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Dumortierite

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Chemical Formula

AlAl6O3BSi3O18

Composition

Aluminum borosilicate

Color

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

Streak

White

Hardness

7

Crystal System

Orthorhombic

Crystal Forms 
and Aggregates

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

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.

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

Popularity (1-4)

3

Prevalence (1-3)

3

Demand (1-3)

3

 

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