A contribution to the study of FTIR spectra of opals

A FTIR spectra collection of a large number of various micro-crystalline (C and CT) and non-crystalline (A) opal samples is here presented. The suite of the investigated specimens consists of the most important natural and synthetic opal samples, with a gemological significance, present on the market today, including both with and without play of color. The opal-C type is clearly identifiable on the basis of the sharp peak at about 620 cm-1, whereas the FTIR spectra of opal-CT and -A differ mainly for the position of the 790 cm-1 SiO band. Other phases, such as i.e. clay minerals, present in opal samples and related to different genesis and localities, can be easily detected by mid-infrared spectroscopy, in addition to X-ray powder diffraction data.