The key difference between schist and gneiss is that schist is made of mudstone or shale, whereas gneiss is made of micas, chlorite, or other platy minerals.
What is Schist?
Schist is a type of metamorphic rock made of mudstone or shale. It is classified as a medium-grade metamorphic rock. This substance has medium to large grains in shapes such as flat and sheet structures roughly in a parallel orientation. We can define this type of minerals as having more than 50% of its structure platy and as elongated mineral structures, including micas and talc. Often, this rock type is finely interleaved with quartz and feldspar. The lamellar minerals we can observe in these rocks include micas, chlorite, talc, hornblende, graphite, etc.
Usually, schist is garnetiferous, and it forms at a high temperature. Moreover, this mineral contains large grains that are larger than phyllite. Schist rock contains geological foliation or metamorphic arrangement in layers which has medium to large grained flakes in a sheet-like orientation which is known as schistosity.
During the process of metamorphism, rocks such as sedimentary rocks, igneous rocks or metamorphic rocks tend to convert into schists and gneiss rocks. However, we cannot sometimes distinguish a rock from another if the metamorphism has been great and the composition of these rocks are originally similar. But it is possible to distinguish a sedimentary or igneous shist from a sedimentary or igneous gneiss. E.g. if the rock has traces of bedding, clastic structure or unconformability, it signs that the original rock was sedimentary.
What is Gneiss?
Gneiss is a type of metamorphic rock made of micas, chlorite, and other platy minerals. It is common and widely distributed. This rock type forms through high temperature and high-pressure metamorphic processes that act on formations consisting of igneous or sedimentary rock. There is another variation of gneiss known as paragneiss, which derives from sedimentary rocks, e.g. sandstone. Usually, this rock type forms at a high temperature and pressure compared to the formation of schist rock. This rock almost always appears with a banded texture that is characterized by alternating darker and lighter band colours. Moreover, it has not distinct foliation.
Typically, gneiss rock is medium to coarse foliated. The rock largely undergoes recrystallization. However, it does not carry large quantities of mica, chlorite, or other platy minerals. Moreover, gneiss rocks that are made from metamorphism of igneous rocks are termed granite gneisses, diorite gneiss, etc.
What is the Difference Between Schist and Gneiss?
During the process of metamorphism, rocks such as sedimentary rocks, igneous rocks or metamorphic rocks tend to convert into schists and gneiss rocks. The key difference between schist and gneiss is that a schist is made of mudstone or shale, whereas a gneiss is made of micas, chlorite or other platy minerals. Moreover, schists form at comparatively low temperatures or pressure conditions, whereas gneiss form at comparatively very high temperature and pressure conditions.
Below is a summary of the difference between schist and gneiss in tabular form.
Summary – Schist vs Gneiss
Schist and gneiss are types of metamorphic rocks. The key difference between schist and gneiss is that schist is made of mudstone or shale, whereas gneiss is made of micas, chlorite or other platy minerals.
1. “Gneiss.” Geology, Available here.