Competitive Exams: Identification of Metamorphic Rocks

Foliation

Grain Size

Hardness

Usual Color

Other

Rock Type

foliated

fine

soft

dark

“tink” when struck

Slate

foliated

fine

soft

dark

shiny; crinkly foliation

Phyllite

foliated

coarse

hard

mixed dark and light

wrinkled foliation; often has large crystals

Schist

foliated

coarse

hard

mixed

banded

Gneiss

foliated

coarse

hard

mixed

distorted “melted” layers

Migmatite

foliated

coarse

hard

dark

mostly hornblende

Amphibolite

nonfoliated

fine

soft

greenish

shiny, mottled surface

Serpentinite

nonfoliated

fine or coarse

hard

dark

dull and opaque colors, found near intrusions

Hornfels

nonfoliated

coarse

hard

red and green

dense; garnet and pyroxene

Eclogite

nonfoliated

coarse

soft

light

calcite or dolomite by the test

Marble

nonfoliated

coarse

hard

light

quartz (no fizzing with acid)

Quartzite

Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ( “shape-changed” ) rocks. Metamorphism acts at two scales, the regional scale and the local scale.