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The Mineral dumortierite

Dumortierite in Quartz

Dumortierite often forms in an attractive blue color and can be used as an ornamental stone. Though it is most commonly perceived as blue, especially in lapidary use, other colors include purple, pink, gray and brown. Dumortierite specimens are composed of dense fibers, giving them a tough durability.

Dumortierite often forms as inclusions in Quartz, and this combination results in a natural blue Quartz gemstone. These are known in the gemstone market as "Dumortierite Quartz" and they are becoming increasingly popular as a minor blue gemstone. Dumortierite is named after French paleontologist Eugene Dumortier (1803-1873).

For additional information, see the gemstone section on Dumortierite.

Chemical Formula

AlAl₆O₃BSi₃O₁₈

Color

Light to dark blue, grayish blue, purple, pink, and brown

Crystal System

Orthorhombic

Properties

Streak
White

Hardness
7

Transparency
Translucent to opaque

Specific Gravity
3.3 - 3.4

Luster
Vitreous, silky, or dull

Cleavage
2,1;3,1

Fracture
Uneven, splintery

Tenacity
Brittle

Other ID Marks
Sometimes fluorescent purple is shortwave ultraviolet light.

Crystal Habits

Never in individual crystals. Most often as radiating sprays, as fibrous masses, columnar, compact, massive, and as grainy splotches on matrix.

Additional Information

Composition
Aluminum borosilicate

In Group
Silicates; Nesosilicates

Striking Features
Color, crystal habits, and hardness

Environment
In high temperature, aluminum-rich regional metamorphic rocks, in boron-rich granite pegmatites, and in hydrothermal replacement deposits.

Rock Type
Igneous, Metamorphic

Varieties

Dumortierite Quartz

- Dumortierite formed as dense inclusion within Quartz.

Magnesiodumortierite

- Form of Dumortierite with magnesium replacing some of the aluminum, with a chemical formula of MgAl₆O₃BSi₃O₁₈. Magnesiodumorierite is recognized by the IMA as a distinct mineral species.

Uses

Dumortierite may be used as a minor gemstone. Dumortierite-included Quartz, often known as "Dumortierite Quartz", is an increasingly popular minor blue gemstone. Dumortierite is also mined for spark plug ceramics as an electric insulator.

Noteworthy Localities

Dumortierite is not a common mineral, and only a handful of localities have produced quality specimens. The type locality where this mineral was first discovered is the Ducarre's Quarry, Beaunant, Rhône, France. Small embedded fibrous groups of Dumortierite have been found in the Waldviertel area, Lower Austria, Austria; and violet-blue acicular crystals from Chiavenna, Val Schiesone, Lombardy, Italy. A deep blue form in relatively large crystals comes from Sahavina and Ambositra, Antananarivo Province, Madagascar. Fine radiating Dumortierite needles have come from Cucucha, Canta Province, Peru; and a deep blue Dumortierite included in Quartz from Macaúbas, Bahia, Brazil (where it is known locally as "Bahia Blue Quartz".)

In the U.S., blue and lavender Dumortierite in dense fibrous form comes from the Dehesa Dumortierite Deposit, near Alpine, San Diego Co., California. Other California localities include the Temescal Wash, Corona, Riverside Co.; and Ogilby, Cargo Muchacho Mountains, Imperial Co., California. Dumortierite was also found in Oreana, Pershing Co., Nevada; and in the Clip Mine, Yuma Co., Arizona.