Alluaudite

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Alluaudite
Alluaudite 2 Sodium iron manganese phosphate Pleasant Valley Mine near Fourmile Custer County South Dakota 2264Spp.jpg
Alluaudite
General
CategoryPhosphate minerals
Formula
(repeating unit)
(Na,Ca)Mn2+(Fe3+,Mn2+,Fe2+,Mg)2(PO4)3
Strunz classification8.AC.10
Dana classification38.2.3.6
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Unit cella = 11.03 Å, b = 12.53 Å
c = 6.4 Å; β = 97.57°; Z = 4
Identification
ColorDirty yellow to brownish yellow, grayish green; superficially dull greenish black, brownish black, black, when altered
Crystal habitPlaty to radiating fibrous, nodular, granular, massive
TwinningPolysynthetic
CleavageDistinct/ good on {100} and {010}, good on {110}
Mohs scale hardness5 - 5.5
StreakBrownish yellow
DiaphaneityTranslucent
Specific gravity3.4 - 3.5
Optical propertiesBiaxial (+)
Refractive indexnα = 1.782 nβ = 1.802 nγ = 1.835
Birefringenceδ = 0.053
PleochroismX = pale olive-green, straw-yellow to greenish yellow; Z = pale olive-greenish to brownish yellow
2V angleMeasured: 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]

References[edit]