a 2 cm(-1) difference with its position in pure moganite. We present a study of the 503 cm(-1) band's behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 A degrees C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm(-1) Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm(-1) in chalcedony, assigned to 'free' Si-O vibrations of non-bridging Si-OH that oscillate with a higher natural frequency than bridging Si-O-Si (at 464 cm(-1)). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si-OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.
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