Mineralization processes in some samples from the fossil forest of Dunarobba (Umbria, Central Italy)

Two isolated twigs (samples A and B) from the site of the fossil forest of Dunarobba (Central Italy) were examined to assess both the degree of degrad ation of the organic fraction and the composition of the inorganic componen t. Sample A, although showing the presence of minute crystals, is mineralis ed only to little larger extent with respect to the trunks of the fossil fo rest and exhibits a wood composition almost comparable to that of the fossi l forest. Main minerals are silica, calcite, chloroapatite, chloromagnesite , and clays. Residual terpenes perfectly match, as either components or pro portions, those found in the fossil forest trunks. Sample B is much more mi neralised and physical parts are hard (sample BII, 84% mineralisation) alon g with others that are quite loose (sample BI, 62% mineralisation). Goethit e constitutes the major mineral in both parts. Calcite, on the contrary, is higher in BII than in BI. In agreement with the literature, goethite is pr esent in the interstitial spaces of the cell walls whereas calcite is found in the cell lumina. The wood components are limited to lignin and terpenes ; being absent cellulose or other polysaccharides. BI contains the same typ es of terpenes present in the fossil trunks, whereas only diterpenes have b een found in BII. The samples B contain, with respect to either sample A or fossil trunks, a lower content of fully or partly aromatic sesquiterpenes and diterpenes. This fact is probably due to the oxidative environment of i ron deposition. The difference in the mineralogical structures exhibited by the two samples must be searched in the different environmental conditions under which the samples were maintained. In particular, an essential role must have been played by water sources much richer in soluble salts and mor e differentiated than the current spring waters of the region.