Mineral Information and Data:

Boracite
Mg3B7O13Cl [CNMNC approved formula]

Named after its composition containing boron.

IMA status : Valid species (Pre-IMA 1789) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
25.06.01.01  
  (25) Anhydrous Borates Containing Hydroxyl or Halogen
  (25.06)
  (25.06.01) Boracite Group (Orthorhombic: Rca21)
  
Nickel-Strunz
(10th edition) :
06.GA.05
  (06) Borates
  (06.G) Heptaborates and other Megaborates
  (06.GA) Tekto-heptaborates
  
Crystal system: Orthorhombic System
Point group (H-M): mm2 — pyramidal
Space group: Pca21
Unit cell: a = 8.577(6) Å, b = 8.553(8) Å, c = 12.09(1) Å, Z=4
Crystal Habit: Euhedral crystals, to 2.5 cm, (referred to pseudotetrahedral morphology) show prominent {001}, {110}, {111}, {111}, and a dozen other modifying forms; spherulitic, plumose to fibrous, fine granular aggregates.
Twinning: On {111}, as penetration twins
  
Color: Colorless, white, gray, yellow; bluish green, green to dark green if ferroan; colorless in transmitted light
Diaphaneity: Transparent to translucent
Luster: Vitreous to adamantine
Hardness (Mohs): 7 - 7.5
Measured Density: 2.91 – 3.10 g/cm3
Fracture: Conchoidal to uneven
Streak: White
  
Polymorphism and Series: Dimorphous with trembathite; forms a series with ericaite.
Geologic Setting: An uncommon component of bedded sedimentary salt and potash deposits of marine origin, the boron probably derived from nearby volcanic activity.
Mineral Association: Anhydrite, gypsum, halite, sylvite, carnallite, kainite, hilgardite
  
Other Properties: Piezoelectric. Pyroelectric. H2O Soluble. Gemstone Extremely Rare.
Comments: Strongly piezoelectric

 

Boracite
Origin: Schildstein, Luneberg, Germany
Sample size: 2.4 x 2 x 1.3 cm
Photo: Rob Lavinsky

 

Boracite
(zoom-in of above)

 

Boracite
(zoom-in of above)

 

Boracite
Boracite with sylvite
Origin: Boulby mine, Loftus, Cleveland, England
Sample size: 3.5 x 4 x 1.8 cm
Photo: Dan Weinrich

 

Boracite
(zoom-in of above)

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