The Mineral glaucophane

Glaucophane is named from a combination of the Greek words Glaukos, meaning "blue", and Phainelein, meaning "appearance", alluding to its bluish color. Glaucophane forms a series with the less-common Ferro-glaucophane, where Glaucophane is the magnesium-rich end member and Ferro-glaucophane is the iron-rich end member.

Chemical Formula

Na₂Mg₃Al₂Si₈O₂₂(OH)₂

Color

Blue, dark blue, and black with blue overtones

Crystal System

Monoclinic

Properties

Streak Grayish blue

Hardness 5.5 - 6

Transparency Translucent to opaque. Rarely transparent.

Specific Gravity 3.0 - 3.1

Luster Vitreous, pearly

Cleavage 1,2

Fracture Uneven, splintery

Tenacity Brittle

Crystal Habits

Crystals are usually prismatic with a diamond-shaped cross-section. Often in platy groups of small prismatic crystals. Also bladed, columnar, acicular, fibrous, and massive. Crystals are often striated lengthwise.

Additional Information

Composition Basic sodium magnesium aluminum silicate, often with some iron

In Group Silicates; Inosilicates; Amphibole Group

Striking Features Blue color, crystal and cleavage habits

Environment In metamorphic schists and eclogites. Glaucophane is one of the components of blueschist rock, and is responsible for it bluish color.

Rock Type Metamorphic

Varieties

Ferro-glaucophane

 -   Iron-rich variant of Glaucophane, which has iron dominating over the magnesium in a solid solution series. Ferro-Glaucophane is recognized by the IMA as a distinct mineral species with the following chemical formula: Na₂Mg₃Al₂Si₈O₂₂(OH)₂. There can be iron replacing some of the magnesium up to 50 percent.

Gastaldite

 -   Transparent to translucent variety of blue Glaucophane.

Noteworthy Localities

Glaucophane is more common than often perceived, though collector specimens of this mineral are seldom encountered, hence the few localities listed here. Groups of prismatic blue crystals come from Groix Island, Brittany, France; and transparent blue crystals from Rio Oremo, Chiavolino, Biella Province, Italy. In the U.S., Glaucophane is a constituent of the blueschist rocks throughout the Coast Ranges of Northern California, Oregon, and Washington. Dark blue crystals have been found in Ward Creek, Cazadero, Sonoma Co., California.

Common Mineral Associations

Muscovite (Fuschite), Epidote, Actinolite, Jadeite, Almandine, Albite, Lawsonite, Omphacite, Rutile

Distingushing Similar Minerals

Riebeckite - Usually darker in color.
Tourmaline - Harder, lacks good cleavage.

glaucophane Photos

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