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Alluaudite 2 Sodium iron manganese phosphate Pleasant Valley Mine near Fourmile Custer County South Dakota 2264Spp.jpg
Category Phosphate minerals
(repeating unit)
Strunz classification 08.AC.10
Dana classification
Crystal system Monoclinic
Unit cell a = 11.03 Å, b = 12.53 Å, c = 6.4 Å; β = 97.57°
Color Dirty yellow to brownish yellow, grayish green; superficially dull greenish black, brownish black, black, when altered
Crystal habit Platy to radiating fibrous, nodular, granular, massive
Crystal symmetry Monoclinic prismatic
H-M symbol: (2/m)
Space group: C 2/c
Twinning Polysynthetic
Cleavage Distinct/ good on {100} and {010}, good on {110}
Mohs scale hardness 5 - 5.5
Streak Brownish yellow
Diaphaneity Translucent
Specific gravity 3.4 - 3.5
Optical properties Biaxial (+)
Refractive index nα = 1.782 nβ = 1.802 nγ = 1.835
Birefringence δ = 0.053
Pleochroism X = pale olive-green, straw-yellow to greenish yellow; Z = pale olive-greenish to brownish yellow
2V angle Measured: 50° to 90°, calculated: 78°
References [1][2][3]

Alluaudite is a relatively common alkaline manganese iron phosphate mineral with formula (Na,Ca)Mn2+(Fe3+,Mn2+,Fe2+,Mg)2(PO4)3. It occurs as metasomatic replacement in granitic pegmatites and within phosphatic nodules in shales.[2]

It was first described in 1848 for an occurrence in Skellefteå, Västerbotten, Sweden. It was named by Alexis Damour after François Alluaud (II) (1778–1866).[1][2]