|South America||San Martin, Bolognesi, Ancash, Peru||2" x 1 ¾" x 1 ½"|
Rhodonite is, "ideally," MnSiO3 - "ideally" because, although listed that way in many references, rhodonite never occurs with that "pure" formula. A more realistic formula would be (Mn,Ca,Fe,Mg,Zn)SiO3. So, you ask, aren't all those elements other than Mn just impurities? The answer is that it appears not, although only Ca seems to constantly accompany Mn. (By the way, if the element symbols have you confused, Mn is manganese, Ca calcium, Fe iron, Mg magnesium, and Zn zinc.) Fe, Mg, and Zn may be very minor, and Zn may be absent altogether, but Ca is always there to some extent.
So why, since MnSiO3 is a perfectly good, balanced formula, is Ca always present? The picture to the right shows the sites in the crystal structure available to these metal elements. The green sphere represents that atom in the site called "M5." The pink spheres represent M1 through M3, and the yellow sphere is M4. M5 is obviously a larger site, and Ca is the largest of the metal atoms that enter rhodonite; Ca always occupies M5. Fe, Mg, and Zn favor M4. All atoms are not the same size, and the sizes of the sites available governs, to a first approximation, where the atoms reside.
Coupled with this is that rhodonite occurs in areas that have undergone mineralization, and Ca, Fe, Mg, plus-or-minus Zn are present in abundance in such areas. Mother Nature uses what she has available, so (Mn,Ca)SiO3 is the simplest any rhodonite specimen would be.