Green Earth Pigment from the Kada Region, Czech Republic: Use of Rare Fe-rich Smectite

A collection of green earths belonging to traditional artistsʹ pigments was examined in terms of mineralogy and provenance. The studied specimens included both mineralogical reference compounds and selected commercially available artistsʹ pigments, and contained green micas (glauconite or celadonite), chlorite, or smectite as pigmenting agents. The samples were examined by X-ray diffraction, Mssbauer spectroscopy, infrared (IR) spectroscopy, ultraviolet-visible (UV-Vis)-near-IR diffuse-reflectance spectroscopy and voltammetry of microparticles. Particular attention was paid to the Kadan green earth, mined until the 20th century in the West Bohemia deposit. The Greene-Kelly chargereduction test, detailed description of non-basal diffraction patterns and characteristic vibrations in the mid-IR spectra were used to classify the major pigmenting agent of the Kadan green earth as ferruginous smectite with separately diffracting saponite-like clusters. The smectite contains ~15% Fe, mainly in the trivalent form, a detectable fraction of Fe in tetrahedral sites, and it is accompanied by a significant amount of Ti -bearing relict minerals due to its volcanogenic origin. On the contrary, in green micas (glauconite and celadonite) the Ti content is much smaller. Diffuse reflectance spectroscopy was found suitable for distinguishing Fe as a constituent of free Fe oxides from Fe in the clay structure. It was also found to be useful for discriminating between green micas and smectites.