THE MINERAL DUMORTIERITE

Chemistry: Al_{6.5 - 7}(BO₃)(SiO₄)₃(O, OH)₃, Aluminum Boro-silicate Hydroxide.
Class: Silicates
Subclass: Nesosilicates
Uses: As an ornamental and semi-precious stone, in the manufacture of high grade ceramics and porcelain and as mineral specimens.
Specimens

Dumortierite is a boro-silicate mineral that is used as a popular ornamental stone. It has a deep violet to blue color that is very attractive and unusual. Although it is not used as a gemstone due to a lack of clarity, it does have good hardness and a bright color. Massive dumortierite can be carved into cabochons, beads, sculptures, eggs and spheres. A variety of quartz called dumortierite quartz is massive quartz colored blue by included crystals of dumortierite. Dumortierite can be misidentified as other ornamental stones such as sodalite, lazurite and lazulite. Blue sodalite has more white portions and is much lighter in density. Lazurite and lazulite are not fibrous. In China, some dumortierite has been used as an imitation lapis lazuli in carvings.

Dumortierite is related to several other nesosilicate boro-silicates such as grandidierite, harkerite, holtite, kornerupine, magnesiodumortierite, prismatine and werdingite. Dumortierite is far more common than all of these. In fact, it is the most common boro-silicate with the exception of the more common members of the Tourmaline Group. Dumortierite is commonly found in aluminum rich metamorphic rocks in contact metamorphic regions and in some pegmatites. It can alter to the mineral pyrophyllite. Dumortierite is named for the French paleontologist, Eugene Dumortier.

PHYSICAL CHARACTERISTICS:

Color is typically blue to violet, but also pink and brown.
Luster is vitreous to dull.
Transparency: Crystals are translucent to transparent, massive specimens are nearly opaque.
Crystal System is orthorhombic; 2/m 2/m 2/m.
Crystal Habits include prismatic crystals but more commonly massive, columnar and fibrous.
Cleavage good in one direction and poor in four others, but rarely seen.
Fracture is uneven to hackly.
Hardness is 7 - 8.5
Specific Gravity is 3.3 - 3.4
Streak is bluish white.
Other Characteristics: Crystals are pleochroic from red to blue to violet. Some specimens fluoresce a blue color under longwave UV light with a fluorescent yellow matrix and a few specimens from Oreana, Nevada have fluoresced white under shortwave UV light.
Associated Minerals are numerous and include quartz, kyanite, sillimanite, staurolite, andalusite, muscovite, lazulite and pyrophyllite
Notable Occurrences include the type locality of Beaunan, France as well as Quartzite, La Paz County, Arizona; Colorado; Oreana, Nevada; New York, New York and Alpine, San Diego County and Los Angeles County, California, USA; Magadanskaya, Siberia, Russia and Sahatany, Madagascar.
Best Field Indicators include hardness, color, density, fluorescence, fibrous crystal habit and environment.