Schist vs. Gneiss

Main Difference

The main difference between the metamorphic rocks schist and gneiss is that the schist contains flat, sheet-like grains in a pattern and gneiss is in the pattern of layers of sheet-like planar structures.

Difference Between Schist and Gneiss

Comparison Chart

Schist vs. Gneiss

Schist and gneiss are metamorphic rocks. Metamorphic rocks are the rocks made through the process of heat and pressure added onto a parent rock. Both schist and gneiss are made of shale, but they are quite different rocks. Schist is a type of medium-grade metamorphic rock which contains flat, sheet-like grains in a pattern. Gneiss is formed in the pattern of layers of the sheet-like planar structures. These layers alternate in light and dark patterns. The schist is known for its characteristic flaky appearance due to the presence of mineral grains in it. It often looks like flecks embedded in the stone. So the schist is usually preferred in decoration rather than in core construction like other forms of stone. The gneiss is the most common type of rock that is known for its layers. Schist is not commonly used for construction also because it tends to split, from which the stone gets its name. The schist is easier to split in the direction of the grain. The Gneiss is formed through so many processes of heating and cooling. It leads to the formation of layers or sheets. Mostly, these layers alternate in light and dark layers. These alternating dark and light layers are one of the most defining characteristics of the gneiss. According to the grade or level or grade of foliation, the sedimentary protolith transforms in steps. It becomes slate first, then phyllite, schist and finally gneiss. Both schist and gneiss are known to be the foliated metamorphic rocks. They are composed of feldspar and quartz minerals that are inherited from the protolith.

What is Schist?

Schist is a medium grade metamorphic rock. It is formed by the metamorphosis of mudstone, shale, or some types of igneous rock by subjecting to higher temperatures and pressures. The resulting foliation is more distinct and coarser than that of slate due to the higher degree of crystallization of mica minerals ( biotite, chlorite, muscovite). It forms large crystals and is often referred to as schistosity. These large crystals reflect light. That is why schist often has a high luster, and it is shiny. Porphyroblasts are most common in schist. They provide information on the pressure and temperature conditions under which the rock is formed. Due to the extreme formation conditions, schist often shows complex folding patterns. Schist is a stone of lower strength. It is only used as a fill for decorative rock in walls, or non-critical construction uses. Schist is fine-grained. It tends to break into thin slabs in the planar direction. This phenomenon is known as schistosity. The word schist derives from a Greek word that also means ‘to split.’ Schist may appear in any combination of blue, black, brown, grey, green, and silver. Limestone is a better metamorphic host rock for gems as it dissolves more easily for separation of the gem from the rock. The useful properties of schist are resistance to impact, pressure, and water. It is commonly used as a host rock for gemstones, i.e., a matrix within which crystals grow.

Examples

• Kyanite
• Garnet
• Tanzanite
• Emerald
• Andalusite
• Ruby
• Sphene
• Sapphire
• Scapolite
• Iolite
• Chrysoberyl

What is Gneiss?

Gneiss is a metamorphic rock of high grade. It is formed by the metamorphosis of sedimentary rock or granite. The word gneiss comes from a German word meaning ‘sparkling’ or ‘bright.’ Gneiss Displays distinct foliation by representing alternating layers. These layers are composed of different minerals. However, gneiss does not break along the planes of foliation. As less than 50% of the minerals formed in the metamorphism are aligned in thin layers. The layers are often sub-parallel because of the coarseness of the foliation; that is, they do not have a constant discontinuous and thickness. Gneiss is more coarse-grained than schist, with dark and light (‘mafic’ and ‘felsic’) mineral layers. These layers are known as gneissic banding and are much thicker and irregular; thus, a distinct foliation is observed. The dark bands are composed of minerals such as biotite, sillimanite, cordierite, kyanite, andalusite staurolite, and garnet. The light bands are composed of silicate minerals containing lighter elements, silicon, oxygen, aluminum, sodium, and potassium. The colors of gneiss include black, brown, pink, red, and white. Due to its resistance to heat, pressure, wear and scratching, gneiss is industrially used as crushed stone for road construction. Its durability also gives it the ability to be used as a dimension stone. Gneiss is specifically used in building and landscaping developments, as it does not easily split along plane lines. Gneiss is architecturally used in floor tiles, stair treads, countertops, and cemetery monuments. Gneiss is also sometimes named for the minerals they contain, like garnet gneiss and biotite gneiss. Some common types of gneisses include:

• Augen Gneiss
• Henderson gneiss
• Lewisian gneiss
• Archean gneiss
• Proterozoic gneiss

Conclusion

Schist and gneiss are metamorphic rocks. Both are made of shale, but they are quite different rocks.