Gem-quality rhodonite from Val Malenco (Central Alps, Italy)

Abstract

Rhodonite from the Mn-deposit of Monte del Forno is one of the more attractive gemstones, together with andradite and nephrite jade, found  in Val Malenco (Central Alps, Italy). A complete characterization of  this mineral  was provided using classical gemological methods and advanced techniques such as  X-ray powder diffraction, EMPA-WDS chemical analyses, Raman and mid-infrared spectroscopy. Rhodonite ranges in color from pink, white-pink to green and pale brown, due to the occurrence of various accessory minerals including tephroite, spessartine, biotite, magnetite, calcite, quartz, and Mn-oxides and hydroxides. From a detailed microprobe study, performed both on thin section and rough samples, the resulted chemical composition scatters from the ideal formula MnSiO₃, due to the occurrence of other cations, such as Ca, Mg and Fe substituting for Mn. Consistent with results of X-ray powder diffraction and IR and Raman spectroscopy, the particularly low content of Ca in in two points analysis suggests the hypothesis of the occurrence of pyroxmangite, probably as relic phase in a fine intergrowth with rhodonite. Data of rhodonite from the most famous classic deposits of the world (Russia, Japan, USA, Australia, Peru, Brazil and Sweden) are also used for comparison.