Physics and Chemistry of Minerals

, Volume 39, Issue 6, pp 455–464 | Cite as

A hitherto unrecognised band in the Raman spectra of silica rocks: influence of hydroxylated Si–O bonds (silanole) on the Raman moganite band in chalcedony and flint (SiO2)

  • Patrick Schmidt
  • Ludovic Bellot-Gurlet
  • Aneta Slodczyk
  • François Fröhlich
Original Paper

Abstract

Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-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 °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.

Keywords

Moganite Chalcedony Quartz Silica rocks Flint Heat treatment Raman band assignment Silanole (SiOH) 

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Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Patrick Schmidt
    • 1
  • Ludovic Bellot-Gurlet
    • 2
  • Aneta Slodczyk
    • 2
  • François Fröhlich
    • 1
  1. 1.Département de Préhistoire UMR 7194, Muséum national d’histoire naturelleCentre de spectroscopie infrarougeParis Cedex 05France
  2. 2.Laboratoire de dynamique, interactions et réactivité (LADIR) UMR 7075CNRS and UPMC (Université Pierre et Marie Curie, Paris 6)Paris Cedex 05France

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