Mineral Information and Data:

Bertrandite
Be4Si2O7(OH)2 [CNMNC approved formula]

Named honoring the French mineralogist, Emile Bertrand (1844-1909).

IMA status : Valid species (Pre-IMA 1878) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
56.01.01.01  
  (56) Sorosilicate Si2O7 Groups and O, OH, F. and H2O
  (56.01) cations in [4] coordination
  (56.01.01)
  
Nickel-Strunz
(10th edition) :
09.BD.05
  (09) Silicates
  (09.B) Sorosilicates (dimer) structures
  (09.BD) Si2O7 groups, with additional anions; cations in tetrahedral [4] and greater coordination
  
Crystal system: Orthorhombic System
Point group (H-M): mm2 — pyramidal
Space group: Cmc21
Unit cell: a = 8.7135(4) Å, b = 15.268(1) Å, c = 4.5683(3) Å
Crystal Habit: Crystals thin tabular, commonly prismatic to needlelike, to 5 cm; in radial aggregates.
Twinning: Common on {011} or {021}; twins heart-shaped or V-shaped with axes crossing at angles of about 60° and 120°.
  
Color: Colorless to slightly yellow
Diaphaneity: Transparent
Luster: Vitreous, pearly on {001}
Cleavage: Perfect on {001}; distinct on {100}, {010}, and {110}
  
Geologic Setting: In fissures in granites and associated pegmatites and in miarolitic cavities in greisens; commonly an alteration product of beryl, more rarely as a primary mineral.
Mineral Association: Beryl, phenakite, herderite, tourmaline, muscovite, °uorite, quartz
Ore deposits: The major ore mineral for beryllium in the U.S.
  
Other Properties: Gemstone Extremely Rare.

 

Bertrandite
Bertrandite twin
Origin: Greenwood, Oxford County, Maine, U.S.A.
Sample size: 2.5 x 1.5 x 1.5 cm
Photo: Rob Lavinsky

 

Bertrandite
(zoom-in of above)

 

Bertrandite
Bertrandite on rhodochrosite
Origin: Kounrad, Karaganda , Kazakhstan
ex. Carnegie Mineralogical Museum
Sample size: 4.7 x 2.7 x 2.6 cm
Photo: Rob Lavinsky

 

Bertrandite
(zoom-in of above)

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