Mineral Information and Data:

Atelestite
Bi2O(AsO4)(OH) [CNMNC approved formula]

Named derivation not explicitly stated. Probably from the Greek for "incomplete," as the composition was undetermined when the mineral was first described.

IMA status : Valid species (Pre-IMA 1832) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
41.11.05.01  
  (41) Anhydrous Phosphates Containing Hydroxyl or Halogen
  (41.11) with miscellaneous formulae
  (41.11.05)
  
Nickel-Strunz
(10th edition) :
08.BO.15
  (08) Phosphates, Arsenates, Vanadates
  (08.B) Phosphates, etc. with Additional Anions, without H2O
  (08.BO) With only large cations, (OH, etc.):RO4 about 1:1
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 10.88Å, b = 7.42Å, c = 6.98Å β = 107.2°
Crystal Habit: As tabular to prismatic crystals, to 2 mm. Also as mammillary crystalline or smooth spherical aggregates.
  
Color: Sulfur-yellow, pale yellow, yellowish green, black; in transmitted light, pale yellow to colorless
Diaphaneity: Transparent to translucent
Luster: Resinous to adamantine
Hardness (Mohs): 4.5 - 5
Measured Density: 7.14 g/cm3
Cleavage: {100}, indistinct
Fracture: Sub-conchoidal
  
Geologic Setting: As a rare secondary mineral in the oxidized zone of bismuth- and arsenic-bearing mineral deposits.
Mineral Association: Bismutite, eulytite, erythrite, bismutostibiconite, beyerite, preisingerite, walpurgite, mixite, conichalcite, torbernite, quartz
  

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