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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 8.AC.10
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/c
Unit cell a = 11.03 Å, b = 12.53 Å
c = 6.4 Å; β = 97.57°; Z = 4
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
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]