Glaucophane Mineral Overview
Mineral name: Glaucophane
Mineral class: Silicate (inosilicate, amphibole group)
Known Facts
- Chemical formula: Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂ (simplified; composition can vary)
- Crystal system: Monoclinic
- Mohs hardness: 6–6.5
- Color range: Blue, lavender-blue, gray-blue
- Luster: Vitreous to silky
- Streak: Grayish-white to pale blue
- Cleavage: Perfect in two directions (at ~56° and 124°)
- Fracture: Splintery
- Specific gravity: Approximately 3.0–3.2
- Common locations: Japan, California (USA), Greece, Italy, New Caledonia
- Uses: Geological indicator mineral, collector specimens
- Similar minerals: Hornblende, riebeckite, crossite
Physical and Optical Characteristics
Glaucophane is a distinctive blue amphibole mineral, often occurring as:
- Slender prismatic crystals
- Fibrous or bladed aggregates
- Massive or schistose forms
Its color ranges from pale lavender-blue to deep blue, which is one of its most recognizable features.
The mineral shows:
- Vitreous luster on fresh crystal faces
- Silky appearance in fibrous aggregates
- Strong directional cleavage, typical of amphiboles
The cleavage occurs in two directions intersecting at approximately 56° and 124°, a key identifying feature of amphibole minerals.
Chemical Composition and Structure
Glaucophane is a sodium-rich amphibole, with a general formula:
- Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂
Its structure consists of:
- Double chains of silicate tetrahedra (Si₈O₂₂)
- Sodium, magnesium, and aluminum occupying cation sites
- Hydroxyl groups (OH) within the structure
There is compositional variation, and glaucophane forms a series with related amphiboles such as crossite and riebeckite, depending on iron and magnesium content.
Formation and Geological Occurrence
Glaucophane forms under high-pressure, low-temperature metamorphic conditions, which are relatively uncommon.
It is the defining mineral of:
- Blueschist facies metamorphism
Typical formation settings include:
- Subduction zones, where oceanic crust is forced deep into the Earth
- Metamorphism of basaltic rocks under الضغط العالي ودرجة حرارة منخفضة (high pressure and relatively low temperature)
Common associated minerals include:
- Lawsonite
- Epidote
- Jadeite
- Chlorite
These mineral assemblages indicate specific pressure–temperature conditions.
Common Locations
Notable occurrences of Glaucophane include:
- Japan: Classic blueschist localities
- United States: California (Coast Ranges)
- Greece: Island of Syros
- Italy: Alpine metamorphic belts
- New Caledonia: Subduction-related terrains
These regions are known for well-developed blueschist metamorphic rocks.
Uses and Practical Significance
Glaucophane has limited practical use but is highly important in geology:
- Indicator mineral: Identifies blueschist facies and subduction-related metamorphism
- Tectonic studies: Helps reconstruct pressure–temperature histories
- Collector specimens: Valued for its blue color
It has no significant industrial or commercial applications.
Similar and Related Minerals
Glaucophane may be confused with other amphiboles:
- Hornblende: Typically darker (green to black) and forms in different environments
- Riebeckite: Blue amphibole but usually iron-rich and found in different geological settings
- Crossite: Intermediate composition between glaucophane and riebeckite
Distinguishing features of Glaucophane include:
- Blue coloration
- Occurrence in blueschist facies rocks
- Specific chemical composition
Identification Notes for Collectors
- Look for blue amphibole crystals in metamorphic rocks
- Check for two cleavage directions at characteristic amphibole angles
- Note association with blueschist minerals like lawsonite and jadeite
- Consider geological context (subduction-related metamorphism)
- Use laboratory analysis for precise identification when needed
Glaucophane is a key mineral in metamorphic geology, recognized for its blue color and association with high-pressure, low-temperature environments typical of subduction zones.