Rhodonite

Rhodonite
Rhodonite from San Martín Mine, Chiurucu, Huallanca District, Bolognesi Province, Ancash, Peru Specimen size: 53 mm × 52 mm × 40 mm (2.1 in × 2.0 in × 1.6 in)
General
Category	Inosilicate
Formula (repeating unit)	(Mn2+, Fe2+, Mg, Ca)SiO3
IMA symbol	Rdn
Strunz classification	9.DK.05
Dana classification	65.04.01.01
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H–M symbol)
Space group	P1
Unit cell	a = 9.758 Å, b = 10.499 Å, c = 12.205 Å; α = 108.58°, β = 102.92°, γ = 82.52°; Z = 20
Identification
Color	Pink, rose-pink to brownish red, red, gray and yellow
Crystal habit	Tabular crystals, massive, granular
Twinning	Lamellar, composition plane {010}
Cleavage	Perfect on {110} and {110}, (110) ^ (110) = 92.5°; good on {001}
Fracture	Conchoidal to uneven
Mohs scale hardness	5.5–6.5
Luster	Vitreous to pearly
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.57–3.76
Optical properties	Biaxial (+)
Refractive index	nα = 1.711–1.738, nβ = 1.714–1.741, nγ = 1.724–1.751
Birefringence	δ = 0.013
Pleochroism	Weak
2V angle	58° to 73° (measured), 58° (calculated)
Alters to	Exterior commonly black from manganese oxides
References

Rhodonite is a manganese inosilicate, with the formula (Mn, Fe, Mg, Ca)SiO₃, and member of the pyroxenoid group of minerals, crystallizing in the triclinic system. It commonly occurs as cleavable to compact masses with a rose-red color (its name comes from Ancient Greek ῥόδον (rhódon) 'rose'), often tending to brown due to surface oxidation. The rose-red hue is caused by the manganese cation (Mn²⁺).

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 is 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.

Pink rhodonite contrasting with black manganese oxides is sometimes used as gemstone material as seen in this specimen from Humboldt County, Nevada.

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.

Rhodonite has also been worked as an ornamental stone. In the iron and manganese mines at Pajsberg near Filipstad and Långban in Värmland, 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.

Rhodonite is the official gemstone of the Commonwealth of Massachusetts.

See also

• Enstatite, with MgSiO₃ and FeSiO₃ end-members
• Khondalite
• Wollastonite, CaSiO₃

References

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Rhodonite". Encyclopædia Britannica (11th ed.). Cambridge University Press.