Alluaudite
>
Alluaudite is a relatively common alkaline manganese iron phosphate mineral with the chemical formula Mn²⁺(Fe³⁺, Mn²⁺, Fe²⁺, Mg)₂(PO₄)₃. It occurs as metasomatic replacement in granitic pegmatites and within phosphatic nodules in shales. Wikipedia
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Platy to radiating fibrous, nodular, granular, massive
Crystal system: Monoclinic
Formula (repeating unit): (Na,Ca)Mn2+(Fe3+,Mn2+,Fe2+,Mg)2(PO4)3