Investigations on the green dolomite from Malentrata (Pomarance, Tuscany, Italy)

A peculiar green variety of dolomite has been described in several deposits across Europe, including Miemo and Malentrata (Tuscany, Italy), Tarasp (Grisons, Switzerland), and Vučkovica (Serbia). This variety has attracted the interest of researchers and collectors due to its aesthetic qualities, which have led to its use as a semi-precious gemstone. In the present study, a thorough characterization of the green dolomite sampled from the Malentrata magnesite mine (Pomarance, Tuscany, Italy) was carried out. The investigation employed a range of advanced analytical techniques, including X-ray powder diffraction (XRPD), electron probe micro-analysis (EPMA), X-ray fluorescence (XRF), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), differential thermal analysis (DTA/TG), optical spectroscopy (UV-Vis-NIR), and electron paramagnetic resonance (EPR) spectroscopy. The results revealed two main features of the Malentrata green dolomite: (1) an anomalous reduction in unit cell volume, caused by an excess of small ionic radius metal cations (Mg and Fe) relative to calcium, and (2) the origin of the green color, primarily attributed to the electronic transitions of iron ions, likely in combination with those of chromium. The substitution of cations in the crystal structure leads to a reduction in unit cell volume, a feature that distinguishes this dolomite from the typical variety. The presence of Fe²⁺ and Fe³⁺ proved to be key to the green coloration, with an additional contribution from Cr³⁺, confirming that the color is not due to a single ionic species, but rather to an interaction between various transition metal cations. Chemical analyses showed that the green dolomite from Malentrata also contains small amounts of transition metals, including manganese (Mn), chromium (Cr), and nickel (Ni), which give the mineral additional unique characteristics. These metals are homogeneously distributed within the sample and contribute to its distinctive chemical composition, highlighting the importance of complete characterization to understand the natural variability of the material. In conclusion, the green dolomite from Malentrata represents a material of geological and chemical interest, with distinctive optical properties. The detailed understanding of its composition and structure, achieved through a multi-method approach, paves the way for future investigations in the Malentrata area, providing a solid basis for the study of local geological processes.