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Metamorphic Rocks
How do rocks change?
Metamorphism causes changes in:
Texture
Mineralogy
• Texture
• The processes of compaction and recrystallization change the texture of
rocks during metamorphism.
• Compaction
– The grains move closer together.
– The rock becomes more dense.
– Porosity is reduced.
– Example: clay to shale to slate
• Recrystallization
Growth of new crystals. No changes in overall chemistry. New crystals grow
from the minerals already present. A preferred orientation of minerals
commonly develops under applied pressure. Platy or sheet-like minerals
such as muscovite and biotite become oriented perpendicular to the
direction of force. This preferred orientation is called foliation.
Metamorphic Textures
Foliation is a broad term referring to the alignment of sheet-like minerals.
Types of foliation:
Schistosity - alignment of large mica flakes, as in a mica schist derived
from the metamorphism of shale.
Slaty cleavage - alignment of very fine-grained micas, as in a slate
derived from the metamorphism of shale.
Phyllitic structure - alignment of fine-grained micas, as in a phyllite.
Gneissic banding - segregation of light and dark minerals into distinct
layers in the rock, as in a gneiss.
As shale is subjected to increasing grade of metamorphism (increasing
temperatures and pressures), it undergoes successive changes in texture
associated with an increase in the size of the mica grains.
Slate - very fine grained rock. Resembles shale. Has slaty cleavage
which may be at an angle to the original bedding. Relict bedding may be
seen on cleavage planes. Often dark gray in color. "Rings" when you
strike it. (Unlike shale, which makes a dull sound. Temperature about 200
degrees C; Depth of burial about 10 km.
Phyllite - fine-grained metamorphic rock. Has a
frosted sheen, resembling frosted eye shadow.
This is no coincidence. Cosmetics commonly
contain ground up muscovite (ground to a size
similar to that occurring naturally in phyllite.)
Schist - metamorphic rock containing abundant obvious
micas, several millimeters across. Several types of schist
may be recognized, based on minerals which may be
present:
mica schist
garnet schist
chlorite schist
kyanite schist
talc schist
Schist
Gneiss - (pronounced "nice") - a banded or striped rock with
alternating layers of dark and light minerals. The dark layers
commonly contain biotite, and the light layers commonly contain
quartz and feldspar.
Migmatite - a very high grade metamorphic rock that has
been subjected to such high temperatures that it has partially
melted. It is intermediate between the metamorphic and the
igneous rocks. Look for swirled banding. The light colored
minerals have undergone melting and flow. The dark colored
minerals have been contorted by flow. Example - the
Lithonia Gneiss in the area southeast of Atlanta.
Lineation refers to the alignment of elongated, rod-like minerals
such as amphibole, pyroxene, tourmaline, kyanite, etc. Lineation
is a texture commonly seen in the metamorphic rock amphibolite
derived from the metamorphism of basalt.
Amphibolite - Abundant amphibole is present; may be
lineated. Usually black. The parent rock is basalt. The grade of
metamorphism is HIGH. Has been subjected to higher
temperatures and pressures than metabasalt, greenstone, or
greenschist.
Non-foliated or granular metamorphic rocks are those which are
composed of equidimensional grains such as quartz or calcite. There
is no preferred orientation. The grains form a mosaic.
Examples: quartzite derived from the metamorphism of quartz
sandstone, and marble derived from the metamorphism of limestone
or dolostone.
Note: Not all quartzites and marbles are pure. Some contain impurities
that were originally mud interlayered with or mixed with the original
quartz sand or lime mud. These clay impurities metamorphose to
layers of micas or other minerals, which may give marble (in
particular) a banded, gneissic appearance, or which may give a slight
foliation to some quartzites.
Quarzite
Marble - fizzes in acid because its dominant minerals is
calcite (or dolomite). The parent rock is limestone (or
dolostone).
Quartzite - interlocking grains of quartz. Scratches glass. The rock fractures
through the grains (rather than between the grains as it does in sandstone).
The parent rock is quartz sandstone.
Serpentinite - A dark green, dense, tough, massive, hard rack. May contain
veins of asbestos. The parent rock is peridotite, an ultramafic rock.
Hornfels - A fine-grained, tough, dense, hard, massive rock. Usually
(but not always) dark in color. Finer grained than basalt, which it may
superficially resemble. This rock forms through contact
metamorphism. The parent rock is commonly siltstone or basalt,
but may be other types of rock.
Metaconglomerate and stretched pebble
metaconglomerate - the parent rock is conglomerate. The
clasts are fairly easily recognized. May be more difficult to
recognize if the clasts have been stretched.
Metabreccia
Mineral Changes
Mineral changes in metamorphic rocks
Recrystallization - rearrangement of crystal structure of existing minerals.
Commonly many small crystals merge to form larger crystals, such as the clay in
shale becoming micas in slate, phyllite, and schist.
Formation of new minerals - there are a number of
metamorphic minerals which form during metamorphism and are
found exclusively (or almost exclusively) in metamorphic rocks:
Garnet - dark red dodecahedrons (12 sides)
Staurolite - brown lozenge-shaped minerals, commonly twinned to
form "fairy crosses".