Alluaudite

Alluaudite
Alluaudite
General
Category	Phosphate minerals
Formula	(Na,Ca)Mn2+(Fe3+,Mn2+,Fe2+,Mg)2(PO4)3
IMA symbol	Ald
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

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.

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). The mineral structure was first described in 1955.