Alluaudite

Alluaudite is a relatively common alkaline manganese iron phosphate mineral with the chemical 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.[3]

Alluaudite
Alluaudite
General
Category	Phosphate minerals
Formula (repeating unit)	(Na,Ca)Mn2+(Fe3+,Mn2+,Fe2+,Mg)2(PO4)3
IMA symbol	Ald[1]
Strunz classification	8.AC.10
Dana classification	38.2.3.6
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
Identification
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	[2][3][4]

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).[2][3] The mineral structure was first described in 1955.[5]