Boracite

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 insoluble in water (not to be confused with borax, which is soluble in water).

Boracite
General
CategoryTektoborates
Formula
(repeating unit)		Mg3B7O13Cl
Strunz classification6.GA.05
Dana classification25.06.01.01
Crystal systemOrthorhombic
Crystal classPyramidal (mm2)
(same H-M symbol)
Space groupPca21
Unit cella = 8.577(6) Å,
b = 8.553(8) Å,
c = 12.09(1) Å; Z = 4
Identification
Formula mass392.03 g/mol
ColorColorless, white, gray, brown, orange, yellow, pale green, dark green, blue-green, or blue; colorless in transmitted light
Crystal habitCrystalline, disseminated (pseudocubic)
TwinningRarely as penetration twins
CleavageNone
FractureIrregular/uneven, conchoidal
TenacityBrittle
Mohs scale hardness7 - 7.5
LusterVitreous - adamantine
StreakWhite
DiaphaneitySubtransparent to translucent
Specific gravity2.95
Density2.91 - 3.10
Optical propertiesBiaxial (+)
Refractive indexnα=1.658 - 1.662,
nβ=1.662 - 1.667,
nγ=1.668 - 1.673
Birefringenceδ = 0.010–0.011
PleochroismNone
2V angle82°
Dispersion0.024 (weak)
Ultraviolet fluorescenceNone
SolubilityVery slowly soluble in H2O; Slowly but completely soluble in HCl
References[1][2][3][4]

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 derived from its boron content (19 to 20% boron by mass).

See also

References
  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" (PDF). 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.
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