Triploidite

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]

Triploidite
Radial reddish triploidite from the Branchville Quarry, Branchville, Fairfield County, Connecticut, US (7.0 x 4.8 x 2.6 cm)
General
Category	Phosphate minerals
Formula (repeating unit)	(Mn,Fe)2PO4OH
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
Identification
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	​4 1⁄2 - 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]

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]