Here’s a magnificent sample of coarsely-crystalline glaucophane schist (blueschist) from the island of Siphnos in the Aegean Sea. It’s got large, very dark blue bladed crystals of glaucophane (Na2(Mg3Al2)Si8O22(OH)2 - sodium magnesium hydroxy-aluminosilicate), a common mineral found in blueschist facies metamorphic rocks. This rock has been significantly altered from its precursor (protolith). Published research on Siphnos metamorphics has indicated that the protolith of this rock was likely basaltic and andesitic lavas contaminated by marine shales.
Many of the islands in the Aegean Sea between Greece and Turkey have high-grade metamorphic rocks, the result of a tectonic collision between the Apulia Microplate with the Eurasian Plate in the late Mesozoic to early Cenozoic.
Locality: northwestern end of Siphnos Island, western Cycladic Islands, southwestern Aegean Sea (~vicinity of 37º 01’ North, 24º 40’ East).
Geologic unit & metamorphic age: lower unit of the Attic-Cycladic Crystalline Complex; high-pressure metamorphism in the Late Cretaceous to Middle Eocene (78 to 42 m.y.).
Glaucophane schist (cut surface, 4.9 cm across) from Siphnos Island in the Aegean Sea. Very dark blue blades = glaucophane; pale yellowish-green blades = kyanite; pale greenish matrix = ? (epidote or amphibole or pyroxene or lawsonite).
Glaucophane schist (rough surface, 3.7 cm across) from Siphnos Island, Aegean Sea. Same mineral suite as above.
Glaucophane schist (rough surface, 4.4 cm across at its widest) from Siphnos Island, Aegean Sea. Same mineral suite as above.
Glaucophane schist (rough surface, 6.8 cm across at its widest) from Siphnos Island, Aegean Sea. Same mineral suite as above, plus dark red pyrope garnets.
Info. synthesized from Tony Peterson (pers. comm.) and various Siphnos geology publications.