| Boracite | |
|---|---|
 | |
| General | |
| Category | Tektoborates |
| Formula | Mg3B7O13Cl |
| IMA symbol | Boc[1] |
| Strunz classification | 6.GA.05 |
| Dana classification | 25.06.01.01 |
| Crystal system | Orthorhombic |
| Crystal class | Pyramidal (mm2) (sameH-M symbol) |
| Space group | Pca21 |
| 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/mol |
| Color | Colorless, white, gray, brown, orange, yellow, pale green, dark green, blue-green, or blue; colorless in transmitted light |
| Crystal habit | Crystalline, disseminated (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) |
| Ultravioletfluorescence | None |
| Solubility | Very slowly soluble in H2O; Slowly but completely soluble in HCl |
| References | [2][3][4][5] |
Boracite is amagnesiumborate mineral with formula:Mg3B7O13Cl. It occurs as blue green, colorless, gray, yellow to white crystals in theorthorhombic - pyramidalcrystal system. Boracite also shows pseudo-isometric cubical andoctahedral forms. These are thought to be the result of transition from an unstable high temperatureisometric form on cooling. Penetrationtwins are not unusual. It occurs as well formed crystals and dispersed grains often embedded withingypsum andanhydrite crystals. It has aMohs hardness of 7 to 7.5 and aspecific 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 aconchoidal fracture and does not showcleavage. It is insoluble in water (not to be confused with borax, whichis soluble in water).
Boracite is typically found inevaporite sequences associated withgypsum,anhydrite,halite,sylvite,carnallite,kainite andhilgardite. It was first described in 1789 for specimens from itstype locality ofKalkberg hill, Lüneburg, Lower Saxony, Germany. It is also found nearSussex, New Brunswick.[6]
The name is derived from itsboron content (19 to 20% boron by mass).
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