What Is a Foliated Metamorphic Rock?

Differential pressure typically results in the development of foliation in metamorphosed rocks. Metamorphic rocks have a variety of foliation examples; however, each is dependent on the minerals that define the foliation.

The most obvious is gneissic banding, which can be identified by the alternating dark and light mineral bands throughout the rock (Figure 13.9). The resulting metamorphic rock is called gneiss (pronounced “nice”, with a silent g). These bands are not always flat but may be bent or folded and are still considered gneissic banding even though the bands are not horizontal. The typical minerals seen in the dark colored bands are biotite micas and amphiboles, whereas the light-colored bands are typically quartz or light-colored feldspars. The protolith for gneiss can be any rock that contains more than one mineral, such as shale with its clay minerals, or an igneous rock with both dark-colored and light-colored silicate minerals, like granite. For gneissic foliation to develop, temperatures and pressures need to be quite high; for this reason, gneiss represents a high-grade of metamorphism.

Left, close-up of a black and white rock with thick stripes. Right, close-up of a black and white rock with thin stripes. — **Figure 13.9:** Gneiss with gneissic banding. (CC-BY 2.0; James St. John via Flickr; CC-BY 2.0; James St. John via Flickr)

Sometimes a metamorphic rock is seen with mostly amphibole minerals that define the foliation pattern. The resulting rock is known as an amphibolite. Occasionally, amphibolites also have layers of light-colored plagioclase minerals present; in this case, the rock is known as an amphibole gneiss. Amphibolites may not always have a foliation. The protolith for an amphibolite is typically a rock with a large amount of dark silicate minerals, such as a mafic or ultramafic igneous rock. Amphibolites require higher temperatures and pressures to form and are also high-grade metamorphic rocks.

Metamorphic rocks with schistose foliation exhibit layering of platy minerals, such as muscovite or biotite micas. Differential stress forces the realignment of the micas, such that their cleavage faces are oriented in the same direction; this results in a rock that sparkles as light is reflected off its minerals’ cleavage faces. The sparkly metamorphic rock is called schist, and typically the name will reflect which mica is present (Figure 13.10). By convention, when naming a metamorphic rock, the mineral in the lowest quantity is mentioned first: for example, garnet muscovite schist. The sedimentary rock shale is usually the protolith for schist; during metamorphism, the very tiny clay minerals in shale recrystallize into micas that are large enough to see unaided. Temperatures and pressures necessary for schistose foliation are not as high as those for gneiss and amphibolite; therefore, schists represent an intermediate-grade of metamorphism.

Left top, close-up of a rock with flaky, muscovite mica crystals. Left bottom, side-view of the rock, with clear layering. Right, close-up of a rock with large garnet crystals. — **Figure 13.10:** *Left*, mica schist. *Right*, garnet-mica schist. Notice the schistose foliation. (*Left:* CC-BY 4.0; Chloe Branciforte, own work; *Right:* CC-BY 2.0; James St. John via Flickr)

Metamorphic rocks with phyllitic foliation exhibit a layering of wavy platy minerals, such as muscovite or biotite micas; however, unlike schist, individual crystals are too small to be seen without magnification (Figure 13.11). The resulting metamorphic rock is called p hyllite. Phyllite is a low-medium grade regional metamorphic rock in which the clay minerals and chlorite have been at least partly replaced by muscovite and biotite. This gives the surfaces of phyllite a satiny or phyllitic luster, brighter than the surface of slate (Figure 13.12). It is also common for the differential stresses under which phyllite forms to have produced a set of folds in the rock, making the foliation surfaces wavy or irregular, in contrast to the often perfectly flat surfaces of slaty cleavage.

Top, close-up of a rock with distinct pearly luster. Bottom, side-view of the rock, with clear layering. — **Figure 13.11:** The metamorphic rock phyllite. Note the phyllitic luster and foliation planes. (CC-BY 4.0; Chloe Branciforte, own work)

The last example of foliation, slaty foliation (cleavage), is defined by the alignment of minerals too small to see, though foliation planes are still visible (Figure 13.12). The resulting metamorphic rock is called slate. Slate forms through low-grade metamorphism of a shale protolith. The clay sized minerals in the shale recrystallize into very tiny micas which are larger than the clay minerals yet still too small to be visible. Because these tiny micas are aligned, however, they control how the slate breaks, and the rock tends to break parallel to the mica alignment. In certain rare instances, some fossils from the original shale may be preserved and visible in slate. Slate has great economic value in the construction industry; due to its ability to break into thin layers and impermeability to water, slate is used as roofing tiles and flooring. In addition to the construction industry, slate helped transform classrooms when the slate blackboard (or chalkboard) was introduced. Has anyone ever moved a pool table? They’re so heavy because they are full of rock! Many have slate decks underneath the felt covering.

Top left, close-up of a rock with distinct sheen. Bottom, left side-view of the rock, with clear layering. Right, close-up of a red rock surface. — **Figure 13.12:** The metamorphic rock slate. Notice the slaty cleavage (bottom left). (*Left:* CC-BY 4.0; Chloe Branciforte; CC-BY 2.0; *Right:* James St. John via Flickr)

**Table 13.1:** Foliated metamorphic rock chart.
Metamorphic Rock	Grain Size	Metamorphic Environment & Grade	Protolith
Slate	Very fine	Regional Very low, 150°C-300°C (300-570°F)	Mudrock, shale
Phyllite	Fine, with wavy layer and phyllitic luster	Regional Low, 300°C-450°C (570-840°F)	Mudrock, shale
Schist	Medium to coarse	Regional Intermediate, 450°C-550°C (840-1020°F)	Mudrock, shale
Amphibolite Gneiss*	Coarse with light & dark bands	Regional High, >550°C (>1020°F)	Mudrock, granite, diorite, *basalt