alluaudite

{{Infobox mineral | name = Alluaudite | image = Alluaudite 2 Sodium iron manganese phosphate Pleasant Valley Mine near Fourmile Custer County South Dakota 2264Spp.jpg | alt = | caption = Alluaudite | category = Phosphate minerals | formula = {{chem2|(Na,Ca)Mn^{2+}(Fe^{3+},Mn^{2+},Fe^{2+},Mg)2(PO4)3}} |IMAsymbol=Ald | molweight = | strunz = 8.AC.10 | dana = 38.2.3.6 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = 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 | habit = Platy to radiating fibrous, nodular, granular, massive | twinning = Polysynthetic | cleavage = Distinct/ good on {100} and {010}, good on {110} | fracture = | tenacity = | mohs = 5 – 5.5 | luster = | streak = Brownish yellow | diaphaneity = Translucent | gravity = 3.4 – 3.5 | density = | polish = | opticalprop = Biaxial (+) | refractive = 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 = Measured: 50° to 90°, calculated: 78° | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | references = }}

Alluaudite is a relatively common alkaline manganese iron phosphate mineral with the chemical formula {{chem2|(Na,Ca)Mn^{2+}(Fe^{3+},Mn^{2+},Fe^{2+},Mg)2(PO4)3}}. It occurs as metasomatic replacement in granitic pegmatites and within phosphatic nodules in shales.