Mineral Information and Data:

Alluaudite
(Na,Ca)2(Mn,Mg,Fe2+)(Fe3+,Mn2+)2(PO4)3 [CNMNC approved formula]

Named in 1848 for Francois Alluaud, mining engineer of Limoges, France, who discovered the mineral.

IMA status : Redefined IMA Approved; 0 ()
CLASSIFICATION 
Dana
(8th edition) :
38.02.03.06  
  (38) Anhydrous Phosphates, etc
  (38.02) (A+ B++)5 (XO4)3
  (38.02.03) Alluaudite-Wyllieite Group (Alluaudite Subgroup)
  
Nickel-Strunz
(10th edition) :
08.AC.10
  (08) Phosphates, Arsenates, Vanadates
  (08.A) Phosphates, etc. without additional anions, without H2O
  (08.AC) With medium-sized and large cations
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 12.004 Å, b = 12.533 Å, c = 6.404 Å, b = 114°22
Crystal Habit: Massive, compact granular, radiating fibrous, and globular aggregates
Twinning: polysynthetic with {-101} as composition plane, seen in thin section
  
Color: Dull greenish black, green, dirty yellow, brownish yellow to brownish black
Diaphaneity: Subtranslucent to opaque
Hardness (Mohs): 5 - 5.5
Measured Density: 3.45 g/cm3
Cleavage: 3; {010}, {110} and {110} good
Streak: Dirty yellow to brownish
  
Polymorphism and Series: Forms a series with ferroalluaudite.
Geologic Setting: Very common in granite pegmatites, formed by sodium metasomatism of triphylite–lithiophilite, heterosite–purpurite, or ferrisicklerite; in phosphatic nodules in shales.
Mineral Association: Triphylite, arrojadite, satterlyite, wicksite, wolfeite, pyrite
  
Comments: Soluble in acids.
Alters to Heterosite and Purpurite.

A confusing change of nomenclature occurred regarding the species in this series. Ferroalluaudite was formerly Alluaudite and the present day Alluaudite was formerly Mangan-Alluaudite.

 

Alluaudite
Origin: Helen Beryl Mine, Custer Co., South Dakota, U.S.A.
Sample size: 2.5 x 3.5 cm
Photo: Diederik Visser

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