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 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". Mineral Changes