Tremolite

Description

Tremolite is a calcium magnesium silicate hydroxide mineral belonging to the amphibole supergroup. It is the magnesium-rich end-member of the tremolite-ferro-actinolite solid solution series. Its chemical formula is Ca2Mg5Si8O22(OH)2. Tremolite typically occurs in metamorphic rocks, often as long, slender, prismatic crystals, or in fibrous, columnar, or granular aggregates. It is known for its variable forms, ranging from compact, non-asbestiform varieties to highly fibrous, asbestiform varieties. The asbestiform variety, often referred to as tremolite asbestos, is a significant health hazard due to its needle-like fibers.

How to Identify

Color

Typically white, gray, light green, or colorless. Can be dark green if iron is present (transitioning towards actinolite).

Luster

Vitreous (glassy) to silky, especially in fibrous varieties.

Texture

Can be prismatic, columnar, fibrous, or granular. Fibrous varieties are often soft and flexible.

Crystal Form

Monoclinic system, typically forming long, slender, prismatic crystals with a diamond-shaped cross-section (due to amphibole cleavage angles of approximately 56 and 124). Also occurs as radiating aggregates or massive forms.

Cleavage

Perfect cleavage in two directions intersecting at approximately 56 and 124, characteristic of amphiboles.

Geological Environment

Metamorphosed dolomitic limestones, marbles, talc schists, and ultramafic rocks. Often associated with minerals like calcite, dolomite, talc, diopside, and serpentine.

Key Facts

• Hardness: 5-6 on the Mohs scale
• Specific Gravity: 2.9-3.2 g/cm
• Crystal System: Monoclinic
• Color: White, gray, light green, colorless
• Luster: Vitreous to silky
• Transparency: Transparent to translucent to opaque
• Fracture: Uneven to subconchoidal
• Cleavage: Perfect in two directions at approximately 56 and 124
• Composition: Calcium magnesium silicate hydroxide (Ca2Mg5Si8O22(OH)2)

Quick Check

• Color: White to light green or gray
• Luster: Vitreous to silky
• Streak: White

Physical Characteristics

• Crystal Habit: Prismatic, columnar, fibrous, radiating, massive, granular
• Cleavage Type: Perfect on {110} (two directions at ~56 and 124)
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle (non-fibrous); flexible (fibrous)
• Luster Type: Vitreous to silky

Formation

Tremolite is a member of the amphibole group, specifically the calcium amphibole subgroup. It forms primarily through regional and contact metamorphism of siliceous dolomites and limestones, where magnesium-rich fluids react with silica and calcium carbonate. It can also form in ultramafic rocks undergoing metamorphism. The presence of water is crucial for its formation, as it is a hydrous mineral. Typical formation temperatures range from approximately 400C to 700C, and pressures can vary widely depending on the metamorphic facies (e.g., greenschist to amphibolite facies).

Usage

Historically, fibrous tremolite was used as a form of asbestos, particularly in insulation, fireproofing, and construction materials. However, due to its severe health hazards, its use has been largely banned or severely restricted globally. Non-fibrous tremolite has limited industrial uses. High-quality, translucent tremolite can be cut and polished as a minor gemstone or ornamental stone, sometimes mistaken for nephrite jade. It is also of scientific interest for understanding metamorphic processes.

Age Distribution

Tremolite forms in metamorphic environments, so its age is tied to the metamorphic events that produced it, ranging from Precambrian to Cenozoic depending on the specific geological setting.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicates

Group

Amphibole Group, Calcium Amphibole Subgroup

Crystal System

Monoclinic

Chemical Formula

Ca2Mg5Si8O22(OH)2

Composition

Calcium Magnesium Silicate Hydroxide

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