Alluaudite

Alluaudite
Alluaudite
General
Category	Phosphate minerals
Formula (repeating unit)	(Na,Ca)Mn^2+(Fe^3+,Mn^2+,Fe^2+,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]

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]

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]

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. Alluaudite on Mindat.org
3. "Alluaudite in the Handbook of Mineralogy" (PDF). Archived from the original (PDF) on 2016-03-03. Retrieved 2009-12-21.
4. Alluaudite data on Webmineral
5. Nowagiel, Maciej; Samsel, Mateusz J.; Pietrzak, Tomasz K. (2021-09-01). "Towards the High Phase Purity of Nanostructured Alluaudite-Type Glass-Ceramics Cathode Materials for Sodium Ion Batteries". Materials. 14 (17): 4997. Bibcode:2021Mate...14.4997N. doi:10.3390/ma14174997. PMC 8434363. PMID 34501086.