Gneiss
The gneiss is a metamorphic Roche containing quartz, Mica, Plagioclase S and sometimes of the Feldspath, all sufficiently large to be identified with the naked eye. The Foliation, always present, is sometimes marked by the alternation of small clear beds and fine darker levels (one speaks then about metamorphic litage).
The clear beds are made up mainly of quartz, Plagioclase S and Feldspath. They have a granoblastic structure. The dark beds are micaceous S with the possible presence of Amphibole S. They have a lepidoblastic structure.
This structure in layers or beds is generally due to a ductile Déformation which occurred at the same time as the Métamorphisme. The plans thus defined are called Foliation, and correspond to the plan of flatness of the rock. This one was often stretched at the same time as flattened (a little like a metal in a Laminoir). One then observes a Linéation of stretching, marked by the lengthening of minerals, within foliation.
A gneiss coming from the Métamorphisme of the Granite is a orthogneiss . If it is resulting from the metamorphism of a sediment Argile ux it is a paragneiss .
A gneiss œillé is a gneiss in which phenocrysts of feldspar are preserved or recristallized. It is often the sign which it is about an orthogneiss.
One generally finds gneisses in the major parts, eroded, of the Assembly lines.
See
- Leptinite
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