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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. |
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Chemical Formula |
Na2Mg3Al2Si8O22(OH)2 |
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Composition |
Basic sodium magnesium aluminum silicate, often with some iron
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| Variable Formula |
Na2(Mg,Fe2+)3Al2Si8O22(OH)2 |
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Color |
Blue, dark blue, and black with blue overtones |
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Streak |
Grayish blue |
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Hardness |
5.5 - 6 |
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Crystal System |
Monoclinic |
Crystal Forms
and Aggregates |
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.
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Transparency |
Translucent to opaque. Rarely transparent. |
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Specific Gravity |
3.0 - 3.1 |
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Luster |
Vitreous, pearly |
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Cleavage |
1,2 |
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Fracture |
Uneven, splintery |
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Tenacity |
Brittle |
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In Group |
Silicates; Inosilicates; Amphibole Group |
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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.
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| Rock Type |
Metamorphic |
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Popularity (1-4)
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3 |
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Prevalence (1-3)
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2 |
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Demand (1-3) |
3 |
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- 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.
- Transparent to translucent variety of blue Glaucophane.
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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.
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Riebeckite - Usually darker in color.
Tourmaline - Harder, lacks good cleavage.
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