Temperature dependent crystallographic transformations in chalcedony, SiO2, assessed in mid infrared spectroscopy

Chalcedony consists of hydroxylated 50-100 nanometre measuring alpha-quartz (SiO2) crystallites that lose their surface silanole groups (Si-OH) upon heating between 350 degrees C and 600 degrees C. The loss of the chalcedony's approximate to 1% of silanole groups allows for the healing of water related defects in the crystallites. We investigated these crystallographic transformations using Fourier Transform mid Infrared Spectroscopy in direct transmission, Attenuated Total Reflection (ATR) and the reflectivity. We found that an absorption band that is specific for chalcedony at 555 cm(-1) disappears gradually upon heating between 350 degrees C and 600 degrees C. The reduction of the band is correlated to the loss of surface silanoles. This result leads to the assignment of the band to free Si-O vibrations in non bridging Si-OH groups that have a lower natural frequency than Si-O vibrations in bridging Si-O-Si. The recognition of a silanole signal in the mid infrared allows for an easy, cheap and rapid recognition of hydroxyl in chalcedony. (C) 2011 Elsevier B.V. All rights reserved.