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Home » Minerals » Rhodonite

Rhodonite


A pink to red manganese silicate used as a gemstone and minor ore of manganese.



What is Rhodonite?



Rhodonite is a pink manganese silicate of variable composition that often contains significant amounts of iron, magnesium, and calcium. It has a generalized chemical composition of (Mn,Fe,Mg,Ca)SiO3. Rhodonite is often associated with black manganese oxides which may occur as dendrites, fracture-fillings, or matrix within the specimen. Other names for rhodonite include: manganese spar, manganolite, and rhodarsenide.


Geologic Occurrence



Rhodonite is usually found in metamorphic rocks associated with other manganese minerals. It is also found in rocks that have been altered by contact metamorphism, hydrothermal and metasomatic processes. It is usually massive to granular in occurrence. Rarely, it is found as red triclinic crystals.

Rhodonite is an uncommon mineral. It is found in a few small deposits across the world. Sources of rhodonite include: Russia, Canada, Australia, Brazil, Sweden, Peru, and England. In the United States it has been found in North Carolina, Colorado, New Jersey, and has been named as the state gem of Massachusetts.


Physical Properties



Rhodonite's diagnostic properties are its pink to red color, hardness, high specific gravity, perfect cleavage, and its close association with black manganese oxides. It is sometimes confused with rhodochrosite, which is softer and effervescent in hydrochloric acid, or thulite, which is usually not associated with black manganese oxides. The physical properties of rhodonite are summarized in the table below.

Physical Properties of Rhodonite

Chemical Classification silicate
Color pink, red, reddish-brown to brown when weathered
Streak white
Luster pearly to vitreous
Diaphaneity transparent to translucent
Cleavage perfect, two directions, 90 degrees
Mohs Hardness 5.5 to 6.5
Specific Gravity 3.5 to 3.7
Diagnostic Properties pink color, cleavage, specific gravity, frequent association with black manganese oxide
Chemical Composition (Mn2+,Fe2+,Mg,Ca)SiO3
Crystal System triclinic
Uses decorative stone, gemstones


Uses of Rhodonite



Rhodonite was once used as ore of manganese in India. Today its only uses are as lapidary materials and as mineral specimens. High-quality crystals of rhodonite can sell for very high prices. Good massive pink- to red-colored material is used as an ornamental stone or gem rough. It is typically used to make cabochons, beads, small sculptures, tumbled stones, and other lapidary projects.

Rhodonite specimen from Minas Gerais, Brazil
A nice cluster of rhodonite crystals from Minas Gerais, Brazil. Specimen is about 2.5 centimeters in height. Photograph by Rob Lavinsky of iRocks.com, used here under a Creative Commons License.

Rare, well-formed, transparent crystals are highly sought after by mineral collectors. Damaged crystals of good quality are sometimes cut into faceted stones. Most of these are acquired by collectors because their cleavage and low hardness make them too fragile for use in jewelry.


Compositional and Structural Variations



Specimens of rhodonite that contain up to 20% calcium oxide are usually grayish brown in color and are known as "bustamite." "Fowlerite" is the name given to specimens that contain up to 7% zinc oxide. Specimens with a brown color have usually been altered by weathering.

Rhodonite is one of two minerals with a chemical composition of manganese silicate. The other is a high-temperature, low-pressure polymorph known as "pyroxmangite."


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Rhodonite cabochon
A cabochon cut from pink rhodonite in a matrix of black manganese oxide. The pink color of this specimen is especially nice, and the manganese oxide is typical. This cabochon is about 28 millimeters in diameter.


faceted rhodonite
Rhodonite is rarely seen as a faceted stone. Excellent crystals of rhodonite are very rare and usually sell for such high prices to mineral collectors that very few are faceted. Occasionally, broken or second-quality crystals are faceted. Because of their cleavage and low hardness, faceted specimens of rhodonite are more in demand as "collector gems" than for use in jewelry.


Rhodonite specimen from Humboldt County, Nevada
Rhodonite with its characteristic matrix and fracture-filling of manganese oxide. This specimen from Humboldt County, Nevada was photographed by Chris Ralph of Nevada-Outback-Gems.com and is used here as a public domain image.




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