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Glaucophane

Picture Gallery of Amphibole Minerals

Glaucophane is a blue amphibole mineral found exclusively in metamorphic rocks of the blueschist or higher facies. Here it is in eclogite. (more below)
The ultimate blueschist mineral
Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)
Glaucophane is the sodium-rich blue mineral making up most of this schist. The name glaucophane means "showing blue," and viewed through the magnifier in bright light it is blue indeed. Glaucophane is a defining mineral of blueschist.

Glaucophane is usually found in basalts that have undergone the high-pressure, low-temperature metamorphism typical of subduction zones. Exactly what degree of temperature and pressure must be determined from the other minerals that accompany it, as it is stable over a range of conditions. Its general composition is Na2(Mg,Fe3+)3Al2(OH)2[Si8O22]. The Si8O22 group in the formula marks it, like other amphiboles, as a double-chain silicate.

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