Mineral Information and Data:

Arseniosiderite
Ca2Fe3+3O2(AsO4)3 • 3H2O [CNMNC approved formula]

Named for the essential "arsenic" in the composition, and "sideros," Greek for "iron.".

IMA status : Valid species (Pre-IMA 1842) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
42.08.04.03  
  (42) Hydrated Phosphates Containing Hydroxyl or Halogen
  (42.08) where (AB)7 (XO4)4 Zq · x(H2O)
  (42.08.04) Mitridatite Group
  
Nickel-Strunz
(10th edition) :
08.DH.30
  (08) Phosphates, Arsenates, Vanadates
  (08.D) Phosphates, etc
  (08.DH) With large and medium-sized cations, (OH, etc.):RO4 < 1:1
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 17.76Å, b = 19.53Å, c = 11.3Å β = 96°
Crystal Habit: Euhedral crystals very rare, to 2 mm; usually as flattened fibers, in radial aggregates, and felted to granular masses.
  
Color: Golden yellow, yellowish brown, reddish brown, brownish black, black
Diaphaneity: Opaque, translucent
Luster: Submetallic to silky
Hardness (Mohs): 4.5
Measured Density: 3.58 - 3.6 g/cm3
Cleavage: On {001}, perfect
Streak: Ocher-yellow
  
Geologic Setting: A rare secondary mineral formed by the oxidation of earlier arsenic-bearing minerals, typically scorodite or arsenopyrite.
Mineral Association: Beudantite, carminite, scorodite, dussertite, pharmacolite, pitticite, adamite, erythrite, arsenopyrite, lollingite
  

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