A pink to red manganese silicate used as a gemstone and minor ore of manganese.
Article by: Hobart M. King, Ph.D., RPG
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" and "manganolite."
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 of Rhodonite
|Color||Pink, red, reddish brown to brown when weathered|
|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|
|Uses||Decorative stone, gemstones|
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 on this page.
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.
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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