Dictionary: rhodonite   (rōd'n-īt')

A pink to rose-red mineral, essentially a glassy crystalline manganese silicate, MnSiO₃, used as an ornamental stone.

[From Greek rhodon, rose.]

A mineral inosilicate with composition MnSiO₃. Hardness is 5.5–6 on Mohs scale, and specific gravity is 3.4–3.7. The luster is vitreous and the color is rose red, pink, or brown. Rhodonite is similar in color to rhodochrosite, manganese carbonate, but it may be distinguished by its greater hardness and insolubility in hydrochloric acid. It has been found at Langban, Sweden; near Sverdlovsk in the Ural Mountains; and at Broken Hill, Australia. Fine crystals of a zinc-bearing variety, fowlerite, are found at Franklin, New Jersey. See also Silicate minerals.

(click to enlarge)
Rhodonite from Pajsberg, Swed. (credit: Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard)

Silicate mineral that occurs in various manganese ores, often with rhodochrosite. A manganese silicate, MnSiO₃, with small amounts of iron and calcium, it is found in the Ural Mountains, Sweden, Australia, California, New Jersey, and elsewhere. Rhodonite is the primary source of some important manganese oxide deposits, such as the manganese ores of India. Fine-grained rhodonite of clean, pink colour is a desirable gem and ornamental stone.

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Environment

A mineral of metamorphic rocks, related to manganese occurrences; rarely associated with ore veins.

Usually massive, sometimes in fine-grained masses. Good several-cm, equidimensional crystals were found embedded in calcite, in the early days of Franklin, New Jersey. Smaller, flattened, acute-angled crystals of deeper hue are also found there. Similar rich-hued, very cleavable material has been found in Congonhas, Brazil. However, the most significant crystal occurrence of rhodonite is one of large, gemmy, chunky red crystals embedded in galena, at Broken Hill, New South Wales, Australia, with very similar pyroxmangite (MnSiO ₃ ) and bustamite ([Mn,Ca] ₃ Si ₃ O ₉ ).

Pink to grayish, blackening rapidly on the surface through weathering. Luster glassy; hardness 5Ɖ-6; specific gravity 3.4-3.7; fracture splintery, very tough in massive state; cleavage prismatic at about 88° and 92°. Crystals brittle; transparent to translucent.

Manganese silicate (54.1% MnO, 45.9% SiO ₂ , with Ca partially replacing Mn).

Fuses to a brown glass. Gives manganese test in borax bead.

The pink material is likely to be mistaken only for rhodochrosite (but it is much harder than the manganese carbonate) and for feldspar, which gives no manganese test or easy fusion. The gemmy red crystals are practically indistinguishable from the similar, closely related iron-bearing pyroxmangite. Bustamite is less intensely red. A black stain quickly develops on exposed internally pinkish boulders and is really diagnostic.

The world's leading specimen locality for large crystal masses is Franklin, New Jersey, a mine from which have come specimens that add luster to museums of the world; the crystals were worked out by careful excavation from a matrix of enclosing calcite. Massive rhodonite is found at Plainfield, Massachusetts, and at numerous western and foreign localities -- California, Brazil (Minas Gerais), Siberia, Australia (Tamworth, New South Wales), and Tanzania, to name a few.

Good and sometimes transparent red crystals are found with other silicates distributed through sulfide ores at Broken Hill, New South Wales (in Japanese manganese mines, pyroxmangite predominates). Masses suitable for carving are found in the Ural Mountains. Small late Franklin-type wedge crystals are found at Pajsberg and Langban, Sweden.

Rhodonite crystals in rock

A piece of rounded rhodonite

Rhodonite is a manganese inosilicate, (Mn,Fe,Mg,Ca)SiO₃ and member of the pyroxene group of minerals, 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 has 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.

Pink rhodonite contrasting with black manganese oxides is sometimes used as a gem material as seen in this specimen from Humboldt Co., Nevada, USA

Rhodonite is the official gem of Massachusetts [1].

References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

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