The BlueSchists of the island of Groix

Under BlueSchist facies conditions, the blue glaucophane-rich metabasites (metabasalts or metagabbros) are called BlueSchists or, where glaucophane is particularly abundant, glaucophanites. On the island of Groix, these rocks form boudins of variable size in the micaschistes :

The rocks of the "Lavoir" site on the island of Groix show a paragenesis typical of these glaucophanites. The dark blue millimetre levels are rich in Glaucophane (blue amphibole) and the yellow levels are rich in epidote.

The colour contrast is sharp in sunlight. Notice in the middle of the picture, the small fuchsite crystal, a green chromium mica. At the top of the photo, an orange quartz level is also visible.

Blue amphibole forms centimetric thick rods

The rock may also have contained large centimetric prisms of lawsonite ...

Microscopic observation shows a rhombus-shaped section of lawsonite now completely pseudomorphosed, (i.e. transformed into other minerals while retaining its original form) into paragonite (white sodic mica), epidote +/- chlorite.

The photo in Plane Polarized Light is around 1 cm wide. Around the relict crystal of lawsonite, glaucophane, epidote and garnet constitute the essential minerals of this glaucophanite.

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