Metamorphic rocks are primarily classified based on their texture and composition, broadly divided into foliated and non-foliated types.
Foliated Metamorphic Rocks
Foliated metamorphic rocks exhibit a layered or banded appearance. This foliation is caused by the parallel alignment of platy minerals, such as mica, under directed pressure during metamorphism. The degree of foliation can vary from subtle to very pronounced.
- Slate: Formed from shale or mudstone; fine-grained with excellent parallel cleavage.
- Phyllite: Formed from shale; has a sheen due to the alignment of mica crystals, which are larger than in slate.
- Schist: Medium- to coarse-grained; visible platy minerals (e.g., mica, chlorite) dominate, resulting in a scaly or flaky appearance.
- Gneiss: Coarse-grained; characterized by distinct banding of light and dark minerals (e.g., feldspar and quartz alternating with biotite and amphibole).
The type of foliated rock formed depends on the temperature, pressure, and parent rock composition during metamorphism.
Non-Foliated Metamorphic Rocks
Non-foliated metamorphic rocks do not exhibit a layered or banded texture. This can be due to several factors, including:
- Lack of directed pressure during metamorphism.
- Dominance of minerals that do not have a platy or elongated shape (e.g., quartz, calcite).
- Metamorphism primarily caused by heat (contact metamorphism).
Common examples of non-foliated metamorphic rocks include:
- Marble: Formed from limestone or dolostone; composed primarily of calcite or dolomite crystals. Its texture is typically granular.
- Quartzite: Formed from sandstone; composed primarily of quartz grains that have been fused together. It is very hard and durable.
- Hornfels: Fine-grained, dense rock formed from a variety of parent rocks subjected to high temperatures during contact metamorphism.
Compositional Classification
In addition to texture, metamorphic rocks can also be classified based on their mineral composition. For example, a quartzite is defined by its high quartz content, while a marble is defined by its high calcite or dolomite content. The original composition of the parent rock plays a significant role in determining the final mineral assemblage of the metamorphic rock.
In summary, while the primary classification relies on foliation versus non-foliation, considering the specific minerals present allows for a more complete characterization of a metamorphic rock.
