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'Glaucophane' is a mineral belonging to the amphibole group, chemical formula Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂. It is named from its typical blue color (in Greek, ''glaucophane'' means "blue appearing"). Glaucophane crystallizes in the monoclinic system.
The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckite are the only common amphibole minerals that are typically blue. Glaucophane forms a solid solution series with ferroglaucophane; Na₂(Fe,Mg)₃Al₂Si₈O₂₂(OH)₂. Glaucophane is the magnesium-rich member and ferroglaucophane is the iron-rich member. Ferroglaucophane is similar to glaucophane but is denser, generally darker in color with a diminished pearly luster. Glaucophane's hardness is 5 - 6, and its specific gravity is approximately 3 - 3.2.
Glaucophane forms in metamorphic rocks that are either particularly rich in sodium or that have experienced low temperature-high pressure metamorphism such as would occur along a subduction zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle. It is glaucophane's color that gives the blueschist facies its name. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.
There is also a rare amphibole called Holmquistite, chemical formula Li₂Mg₃Al₂Si₈O₂₂(OH)₂, which occurs only in lithium-rich continental rocks. For many years, Holmquistite was mistaken for Glaucophane, as the two look identical in thin section.

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★ List of Minerals