Rhodonite-Pyroxmangite from Tanatz Alp, Switzerland

Abstract

R hodonite is the Mn-rich member of the pyroxenoid group; it is triclinic with a structure made up of silicate chains parallel to the c-axis, in turn composed of a repeating sequence of five tetrahedral units. The chains alternate with M octahedral sites, designated M1 though M5, where divalent cations reside (Deer et al., 1997, and references therein). The ideal chemical formula is MnSiO 3. Rhodonite of this composition has been synthesized but has never been found in nature, where Ca, Mg, and Fe 2+ replace Mn (Ito, 1972). Rhodonite was first discovered in 1790 in the Ural Mountains near Sidelnikovo. The mineral was named in 1819 by the German naturalist Christoph Friedrich Jasche. The name is derived from the Greek rhodo, which means "rose," because of its characteristic pink color (figure 1). The Hermitage Museum in Saint Petersburg preserves many precious and decorative objects made with rhodonite that belonged to the Russian aristocracy, especially the czars. Even today, Russian children exchange rhodonite eggs for Easter as a gesture of friendship. In the United States there are important rhodonite mines in New Jersey and in Massachusetts, which declared it the official state gem in 1979. Rhodonite is collected as a lapidary material and ornamental stone and is used to make cabochons, beads, small sculptures, tumbled stones, and other objects. High-quality crystals of this mineral can be very expensive. Rhodonite is generally found in massive translucent to opaque aggregates, and the best-quality gems are transparent. The crystals have perfect cleavage in two directions and low hardness (5.5-6 Mohs), making it one of the most difficult gemstones to cut. For this reason, faceted rhodonites are typically sold as collectible gems rather than for jewelry use. Rhodonite is very similar to its polymorph, pyroxmangite, and to rhodochrosite (MnCO 3