Dumortierite

{{Short description|Aluminum boro-silicate mineral}}

{{infobox mineral

| name = Dumortierite

| category = Nesosilicate

| boxwidth =

| image = Dumortiérite.JPG

| alt =

| caption = Dumortierite from Tuléar Province (Toliara), Madagascar

| formula = Al₇BO₃(SiO₄)₃O₃ or Al_6.5-7BO₃(SiO₄)₃(O,OH)₃

| IMAsymbol = Dum{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 9.AJ.10

| dana =

| system = Orthorhombic

| class = Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)

| symmetry = Pmcn (no. 62)

| unit cell = a = 11.77 Å, b = 20.21 Å
c = 4.71 Å; Z = 4

| color = Blue, greenish-blue, violet-blue, pale blue, red

| colour =

| 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

| tenacity =

| mohs = 7–8.5

| luster = Vitreous to dull

| streak = White

| diaphaneity = Transparent to translucent

| gravity = 3.3–3.4

| density =

| polish =

| opticalprop = Biaxial (−)

| refractive = n_α = 1.659 – 1.678 n_β = 1.684 – 1.691 n_γ = 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 = Measured: 20° to 52°, calculated: 30°

| dispersion = r > v; strong

| extinction =

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| melt =

| fusibility =

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| references = http://webmineral.com/data/Dumortierite.shtml Webmineral datahttp://rruff.geo.arizona.edu/doclib/hom/dumortierite.pdf Handbook of Mineralogyhttp://www.mindat.org/min-1329.html Mindat.org

}}

File:Spectical Bracelet in Dumortierite and Gold by Cora Sheibani.jpg

Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, Al₇BO₃(SiO₄)₃O₃. It 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 results 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).{{Cite EB1911|wstitle=Dumortierite|volume=8|page=667|short=y}} 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, Peru, Poland, Russia, Indonesia, and Sri Lanka.