METAMORPHIC ROCKS
Formed in the solid state in response to changes in temperature, pressure and chemical conditions imposed at depth below the level of weathering and lithification
Part I: Foliated Metamorphic Rocks
Classification of Foliated Metamorphic Rocks
| Foliation |
|---|
| Slaty Cleavage |
Intermediate* |
Schistosity |
Gneissic Layering |
|---|
| Rock Name | Slate | Phyllite | Schist | Gneiss |
|---|
| Parent Rock | Mudstone/Shale | Mudstone/Shale | Clay-rich Rock | Variable |
|---|
*Note: Phyllite has a texture that is intermediate between slate and schist. This results in slaty cleavage and shiny cleavage surfaces but grains that are still microscopic.
SLATE
Slate is a low-grade foliated metamorphic rock formed by regional metamorphism. It is similar in appearance to mudstone and shale due to the low grade of metamorphism but can be distinguished by its slaty cleavage and more dense, compact nature.
Grade: low
Parent Rock: clay-rich mudstone or shale
Nature of Metamorphism: regional with directed stress
Impt. Characteristics: planar alignment of microscopic clay grains cause the rock to break in parallel layers (slaty cleavage).
PHYLLITE
Phyllite is a low-grade foliated metamorphic rock that is intermediate in grade between slate and schist. Aligned near-microscopic crystals of fine-grained muscovite and chlorite give the rock a shiny surface or sheen along cleavage surfaces.
Grade: low
Parent Rock: clay-rich mudstone or shale
Nature of Metamorphism: regional with directed stress
Impt. Characteristics: intermediate in texture between slate and schist with planar alignment of near-microscopic platy minerals which give the rock a shiny reflective appearance.
SCHIST
Schist is a medium-grade foliated metamorphic rock. Regional metamorphism of clay-rich parent rock results in an increase in grain size and preferred planar alignment of large visible platy grains.
Grade: medium
Parent Rock: clay-rich mudstone or shale
Nature of Metamorphism: regional with directed stress
Impt. Characteristics: planar alignment of visible platy minerals (schistosity) such as micas, talc and kyanite.
GNEISS
Gneiss is a high-grade foliated metamorphic rock which forms under extreme temperatures and directed pressure. Gneiss often grades into migmatite (intermixed igneous and metamorphic rock) reflecting the high temperatures of formation of both rock types.
Grade: high
Parent Rock: variable
Nature of Metamorphism: regional with directed stress
Impt. Characteristics: distinct layers of dark- and light-colored minerals (gneissic layering)
Click here to continue to Part II - Nonfoliated Metamorphic Rocks and Part III - Common Metamorphic Minerals
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