Rhodonite

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Rhodonite crystals in rock

A piece of rounded rhodonite

Rhodonite is a member of the pyroxene group of minerals, consisting of manganese inosilicate, (Mn,Fe,Mg,Ca)SiO₃, and crystallizing in the triclinic system. It commonly occurs as cleavable to compact masses with a rose-red color, often tending to brown because of surface oxidation.

Rhodonite crystals often have a thick tabular habit, but are rare. It has a perfect, prismatic cleavage, almost at right angles. The hardness is 5.5 - 6.5, and the specific gravity 3.4 - 3.7; luster is vitreous, being less frequently pearly on cleavage surfaces. The manganese is often partly replaced by iron, magnesium, calcium, and sometimes zinc which may sometimes be present in considerable amounts; a greyish-brown variety containing as much as 20% of calcium oxide is called bustamite; fowlerite is a zinciferous variety containing 7% of zinc oxide.

Rhodonite is a mineral liable to alteration, with the formation of manganese carbonate (rhodochrosite), hydrous silicates or oxides. The compact material, which is cut and polished for ornamental purposes, is often marked in a striking manner by veins and patches of these black alteration products. At Syedelnikova, near Ekaterinburg in the Urals, compact material of a good color occurs in a clay-slate and is extensively quarried: boulders of similar material found at Cummington, in Massachusetts ( cummingtonite ) have also been worked as an ornamental stone. In the iron and manganese mines at Pajsberg near Filipstadt and Langban in Vermland, Sweden, small brilliant and translucent crystals (pajsbergite) and cleavage masses occur. Fowlerite occurs as large, rough crystals, somewhat resembling pink feldspar, with franklinite and zinc ores in granular limestone at Franklin Furnace in New Jersey.

The inosilicate (chain silicate) structure of rhodonite has a repeat unit of five silica tetrahedra. The rare polymorph pyroxmangite, formed at different conditions of pressure and temperature, has the same chemical composition but a repeat unit of seven tetrahedra.