Foliated rocks include slate, phyllite, schist, gneiss, and migmatite. Each type varies in grain size, mineral composition, and foliation intensity, reflecting different metamorphic conditions.
What are Foliated Rocks?
Foliated rock forms when pressure squeezes into the flat or elongated minerals within a rock so they become aligned. These rocks develop a platy, sheet, banded, or attended-like structure that reflects the direction of minerals (Parallel arrangement).
Types of Foliated Rocks:
Some foliated rocks are,
Slate:
This fine-grained, perfectly foliated metamorphic rock exhibits slaty cleavage, which may form at an angle to the original bedding. Cleavage planes may reveal relict bedding. It forms from shale, but during metamorphism, the process alters clay minerals into small, aligned mica minerals. It is often dark gray in color. The metamorphism occurs at a temperature of about 200°C and a burial depth of approximately 10 km.
Phyllite:
The rock has a grained texture and displays a lustrous, silky sheen as light reflects off the chlorite and muscovite micas that compose it. Quartz and albite often accompany these minerals. Its grain size exceeds that of slates but remains finer than that of schists. Phyllites typically appear greenish or red and may begin to show the segregation of some mineral constituents into layers.
Schist:
Schist is a foliated rock with medium to coarse crystalline textures. One can easily identify most of its mineral constituents without using a microscope. The parallel or nearly parallel alignment of micaceous minerals causes foliation. Quartz, feldspar, and micas are the most common minerals in schist. Several types of schist may be recognized based on minerals that make up 30 percent or more of the rocks:
- Mica schist
- Garnet schist
- Chlorite schist
- Kyanite schist
- Talc schist
If no constituent comprises 50 percent, the names of the two most abundant constituents are used, for example, garnetiferous-mica schist.
Gneiss:
It is a medium to coarse-grained banded metamorphic rock with alternating layers of dark and light minerals. The dark layers commonly contain biotite and hornblende, and the light layers commonly contain
quartz and feldspar. The layering is thought to be developed as a result of the segregation of mineral
constituents during metamorphism. In some cases, it corresponds to the original bedding of sedimentary rocks.
Migmatite:
A composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.
