Raman spectroscopy of the joaquinite minerals

Selected joaquinite minerals have been studied by Raman spectroscopy. The minerals are categorised into two groups depending upon whether bands occur in the 3250 to 3450 cm(-1) region and in the 3450 to 3600 cm-1 region, or in the latter region only. The first set of bands is attributed to water stretching vibrations and the second set to OH stretching bands. In the literature, X-ray diffraction could not identify the presence of OH units in the structure of joaquinite. Raman spectroscopy proves that the joaquinite mineral group contains OH units in their structure, and in some cases both water and OH units. A series of bands at 1123, 1062, 1031, 971, 912 and 892 cm(-1) are assigned to SiO stretching vibrations. Bands above 1000 cm(-1) are attributable to the upsilon(as) modes of the (SiO4)(4-) and (Si2O7)(6-) units. Bands that are observed at 738, around 700, 682 and around 668, 621 and 602 cm(-1) are attributed to O-Si-O bending modes. The patterns do not appear to match the published infrared spectral patterns of either (SiO4)(4-) or (Si2O7)(6-)units. The reason is attributed to the actual formulation of the joaquinite mineral, in which significant amounts of Ti or Nb and Fe are found. Copyright (C) 2007 John Wiley & Sons, Ltd.