13.6: What Is a Foliated Metamorphic Rock?
What Is a Foliated Metamorphic Rock?
Differential pressure typically results in the development of foliation Links to an external site. 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 Links to an external site. (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.
Sometimes a metamorphic rock is seen with mostly amphibole minerals that define the foliation pattern. The resulting rock is known as an amphibolite Links to an external site.. 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 Links to an external site., 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.
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 Links to an external site.hyllite Links to an external site.. 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.
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 Links to an external site.. 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) Links to an external site. 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.
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 |