Dumortierite

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Dumortierite
Dumortierite-ch23a.jpg
Dumortierite from Alpine County, California, USA, possibly pseudomorphic after tourmaline (size: 5.7 x 2.7 x 1.1 cm)
General
CategoryNesosilicate
Formula
(repeating unit)
Al7BO3(SiO4)3O3 or Al6.5-7BO3(SiO4)3(O,OH)3[1]
Strunz classification09.AJ.10
Crystal symmetryOrthorhombic 2/m 2/m 2/m
Unit cella = 11.77 Å, b = 20.21 Å,
c = 4.71 Å; Z=4
Identification
ColorBlue, greenish-blue, violet-blue, pale blue, red
Crystal habitAs fibrous or columnar crystals; coarsely crystalline to intimate parallel aggregates of needles; massive
Crystal systemOrthorhombic - Dipyramidal
TwinningCommon on {110}, may produce trillings
CleavageDistinct on {100}, poor on {110}; parting on {001}
FractureFibrous
Mohs scale hardness7 - 8.5
LusterVitreous to dull
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity3.3 - 3.4
Optical propertiesBiaxial (-)
Refractive indexnα = 1.659 - 1.678 nβ = 1.684 - 1.691 nγ = 1.686 - 1.692
Birefringenceδ = 0.027
PleochroismStrong; X = deep blue or violet; Y = yellow to red-violet or nearly colorless; Z = colorless or very pale blue
2V angleMeasured: 20° to 52°, Calculated: 30°
Dispersionr > v; strong
References[1][2][3]
 
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Dumortierite
Dumortierite-ch23a.jpg
Dumortierite from Alpine County, California, USA, possibly pseudomorphic after tourmaline (size: 5.7 x 2.7 x 1.1 cm)
General
CategoryNesosilicate
Formula
(repeating unit)
Al7BO3(SiO4)3O3 or Al6.5-7BO3(SiO4)3(O,OH)3[1]
Strunz classification09.AJ.10
Crystal symmetryOrthorhombic 2/m 2/m 2/m
Unit cella = 11.77 Å, b = 20.21 Å,
c = 4.71 Å; Z=4
Identification
ColorBlue, greenish-blue, violet-blue, pale blue, red
Crystal habitAs fibrous or columnar crystals; coarsely crystalline to intimate parallel aggregates of needles; massive
Crystal systemOrthorhombic - Dipyramidal
TwinningCommon on {110}, may produce trillings
CleavageDistinct on {100}, poor on {110}; parting on {001}
FractureFibrous
Mohs scale hardness7 - 8.5
LusterVitreous to dull
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity3.3 - 3.4
Optical propertiesBiaxial (-)
Refractive indexnα = 1.659 - 1.678 nβ = 1.684 - 1.691 nγ = 1.686 - 1.692
Birefringenceδ = 0.027
PleochroismStrong; X = deep blue or violet; Y = yellow to red-violet or nearly colorless; Z = colorless or very pale blue
2V angleMeasured: 20° to 52°, Calculated: 30°
Dispersionr > v; strong
References[1][2][3]

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).

It is used in the manufacture of high grade porcelain. It is sometimes mistaken for sodalite and has been used as imitation lapis lazuli.

Sources of Dumortierite include Austria, Brazil, Canada, France, Italy, Madagascar, Namibia, Nevada, Norway, Poland, Russia and Sri Lanka.

See also[edit]

References[edit]