Mineral Information and Data:

Bahianite
Al5Sb5+3O14(OH)2 [CNMNC approved formula]

Named for the locality: the region around Rio do Picos das Almas, Serra das Almas, south-central Bahia, Brazil.

IMA status : Approved Species 1974 (IMA1974-027)
CLASSIFICATION 
Dana
(8th edition) :
44.03.07.01  
  (44) Antimonates
  (44.03) with miscellaneous formulae
  (44.03.07)
  
Nickel-Strunz
(10th edition) :
04.DC.05
  (04) Oxides and Hydroxides
  (04.D) Metal:Oxygen = 1:2 and similar
  (04.DC) With medium-sized cations; sheets of edge-sharing octahedra
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 9.406Å, b = 11.541Å, c = 4.41Å β = 90.94°
Crystal Habit: As water-worn, bean-shaped pebbles, to 10 cm, with polycrystalline or radial-fibrous structure; also as curved and striated crystals, diamond-shaped or rectangular, lining pockets in the pebbles.
Twinning: As pseudohexagonal multiple contact twins, chrysoberyl-like.
  
Color: Tan, cream, orange-brown, brown; crystals colorless, tan, pale violet; yellow internal reflections in transmitted light
Diaphaneity: Transparent to translucent
Luster: Adamantine
Hardness (Mohs): 9
Measured Density: 5.46 g/cm3
Cleavage: {100}, perfect
Fracture: Uneven
  
Geologic Setting: In potholes as stream-worn pebbles and as residual concentrates above weathered volcanic rock.
Mineral Association: Quartz, andalusite, kyanite, diaspore, phenakite, eskolaite, cassiterite, gold
  
Other Properties: Gemstone Extremely Rare.

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