Triploidite

Triploidite
Radial reddish triploidite from the Branchville Quarry, Branchville, Fairfield County, Connecticut, USA (7.0 x 4.8 x 2.6 cm)
General
Category	Phosphate minerals
Chemical formula	(Mn,Fe)2PO4OH
Strunz classification	08.BB.15
Crystal symmetry	2/m - Prismatic
Unit cell	a = 12.36Å, b = 13.27Å, c = 9.94Å β = 108.23°
Identification
Color	Red-brown, light pink, yellow-brown; light pink to light brown in transmitted light.
Crystal system	Monoclinic
Cleavage	On {010}, good; on {120}, fair; on {110}, very poor.
Mohs scale hardness	4½ - 5
Luster	Adamantine, vitreous, greasy
Streak	White, off-white.
Diaphaneity	Transparent, translucent
Specific gravity	3.66 - estimated value for (Mn)2PO4OH
Optical properties	Biaxial (+)
Refractive index	nα = 1.725 nβ = 1.726 nγ = 1.730
Birefringence	δ = 0.005
Pleochroism	Weak
Dispersion	r > v weak
Solubility	Soluble 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.

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.

It typically occurs as hydrothermal alteration of primary phosphate minerals in granite pegmatites. It occurs with triplite, lithiophilite, triphylite, eosphorite, dickinsonite and rhodochrosite.

It forms a solid solution series with the iron rich wolfeite.

See also

• Classification of minerals
• List of minerals