Dumortierite

Dumortierite
Dumortierite from Alpine County, California, USA, possibly pseudomorphic after tourmaline (size: 5.7 x 2.7 x 1.1 cm)
General
Category	Nesosilicate
Formula (repeating unit)	Al7BO3(SiO4)3O3 or Al6.5-7BO3(SiO4)3(O,OH)3
Strunz classification	9.AJ.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Unit cell	a = 11.77 Å, b = 20.21 Å c = 4.71 Å; Z = 4
Identification
Color	Blue, greenish-blue, violet-blue, pale blue, red
Crystal habit	As fibrous or columnar crystals; coarsely crystalline to intimate parallel aggregates of needles; massive
Twinning	Common on {110}, may produce trillings
Cleavage	Distinct on {100}, poor on {110}; parting on {001}
Fracture	Fibrous
Mohs scale hardness	7 - 8.5
Luster	Vitreous to dull
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.3 - 3.4
Optical properties	Biaxial (-)
Refractive index	nα = 1.659 - 1.678 nβ = 1.684 - 1.691 nγ = 1.686 - 1.692
Birefringence	δ = 0.027
Pleochroism	Strong; X = deep blue or violet; Y = yellow to red-violet or nearly colorless; Z = colorless or very pale blue
2V angle	Measured: 20° to 52°, Calculated: 30°
Dispersion	r > v; strong
References

Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, Al₇BO₃(SiO₄)₃O₃. Dumortierite crystallizes in the orthorhombic system typically forming fibrous aggregates of slender prismatic crystals. The crystals are vitreous and vary in color from brown, blue, and green to more rare violet and pink. Substitution of iron and other tri-valent elements for aluminium result in the color variations. It has a Mohs hardness of 7 and a specific gravity of 3.3 to 3.4. Crystals show pleochroism from red to blue to violet. Dumortierite quartz is blue colored quartz containing abundant dumortierite inclusions.

Dumortierite was first described in 1881 for an occurrence in Chaponost, in the Rhône-Alps of France and named for the French paleontologist Eugène Dumortier (1803–1873). It typically occurs in high temperature aluminium rich regional metamorphic rocks, those resulting from contact metamorphism and also in boron rich pegmatites. The most extensive investigation on dumortierite was done on samples from the high grade metamorphic Gfohl unit in Austria by Fuchs et al. (2005).