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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