Glaucophane
A blue mineral that defines the blueschist metamorphic facies.
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Glaucophane is both a specific mineral and a mineral group within the sodic amphibole supergroup, a type of double-chain silicate. Its chemical composition is Na2(Mg3Al2)Si8O22(OH)2, and it forms crystals in the monoclinic system.
Characteristics
Glaucophane’s distinctive blue color sets it apart. It belongs to a small group of amphiboles—including riebeckite and the intermediate crossite—that are typically blue. This mineral forms a solid solution series with ferroglaucophane: glaucophane is the magnesium-rich end, while ferroglaucophane is the iron-rich end. Ferroglaucophane is slightly denser, giving it a higher specific gravity, but the two look identical in hand samples and both show strong pleochroism.
Occurrence
The blueschist facies is named for its blue minerals, including glaucophane. This amphibole typically appears in sodium-rich, low-temperature, high-pressure metamorphic rocks like those formed in subduction zones, often from basalt or gabbro. It can also occur in eclogites altered by retrograde metamorphism.
Did You Know?
- For many years, holmquistite was mistaken for glaucophane, as the two look identical in thin section.
- Glaucophane is named for its typical blue color; in Greek, glaucophane means 'blue appearing'.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Glaucophane (CC BY-SA 4.0).
- Word definitions: the , each quoted from its Wikipedia article.
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