Christos G. Kontoyannis, Malvina G. Orkoula and Petros G. Koutsoukos
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.