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igneous protolith

igneous protolith

SRI Petrography ID: 16

gneiss
Stainz, Styria

Names

[de] Magmatisches Ausgangsgestein
[en] igneous protolith

Description

These metamorphic rocks are rocks formed by the alteration of preexisting igneous rocks (igneous protolith) due to changing geological conditions: variations in temperature, pressure, mechanical stress.
The following rock types belong to this group:
amphibolite
gneiss
hornfels
mica schist
serpentinite
soapstone (talc-rock)

Color

variable

Grain Size

variable

Texture

variable, often grain orientation, banding, foliation, layering

Minerals

variable, most common:
index minerals for metamorphic facies: sillimanite, kyanite, staurolite, andalusite, garnet.
Other typical minerals in metamorphites (but also in magmatites and partly in sedimentites): feldspar, quartz, mica, olivine, pyroxene, amphibole

Occurences

all over the world

Classification

Class

METAMORPHIC ROCKS

Narrower Terms

amphibolite

eclogite

gneiss

hornfels

mica schist

serpentinite

soapstone (talc-rock)