Mineral of Custer County, South Dakota. These mineral images are free to use how you wish. — Photo: Dave Dyet http://www.shutterstone.com http://www.dyet.com · Public domain
Alluaudite
A fairly common manganese-iron phosphate found in granite pegmatites and phosphatic nodules in shale.
Intro adapted from Wikipedia (CC BY-SA).
Alluaudite is an IMA-recognized mineral species in the phosphate class, with the chemical formula NaMnFe³⁺₂(PO₄)₃.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Alluaudite structural group.
Key properties: Mohs hardness 5–5.5; specific gravity 3.45; Dirty yellow streak; refractive index 1.806; optically biaxial.
IMA symbol Ald, IMA status: approved, named in 1848.
Very common in granite pegmatites, formed by sodium metasomatism of triphylite- lithiophilite, heterosite-purpurite, or ferrisicklerite; in phosphatic nodules in shales.
It is found associated with Triphylite, arrojadite, satterlyite, wicksite, wolfeite, pyrite.
The mineral was named to honor François Alluaud, mining engineer of Limoges, France, who discovered the species.