Metamorphic Rocks: Classification, Texture, and Formation
Metamorphic rocks are the result of the transformation of a pre-existing rock (protolith) due to changes in environmental conditions. This transformation occurs primarily in the solid state, involving recrystallization, mineral reactions, and structural changes, often facilitated by intergranular fluids. The main factors driving metamorphism are temperature, pressure, and chemically active fluids.
The classification of metamorphic rocks is based on their mineralogical composition, texture (grain size and foliation), and the original rock type.
Metamorphic Rock Texture
Crystallinity Degree (Grain Size)
The average crystal size indicates the metamorphic grade. Higher crystallinity signifies a higher grade:
- Low: Crystals are not visible to the naked eye (e.g., some shales).
- Medium: Crystals are visible with the naked eye or a magnifying glass (e.g., slate).
- High: Crystals are readily visible and large.
Crystal Shape and Distribution
The following textures are typically identified with a microscope:
- Granoblastic: Similar-sized crystals form a mosaic, characteristic of monomineralic rocks (e.g., quartzite, marble) formed under static conditions.
- Lepidoblastic: Parallel stacks of planar minerals (micas), common in mica-rich rocks (e.g., phyllite, mica schist, some gneisses).
- Nematoblastic: Similar to lepidoblastic, but with acicular (needle-like) minerals, often amphiboles.
- Porphyroblastic: Large crystals (porphyroblasts) embedded in a finer-grained matrix. Porphyroblasts are often index minerals that indicate metamorphic conditions.
Microstructures
Under directed pressure, planar or prismatic minerals grow perpendicular to the maximum pressure, creating foliation. Different foliation types exist:
- Slaty Cleavage: Oriented crystallization of microscopic planar minerals, typical of low-grade metamorphism.
- Schistosity: Visible planar minerals due to increased metamorphism. May have a scaly appearance from mica plates.
- Gneissic Banding: Segregation of minerals into light and dark bands during high-grade metamorphism.
- Crenulation Cleavage: Two intersecting foliations from separate deformation events, often with one set folded.
Foliation type correlates with grain size and metamorphic grade: slate (fine-grained), schist (medium-grained), gneiss (coarse-grained).
Mineralogical Composition and Chemical Composition
Metamorphism doesn’t significantly alter the protolith’s chemical composition, resulting in diverse metamorphic rock compositions. Four main series are recognized based on the protolith’s geochemical characteristics.
Classification of Metamorphic Rocks
Metamorphic rocks are broadly classified as foliated or non-foliated:
Foliated Rocks
- Slate: Fine-grained, abundant planar minerals, low-grade metamorphism (protolith: fine-grained clastic rocks).
- Schist: Medium-grained, >20% planar minerals, intermediate metamorphism (protolith: various clastic and volcanic rocks). Subtypes based on index minerals: chlorite schist, staurolite schist, garnet schist, etc.
- Gneiss: Coarse-grained, light and dark bands of granular and elongated minerals, high-grade metamorphism (protolith: igneous rocks [orthogneiss], sedimentary rocks [paragneiss]).
Non-Foliated Rocks
Typically monomineralic with equidimensional crystals:
- Marble: Calcite grains, from metamorphosed limestone or dolostone.
- Quartzite: Quartz grains, from metamorphosed quartz-rich sandstone.
- Hornfels: Contact metamorphosed rocks without planar fabric, may contain index minerals.
