mysqlClass Error 9 in file extras/autoload.inc.php, line 72:
Error #8192: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead
gneiss

gneiss

SRI Petrography ID: 38

Stainz
Steiermark

Names

[de] Gneis
[en] gneiss

Description

Gneiss is a medium- to coarse-grained, semischistose metamorphic rock. It is characterized by alternating light and dark bands differing in mineral composition (coarser grained than schist). The lighter bands contain mostly quartz and feldspar, the darker often contain biotite, hornblende, garnet or graphite. The minerals are orientated due to schistosity.

In behalf of the origin of the source rock of gneiss has to be devided between:
orthogneiss is formed by the metamorphism of igneous rocks (igneous protolith), paragneiss results from the metamorphism of sedimentary rocks (sedimentary protolith).
Gneiss usually has little tendency to split along its planes, some para- or orthogneisses split easily to slabs (“Plattengneis”)

Color

variable in colour
usually alternating light and dark bands (“banded gneiss”)

Grain Size

(medium to) coarse grained

Texture

banded, foliated, layered, aggregates of platy or linear grains forming lenses or blades

Minerals

light bands: quartz, feldspar, muscovite, dark bands: hornblende, biotite, graphite, garnet

Occurences

Gneiss is the main rock in Precambrian regions but occur world-wide in a lot of different metamorphic geological units.

Comments

In building stone industry mostly described as “granite”
Pencil gneiss: contains rod-shaped individual minerals or segregations of minerals
Augengneiss: contains large, lenticular mineral grains or aggregates with the appearance of eyes scattered through the rock.
Schist: similar to gneiss in mineralogical composition but finer grained.

Classification

Class

METAMORPHIC ROCKS

Broader Terms

igneous protolith

Narrower Terms

Austria-gneiss

Brazil-gneiss

Egypt-gneiss

Germany-gneiss

India-gneiss

Sweden-gneiss

Homonyms

gneiss (sedimentary protolith)