Rhodonite was first discovered in 1790 in the Russian Ural Mountains. The mineral’s name is derived from the Greek word “rhodo,” which means rose, because of its characteristic pink color. Rhodonite is also found in Australia, Brazil, Peru and the United States. Rhodonite generally forms as translucent to opaque aggregates, often with dark inclusions of manganese oxide. It is rarely transparent. Rhodonite has perfect cleavage in two directions and low hardness (5.5 to 6 on the Mohs scale), making it a difficult gem material to facet. For this reason, it is usually cut into cabochons and beads or used as an ornamental stone for carving.
Rhodonite 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 MnSiO3. Rhodonite of this composition has been synthesized but has never been found in nature, where Ca, Mg, and Fe2+ 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 (MnCO3). Pyroxmangite has a structure very similar to that of rhodonite, with a repeating sequence of seven tetrahedral units; the distinction from rhodonite therefore requires more accurate investigations with techniques such as X-ray powder diffraction (XRPD) and electron microprobe. Rhodochrosite differs in that it frequently shows white streaks of calcite and is reactive to hot acids, whereas rhodonite is resistant to acids and usually associated with manganese oxides.
Rhodonite and pyroxmangite contain small amounts of Ca, Mg, and Fe2+ substituting for Mn2+; the P-T compositional stability limits have been investigated by many authors, but there are still uncertainties (Ito, 1972; Peters et al., 1973; Ohashi et al., 1975; Maresch and Mottana, 1976; Brown et al., 1980; Akaogy and Navrotsky, 1985; Abrecht, 1988; Takahashi and Hariya, 1995; Zanazzi et al., 2008; Diella et al., 2014, and references therein). The two minerals can generally be distinguished on the basis of their chemical composition, as rhodonite contains a higher amount of Ca (more than 0.05 wt.%) than pyroxmangite. Rhodonite and pyroxmangite often occur together as a bladed intergrowth, in various types of ore deposits and Mn-rich lithologies (Ohashi et al., 1975; Jefferson et al., 1980; Pinckney and Burnham, 1988; Millsteed et al., 2005; Michailidis and Sofianska, 2010).
Deposits of rhodonite are found all over the world. The most important are those at Broken Hill in New South Wales, Australia, where the best single-crystal gems are found (Millsteed et al., 2005; Millsteed, 2006); Franklin, New Jersey, in the United States (Nelson and Griffen, 2005); many areas of the Ural Mountains (Bukanov, 2006; Brusnitsyn, 2010); British Columbia, Canada (Simandl et al., 2001); Huánuco, Peru (Wilson,1989); Minas Gerais, Brazil (Quinn, 2004; Leverett et al., 2008); Pajnsberg, Sweden (Lee, 1958); and several localities in Japan (Ohashi et al., 1975). A variety of rhodonite containing high levels of zinc (up to 10 wt.% ZnO), called fowlerite, was found at Franklin, New Jersey (Nelson and Griffen, 2005).
In the Italian Alps, rhodonite has been found in the Praborna mine near San Marcel in Valle d'Aosta (Mottana, 1986), Feglierec near Alagna Valsesia in Piedmont (Peters et al., 1978), Monte Forno in Malenco Valley, Scerscen in Lombardy (Diella et al., 2014), and Monte Civillina in Veneto (Schiavinato, 1953). Rhodonite is also found in the Apennines chain, in the mines of Gambatesa and Molinello in Graveglia Valley in Liguria (Marchesini and Pagano, 2001), Alpe Ravinella in Strona Valley (Bertolani, 1967), Scortico in the Apuan Alps (Di Sabatino, 1967; Mancini, 1997), and Campiglia Marittima in Tuscany (Capitani et al., 2003). Italian rhodonites are translucent or opaque and microcrystalline with a pale to deep pink color; only those from Monte Forno in Val Malenco (Diella et al., 2014) and Scortico (Apuan Alps) can be truly considered gem-quality since they are transparent enough to be faceted.
The manganese deposits in the Rhetic Alps of Switzerland that contain rhodonite, such as Falotta and Alpe Parsettens in the Oberhalbstein or Fianel and Starlera in Val Ferrera, have been widely investigated (Trommsdorff et al., 1970; Peters et al., 1973, 1978; Wenk and Maurizio, 1978; Brugger and Giere, 1999), but those in the Tanatz Alp (figure 2) are less well known. This work investigates the chemical, physical, and gemological properties of rhodonite from Tanatz Alp.