Quantitative Analysis of Sulfated Calcium Carbonates Using Raman Spectroscopy and X-ray Powder Diffraction

Abstract

A non-destructive method based on the use of Raman spectroscopy (RS) for the determination of the percentage of gypsum in sulfated marble is presented. The Raman spectra of well mixed powder samples of calcite–aragonite, calcite–gypsum and gypsum–aragonite pairs of mixtures were recorded and the characteristic bands at 280 cm ^{- 1} for calcite, 205 cm ^{- 1} for aragonite and 412 cm ^{- 1} for gypsum were used as the basis for the quantitative analysis of specimens in which the most stable calcium carbonate phases, calcite and aragonite, were present. The detection limits were found to be 0.3 mol% for calcite, 0.5 mol% for aragonite and 0.6 mol% for gypsum. For samples containing only one calcium carbonate phase the use of the strong and sharp Raman band at 1085 cm ^{- 1} , common for aragonite and calcite, together with the intensity of the Raman peak at 1006 cm ^{- 1} for gypsum, yielded lower detection limits: calcite 0.1, aragonite 0.1 and gypsum 0.05 mol%. The analysis by RS was compared with X-ray powder diffraction (XRD). In this analysis, the calibration curves were constructed using the relative intensities corresponding to the 113, the 111 and the 12 reflections of the calcite, aragonite and gypsum, respectively. The detection limits for calcite, aragonite and gypsum were 4, 5 and 1–2 mol%, respectively. The potential of using RS for a point-by-point analysis (‘mapping’) of a surface by focusing the laser beam on the selected spots was also demonstrated on a marble sample removed from Athens National Garden, exposed in the open air.