Dumortierite

Description

Dumortierite is a fibrous or columnar borosilicate mineral, typically occurring in aggregates. Its most distinctive feature is its color, which ranges from deep blue to violet, often with reddish-brown or pinkish hues. It is known for its excellent hardness and resistance to weathering. It often forms in radiating or parallel fibrous masses, sometimes appearing as dense, compact aggregates. When polished, it can exhibit a vitreous to dull luster. Its presence in quartz can create striking blue patterns, making 'dumortierite quartz' a popular material.

How to Identify

Color

Typically deep blue, violet-blue, reddish-brown, or pink. The blue and violet hues are most characteristic.

Luster

Vitreous to dull, sometimes silky in fibrous varieties.

Texture

Often fibrous, columnar, or massive. Can be granular or compact.

Crystal Form

Usually occurs as aggregates of prismatic or fibrous crystals, often radiating or parallel. Individual crystals are rare and typically small, forming elongated prisms.

Cleavage

Distinct on {100}, but rarely observed due to its fibrous habit and common occurrence in massive forms.

Geological Environment

Metamorphic rocks (gneisses, schists, quartzites), pegmatites, aplites, and hydrothermal veins, particularly in aluminum-rich and boron-bearing environments.

Key Facts

• Hardness: 7-8 on the Mohs scale
• Specific Gravity: 3.26-3.41
• Crystal System: Orthorhombic
• Color: Blue, violet-blue, reddish-brown, pink
• Luster: Vitreous to dull, silky
• Transparency: Transparent to opaque
• Fracture: Uneven to splintery
• Cleavage: Distinct on {100}, but rarely observed
• Composition: Al7(BO3)(SiO4)3O3

Quick Check

• Color: Blue to violet, reddish-brown, pink
• Luster: Vitreous to dull, silky
• Streak: White to pale blue

Physical Characteristics

• Crystal Habit: Prismatic, fibrous, columnar, radiating aggregates, massive
• Cleavage Type: Distinct on {100}, but often obscured by fibrous habit
• Fracture Type: Uneven to splintery
• Tenacity: Brittle
• Luster Type: Vitreous to dull, silky

Formation

Dumortierite is a borosilicate mineral that forms primarily in aluminum-rich metamorphic rocks, particularly those that have undergone high-temperature, low-pressure metamorphism. It is commonly found in gneisses, schists, and quartzites, often as a product of contact or regional metamorphism. It can also occur in pegmatites and aplites, and as a secondary mineral in some hydrothermal veins. The presence of boron is crucial for its formation, often derived from tourmaline or other boron-bearing minerals during metamorphic processes.

Usage

Dumortierite is primarily used as an ornamental stone and in jewelry due to its attractive blue to violet color. It is often cut into cabochons, beads, and carvings. When found as inclusions in quartz, it forms 'dumortierite quartz,' which is also highly valued for lapidary purposes. Industrially, dumortierite has been investigated for its potential use in high-temperature ceramics and refractories due to its high aluminum and boron content and thermal stability, though this application is less common.

Age Distribution

Precambrian to Cenozoic, typically associated with metamorphic events.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Borosilicate

Group

Silicates

Crystal System

Orthorhombic

Chemical Formula

Al7(BO3)(SiO4)3O3

Composition

Aluminum borosilicate with minor iron and magnesium substitutions.

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