When colour makes minerals unique: the case of the green dolomite from Malentrata (Pomarance, Tuscany, Italy)

A peculiar green variety of dolomite has been described in several deposits in Europe (Miemo and Malentrata, Tuscany, Italy; Tarasp, the Grisons, Switzerland; Vučkovica, Serbia). This variety has attracted the interest of researchers and collectors for its aesthetic quality, which has led to its use as a semi-precious gemstone. In the present study, a thorough multi-method characterisation of the green dolomite sampled at the Malentrata magnesite mine (Pomarance, Tuscany, Italy) has been carried out. The investigation included X-ray powder diffraction, electron probe microanalysis, X-ray fluorescence, X-ray absorption spectroscopy, scanning electron microscopy, thermal analysis, optical spectroscopy (UV-visible-NIR), and electron paramagnetic resonance spectroscopy. As a result, two main pieces of evidence were described: (1) a peculiar reduction in cell volume, due to an anomalous excess of small-ionic-radius metal cations (i.e. Mg and Fe) with respect to Ca, and (2) the origin of the peculiar green colour, mainly determined by the Fe electronic transitions, probably in combination with those of Cr. In fact, the green dolomite of Malentrata contains aliquots of a number of environmentally relevant elements, in particular Mn, Cr, and Ni, whose mobility is consequently affected.