7.3 Classification of metamorphic rocks

Metamorphic Rock Classification

Metamorphic rocks form when existing rocks are transformed by heat, pressure, or both, changing their mineral makeup and texture. Classifying these rocks helps geologists reconstruct the temperature and pressure conditions deep within the Earth, and trace what the original rock was before metamorphism occurred.

The two main criteria for classification are texture (how the minerals are arranged) and mineral composition (which minerals are present).

Classification Criteria

Texture describes the physical appearance and arrangement of minerals in the rock.

• Foliated rocks have a layered or banded appearance because platy minerals (like micas and chlorite) have been forced into parallel alignment by directed pressure.
• Non-foliated rocks have a uniform or granular texture with no preferred mineral orientation. This typically happens when the parent rock lacks platy minerals or when pressure is applied equally in all directions.

Mineral composition depends on two things: what the parent rock was made of, and the metamorphic conditions (temperature and pressure) it experienced. Certain mineral combinations, called mineral assemblages, correspond to specific metamorphic environments. Geologists group these into categories called facies (such as greenschist facies for lower-grade conditions and amphibolite facies for higher-grade conditions).

Foliated Metamorphic Rock Types

Foliated rocks form a progression from low-grade to high-grade metamorphism. As temperature and pressure increase, grain size gets larger and the minerals change.

Slate

• Fine-grained, low-grade rock formed from shale or mudstone
• Develops slaty cleavage, meaning it splits cleanly into thin, smooth sheets
• The individual mineral grains are too small to see without a microscope
• Commonly used as roofing material because of its ability to split into flat sheets

Phyllite

• Fine-grained, low-to-medium-grade rock that forms when slate is subjected to more heat and pressure
• Has a distinctive satiny or silky sheen on its foliation surfaces, caused by tiny mica crystals that have grown just large enough to reflect light
• Grain size is still too fine to identify individual minerals with the naked eye, but the sheen distinguishes it from slate

Schist

• Medium-to-coarse-grained, medium-to-high-grade rock formed from phyllite (or other rocks) under still greater temperature and pressure
• Contains visible mica flakes (muscovite, biotite) arranged in a preferred orientation, giving the rock a sparkly, flaky appearance
• Often named by its most prominent mineral: mica schist, garnet schist, chlorite schist

Gneiss

• Coarse-grained, high-grade rock formed under intense temperature and pressure
• Characterized by distinct banding: alternating layers of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, hornblende)
• Unlike schist, gneiss tends to have a more segregated, striped look rather than a uniformly flaky texture
• Common varieties include biotite gneiss, hornblende gneiss, and granite gneiss

Non-Foliated Metamorphic Rock Types

Non-foliated rocks lack the layered appearance of foliated rocks. They typically form from parent rocks that don't contain many platy minerals, so there's nothing to align into sheets.

Marble

• Forms from limestone or dolostone through recrystallization of calcite or dolomite
• Has a sugary or granular texture made of interlocking carbonate crystals
• Relatively soft compared to other metamorphic rocks, which makes it easy to carve and polish
• Widely used as decorative building stone (Carrara marble from Italy, Yule marble from Colorado)

Quartzite

• Forms from quartz-rich sandstone as quartz grains recrystallize and fuse together
• Extremely hard and resistant to weathering because of the strong bonds between interlocking quartz grains
• You can tell quartzite from sandstone by how it breaks: quartzite fractures through the grains, while sandstone breaks around them
• Used as a source of high-purity silica for glassmaking and abrasives

Hornfels

• Forms from various parent rocks through contact metamorphism, which occurs when rock is baked by heat from a nearby igneous intrusion
• Fine-grained, dense, and massive with a splintery or conchoidal (shell-shaped) fracture
• Mineral composition varies widely depending on the parent rock and the heat source, leading to varieties like spotted hornfels and pyrite hornfels

Parent Rock and Metamorphic Grade

The parent rock (also called the protolith) largely controls which minerals end up in the metamorphic rock:

• Clay-rich rocks (shale, mudstone) → slate → phyllite → schist
• Quartz-rich rocks (sandstone) → quartzite
• Carbonate rocks (limestone, dolostone) → marble
• Felsic igneous rocks (granite) → gneiss

Metamorphic grade refers to the intensity of temperature and pressure conditions. It also shapes the final product:

1. Low-grade metamorphism produces fine-grained minerals and well-developed foliation (e.g., slate)
2. Medium-grade metamorphism results in larger, visible crystals and more diverse mineral assemblages (e.g., schist)
3. High-grade metamorphism produces coarse-grained minerals; foliation may become less pronounced and shift toward banding rather than flaky sheets (e.g., gneiss)