“Green-earths”: vibrational and elemental characterisation of glauconites, celadonites and historical pigments

“Green-earths” have been employed since antiquity and are still used as pigments in the creation of artworks. The minerals responsible for the colour belong to four groups: (i) the clayey micas celadonite and glauconite, undoubtedly the most common; (ii) smectites; (iii) chlorites; (iv) serpentines1. Mineralogical analyses in the field of cultural heritage are rare, and often limited to the identification of the generic class “green-earth”. A multi-disciplinal study of minerals coming from the historical mining localities, besides to clarify the mineralogy of traditional green-earths, could support the correct identification of the mineralogical species employed in the artworks, and could provide a valid tool for the study of the pigments’ origin. This work presents a preliminary study on some raw minerals (glauconite, celadonite, Fe-celadonite) characterized by multi-technical approach. In particular, the attention has been focused on the possible distinction between celadonite and glauconite by μ-Raman, μ-IR and SEM-EDS techniques. Concerning μ-Raman spectroscopy, we identified the characteristic bands of minerals, considering the internal variations in the single species, and distinguishing them from spurious structures, heating effects, dependence on wavenumber of exciting source. A similar procedure has been followed with μ-IR spectroscopy. Vibrational analyses have been correlated with elemental analyses, thanks to the coupled instrument SEM-EDS-SCA that lets to collect EDS and Raman spectra on the same microscopic area. Besides pure minerals, archaeological samples and commercial “green earths” have been analyzed. 1. C.A. Grissom, “Green Earth”, in Artists’ Pigments, Vol. 1, (1986), Ed. R.L. Feller, Cambridge University Press, UK