GLAUCOPHANE SCHIST
HereÕs a magnificent sample of coarsely-crystalline
glaucophane schist (blueschist) from the island of Siphnos in the Aegean
Sea. ItÕs 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 37¼ 01Õ North, 24¼ 40Õ 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.