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Radial reddish triploidite from the Branchville Quarry, Branchville, Fairfield County, Connecticut, US (7.0 x 4.8 x 2.6 cm)
CategoryPhosphate minerals
(repeating unit)
Strunz classification8.BB.15
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/a
Unit cella = 12.36 Å, b = 13.27 Å
c = 9.94 Å; β = 108.23°; Z = 16
ColorRed-brown, light pink, yellow-brown; light pink to light brown in transmitted light.
Crystal habitElongated and striated prismatic crystals; fibrous: columnar to spherulitic
CleavageOn {010}, good; on {120}, fair; on {110}, very poor.
FractureUneven to subconchoidal
Mohs scale hardness4 12 - 5
|re|er}}Adamantine, vitreous, greasy
StreakWhite, off-white.
DiaphaneityTransparent, translucent
Specific gravity3.70 measured
Optical propertiesBiaxial (+)
Refractive indexnα = 1.725 nβ = 1.726 nγ = 1.730
Birefringenceδ = 0.005
Dispersionr > v strong
SolubilitySoluble in acids.

Triploidite is an uncommon manganese iron phosphate mineral with formula: (Mn,Fe)2PO4OH. It crystallizes in the monoclinic crystal system and typically occurs as elongated and striated slender prisms which may be columnar to fibrous. Its crystals may be pinkish to yellowish brown or red-orange.[1]

It was first described in 1878 for an occurrence in the Branchville Quarry, Branchville, Fairfield County, Connecticut. The name is derived from its resemblance to triplite.[2]

It typically occurs as a hydrothermal alteration product of primary phosphate minerals in granite pegmatites. It occurs with triplite, lithiophilite, triphylite, eosphorite, dickinsonite and rhodochrosite.[1][3]

It forms a solid solution series with the iron rich wolfeite.[2][3]

See also