Le terre coloranti e la farina fossile del Monte Amiata. Caratteristiche geologiche, attività estrattiva e significato paleoambientale

Earth pigments from Monte Amiata comprise the famous "terra di Siena" (raw sienna) and "terra di Siena bruciata" (burnt sienna) that have been ones of the yellow-brown and red pigments most widely used by Italian and European painters from the Renaissance to 19th century. These Earth pigments are lacustrine sediments composed mainly of hydrated iron oxide (limonite/goethite) produced by biochemical precipitation of iron-rich waters. The quarrying localities were at Castel del Piano, Arcidosso, Piancastagnaio, and Abbadia San Salvatore. Diatomaceous earths from Monte Amiata are soft, friable, earthy, very fine-grained, siliceous lacustrine sediments usually light in color (white if pure) composed of diatom's fossilized skeletons. The main quarrying localities were at Castel del Piano and Santa Fiora. Two new coring were performed in the lacustrine deposits of the Bagnolo Diatomaceous earth quarry (Santa Fiora). The age of the lacustrine basins comprising the Earth pigments and Diatomaceous earth ore deposits is following the volcanic activity of the volcano of Monte Amiata. The evolution of these lake basins has been reconstructed in four phases. The first phase is represented by the sedimentation of clay and iron hydroxides in well-developed lacustrine basins, during cold climate conditions that are suggested by the pollen predominance of Pinus, Picea, and Betula. During the second phase, the lake conditions continue, but the sedimentation of iron hydroxides basins become different from those where sedimentation of the Diatomaceous earth begins. In the latter, there are abundant plant, ictiological, and entomological fossils that indicate a warm continental climate with great development of Fagus, Abies, and Castanea. The lithological and paleontological characteristics of the Diatomaceous earth suggest a gradual drying of the lake and the transition to a humid fresh climate. The third phase is characterized by the simultaneous end of the lacustrine deposition in all the basins, and by the alluvial sedimentation of volcaniclastic sands and gravel. This sharp environmental change is interpreted as the effect of a climate change that established cold and rainy conditions. The last stage shows a low colluvial and alluvial sedimentation with a substantial stability of the topographic surface that has favored the development of intense pedogenesis.