Eosphorite

Eosphorite is a brown (occasionally pink) manganese hydrous phosphate mineral with chemical formula: MnAl(PO₄)(OH)₂·H₂O.^[5] It is used as a gemstone.^[6]

Eosphorite
General
Category	Phosphate mineral
Formula	MnAl(PO4)(OH)2·H2O
IMA symbol	Eos[1]
Strunz classification	8.DD.20
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cmca
Unit cell	a = 10.455(1), b = 13.501(2) c = 6.928(1) [Å]; β = 90°; Z = 8
Identification
Color	usually pale brown, golden brown, also medium brown to dark brown; occasionally pink, rose red
Crystal habit	Prismatic in radiating sprays or spheres, massive; twinned pseudo-orthorhombic
Twinning	May be observed on {100} and {001}
Cleavage	Poor on {100}
Fracture	Subconchoidal to uneven
Mohs scale hardness	5
Luster	Vitreous, resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.06 – 3.08
Optical properties	Biaxial (-)
Refractive index	nα = 1.628 - 1.639 nβ = 1.648 - 1.664 nγ = 1.657 - 1.671
Birefringence	δ = 0.029 - 0.032
Pleochroism	Visible: X = yellow; Y = pink; Z = pale pink to colorless
2V angle	Measured: 50°
Alters to	Oxidizes to brown or black
References	[2][3][4]

Eosphorite crystallizes in the monoclinic crystal system. It forms slender prismatic crystals which often form radiating or spherical clusters. The crystals often show pseudo–orthorhombic forms due to twinning.^[4]

Eosphorite forms a series with childrenite, the iron rich member, with divalent iron replacing most of the manganese in the crystal lattice. The two endmembers are isostructural but differ in their properties, such as crystal habit, coloration, and optical properties.

It was first described in 1878 for an occurrence in the Branchville Mica Mine in Branchville, Fairfield County, Connecticut, US. Its name is derived from the Greek έωσφορος for "dawn-bearing," because of its pink color.^[3] It occurs worldwide typically as a secondary mineral in phosphate rich granitic pegmatites in association with rhodochrosite, lithiophilite, triphylite, triploidite, dickinsonite, albite, cookeite, apatite, beryllonite, hydroxyl-herderite, and tourmaline.^[4] An attractive combination of eosphorite and rose quartz occurs at Taquaral, Minas Gerais, Brazil.^[7]