Boracite

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Boracite
Boracit mineralogisches museum bonn.jpg
Boracite, Mineralogisches Museum Bonn
General
Category Tektoborates
Formula
(repeating unit)
Mg3B7O13Cl
Strunz classification 06.GA.05
Dana classification 25.06.01.01
Unit cell a = 8.577(6) Å, b = 8.553(8) Å, c = 12.09(1) Å; Z = 4
Identification
Formula mass 392.03 g
Color Colorless, white, gray, brown, orange, yellow, pale green, dark green, blue-green, or blue; colorless in transmitted light
Crystal habit Crystalline, disseminated
Crystal system Orthorhombic - pyramidal, pseudocubic
Twinning Rarely as penetration twins
Cleavage None
Fracture Irregular/uneven, conchoidal
Tenacity Brittle
Mohs scale hardness 7 - 7.5
Luster Vitreous - adamantine
Streak White
Diaphaneity Subtransparent to translucent
Specific gravity 2.95
Density 2.91 - 3.10
Optical properties Biaxial (+)
Refractive index nα=1.658 - 1.662,
nβ=1.662 - 1.667,
nγ=1.668 - 1.673
Birefringence δ = 0.010–0.011
Pleochroism None
2V angle 82°
Dispersion 0.024 (weak)
Ultraviolet fluorescence None
Solubility Very slowly soluble in H2O; Slowly but completely soluble in HCl
References [1][2][3][4]

Boracite is a magnesium borate mineral with formula: Mg3B7O13Cl. It occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. Boracite also shows pseudo-isometric cubical and octahedral forms. These are thought to be the result of transition from an unstable high temperature isometric form on cooling. Penetration twins are not unusual. It occurs as well formed crystals and dispersed grains often embedded within gypsum and anhydrite crystals. It has a Mohs hardness of 7 to 7.5 and a specific gravity of 2.9. Refractive index values are nα = 1.658 - 1.662, nβ = 1.662 - 1.667 and nγ = 1.668 - 1.673. It has a conchoidal fracture and does not show cleavage. It is soluble in water.

Boracite is typically found in evaporite sequences associated with gypsum, anhydrite, halite, sylvite, carnallite, kainite and hilgardite. It was first described in 1789 for specimens from its type locality of Kalkberg hill, Lüneburg, Lower Saxony, Germany. The name is obviously derived from its boron content (19 to 20% boron by mass).

See also[edit]

References[edit]

  1. ^ "Boracite Mineral Data". Webmineral.com. Retrieved 2011-10-27. 
  2. ^ "Boracite Gems". ClassicGems.net. Retrieved 2011-10-27. 
  3. ^ "Boracite mineral information and data". Mindat.org. Retrieved 2011-10-27. 
  4. ^ "Boracite". Mineral Data Publishing. Retrieved 2011-10-27. 
  • Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, 7th edition, v. II, pp.378–381.