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
Category	Tektoborates
Formula (repeating unit)	Mg3B7O13Cl
IMA symbol	Boc[1]
Strunz classification	6.GA.05
Dana classification	25.06.01.01
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-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)
Ultraviolet fluorescence	None
Solubility	Very slowly soluble in H2O; Slowly but completely soluble in HCl
References	[2][3][4][5]

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. It is also found near Sussex, New Brunswick.^[6]

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