Boracite, Mineralogisches Museum Bonn
|Crystal system||Orthorhombic - pyramidal (pseudocubic)|
|Unit cell||a = 8.577(6) Å, b = 8.553(8) Å, c = 12.09(1) Å; Z = 4|
|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|
|Twinning||Rarely as penetration twins|
|Mohs scale hardness||7 - 7.5|
|Luster||Vitreous - adamantine|
|Diaphaneity||Subtransparent to translucent|
|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|
|Solubility||Very slowly soluble in H2O; Slowly but completely soluble in HCl|
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 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).
- Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, 7th edition, v. II, pp.378–381.
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