New insight on the gem quality pink epidote ("Clinothulite") from Val Malenco, Central Alps, Italy

Val Malenco, Central Alps (Sondrio, Italy) is a valley well-known for its variety of minerals, which has attracted the interest of mineralogists and collectors since the end of 19th Century. Some of the stones recovered in Val Malenco have also a significant interest as gemological materials, such as the case of demantoid garnet, nephrite jade, serpentine minerals and rhodonite (Bocchio et al., 2010; Adamo & Bocchio, 2013; Diella et al., 2014; Adamo et al., 2016). Another stone found in Val Malenco, is "clinothulite", a pink colored material, similar to rhodonite as aspect, but fully different from the mineralogical and gemological point of view. This stone was described for the first time in 1934 by Pietro Sigismund, a mineral collector, who labeled it as thulite, i.e. the manganese-bearing variety of zoisite (Dana, 1920), used as gemstone and carving material in the manufacture of jewellery and ornamental objects. More recently, Bianchi et al. (1987) recognized in this stone a monoclinic symmetry and named it "clinothulite" which stands for "monoclinic thulite". The main outcrop of "clinothulite", 70-80 m in length and almost 1 m in thickness, occurs near the locality of Pizzo Tremogge at an altitude of 2920 m and it is enclosed in gneissic rocks and associated to calcium silicate marbles of Paleozoic age. "Clinothulite" is very fine-grained and displays an agreeable pink color, sometimes with white or light green veins, due to the occurrence of a few of accessory minerals such as green epidote, pyroxene, amphibole, calcite, mica and pyrite. This study aims to provide a review and an update of "clinothulite" from Pizzo Tremogge investigating a suite of gem-quality samples (3 faceted gems and 2 rough pieces) by means of traditional gemological tests, XRPD data, combined with quantitative full-phase analysis using the Rietveld method, and EMPA-WDS measurements. All the examined samples (7.58-10.77 ct) are opaque, inert to UV radiation and have a refractive index approximately of 1.69. The density ranges from 3.10 to 3.22 g/cm3, with variations related to the occurrences of the accessory minerals. The cell parameters (a = 8.876, b = 5.595, c = 10.149 Å, ? = 115.46°, V = 455.04 Å3) suggest a monoclinic symmetry and agree with those reported in literature for clinozoisite. The chemical composition compares well with that of thulite samples from literature, both of orthorhombic and monoclinic symmetry, and is very close to the idealized formula of clinozoisite, Ca2Al3[O(OH)(SiO4)(Si2O7)], with an almost stoichiometric content of Ca and Si (Ca ~ 2.000, Si ~ 3.000 apfu). On the contrary, Al is lower than 3.000 apfu and shows a well definite negative correlation with Fe (0.116 to 0.559 apfu) indicating the substitution between these elements. The samples contain also a minor amount of Ti, Cr, Mg and Mn. In particular, the Mn content is low (0.001 to 0.008 apfu.) but it causes the soft pink coloration that makes "clinothulite" an attractive gem material.