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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)
Category Phosphate minerals
(repeating unit)
Strunz classification 8.BB.15
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/a
Unit cell a = 12.36 Å, b = 13.27 Å
c = 9.94 Å; β = 108.23°; Z = 16
Color Red-brown, light pink, yellow-brown; light pink to light brown in transmitted light.
Crystal habit Elongated and striated prismatic crystals; fibrous: columnar to spherulitic
Cleavage On {010}, good; on {120}, fair; on {110}, very poor.
Fracture Uneven to subconchoidal
Mohs scale hardness 4½ - 5
Luster Adamantine, vitreous, greasy
Streak White, off-white.
Diaphaneity Transparent, translucent
Specific gravity 3.70 measured
Optical properties Biaxial (+)
Refractive index nα = 1.725 nβ = 1.726 nγ = 1.730
Birefringence δ = 0.005
Pleochroism Weak
Dispersion r > v strong
Solubility Soluble in acids.
References [1][2][3]

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[edit]