Difference Between Schist And Gneiss

Metamorphism

Rocks that are buried deep in mountainous (orogenic) zones are subjected to high temperatures and pressures, for example due to continental shift. This causes the mineral composition of the original rock, called the protolith, to recrystallize into new structures over thousands of years. This process is described as regional metamorphism.

Other types include contact (caused by baking), hydrothermal (caused by movement of hot fluids) and cataclastic (caused by faulting) metamorphism.

It is important to note that metamorphism does not alter the chemical composition of rocks; rather, it only changes the mineral structure and thus the physical properties. Thus, while it may be difficult to distinguish between certain types of schists or gneisses, classification by chemical methods are reliable.[i]

Foliation

When sedimentary rock (e.g. shale and mudstone) undergoes regional metamorphosis, the clay minerals form platy minerals, also known as ‘micas’.

The results of micas and elongate minerals recrystallizing into parallel arrangements that are perpendicular to the applied pressure, are known as foliation and lineation respectively. Foliated rocks have a layered appearance due to these parallel streaks of different-coloured minerals.

Non-foliated rocks, on the other hand, are composed of minerals that have recrystallized into solid, interlocking networks.

Origin of schist and slate

Sedimentary protolith transforms in a step-wise manner according to the level or grade of foliation; it first becomes slate, then phyllite, schist and finally gneiss.

Both schist and gneiss are thus known as foliated metamorphic rocks. They are composed of quartz and feldspar minerals inherited from the protolith, other assorted minerals unique to each type, as well as garnet porphyroblasts, large crystals which grow within the finer-grained rock.[ii]

Formation & characteristics of gneiss

Formation of gneiss is often associated with the transformation of igneous rock; these are rocks that have been subjected to extreme heat and slow cooling. They are buried deeply in mountains under which tectonic movement generates extreme pressure, causing high grade metamorphism. [iii]Thus gneiss may be formed from sedimentary rock (paragneiss), or igneous rock (orthogneiss).[iv]

Gneiss tends to be more coarse-grained than schist, with light and dark (‘felsic’ and ‘mafic’) mineral layers known as gneissic banding. These layers are much thicker and more irregular than any found in schists, thus a more distinct foliation is observed.

The darker bands are composed of minerals such as biotite, cordierite, sillimanite, kyanite, staurolite, andalusite and garnet, many of which contain magnesium and iron. ii The lighter bands are composed of silicate minerals containing lighter elements, such silicon, aluminium, oxygen, sodium and potassium.iv

Colours include black, brown, pink, red and white.[v]

Formation & characteristics of schist

Schists are formed by medium grade metamorphism of sedimentary rock. ⁱ The mica grains in shale undergoing metamorphosis grow and align, forming large crystals which give the rock a lustrous appearance. These mineral plates, visible to the naked eye, are comprised mainly of chlorite, muscovite and biotite. ii Certain types of schist may be formed from fine-grained igneous rock, such as basalt and tuff. iv

Compared to gneiss, schist is more fine-grained and has a tendency to break into thin slabs in the planar direction, known as schistocity. ii Schist may appear any combination of black, blue, brown, grey, green and silver. v

Commercial uses of gneiss

Gneiss is used industrially as crushed stone for road construction due to its resistance to pressure, heat, wear and scratching. Its durability also lends it the ability to be used as a dimension stone: blocks and slabs used in paving and other building projects. Gneiss is particularly suited to building and landscaping developments, because it doesn’t easily split along plane lines.

Gneiss may be polished and used architecturally in floor tiles, stair treads, countertops, windowsills and cemetery monuments. They are often labelled as ‘granite’. This is technically an incorrect classification, but reduces confusion in basic material identification for consumer convenience. ii

The architectural use of Gneiss dates back to 683 BC, when it was used to erect the stone Sphinx of Taharqo in the Nile Valley. iv

Commercial uses of schist

Schist, a stone of lower strength, is only used as a fill for non-critical construction uses, or decorative rock in walls. Its useful properties are resistance to impact, pressure and water.

It is more commonly used as a host rock for gemstones; that is, a matrix within which crystals grow. Examples of these are garnet, kyanite, tanzanite, emerald, andalusite, sphene, sapphire, ruby, scapolite, iolite and chrysoberyl. Limestone, however, is a better metamorphic host rock for gems, since it dissolves more easily for separation of the gem from the rock. ii

Types & nomenclature of gneiss

Gneiss comes from a German word meaning ‘bright’ or ‘sparkling’. iv Common types of gneisses include Augen gneiss, Henderson gneiss, Lewisian gneiss, Archean gneiss and Proterozoic gneiss. v

Augen gneiss is coarse-grained and originates from granite. It contains lenticular (elliptical-shaped) feldspar porphyroclasts, which, like porphyroblasts, are also large crystals, but older than the rest of the rock matrix. Henderson gneisses are found near the Brevard Shear Zone in North and South Carolina; one form is associated mainly with the Brevard Fault. Lewisian gneiss forms the bedrock of much of Scotland’s Outer Hebrides, western mainland and the Coll and Tiree islands. Archean and Proterozoic gneisses are found in the Baltic shield, named for the age from which they originate. iv

Gneisses are also sometimes named for the minerals they contain, such as garnet gneiss and biotite gneiss. v

Types & nomenclature of schist

Schist comes from a Greek word meaning ‘to split’; this refers to the ease with which planar layers in schist rock tend to split. iv

Schists are usually named for the dominant mineral found in the rock. Common examples include Calc-Silicate schist, Blueschist, Whiteschist, Hornblende schist, Talc schist, Chlorite schist (“greenstone”), Garnet schist and Glaucophane schist. v

Mica schists are the most commonly found, formed from claystones. These may be broadly categorized into the graphitic or calcareous varieties and are easily recognized by their black and white micas. Graphitic schists are thought to represent sediments formed from plant remains. Contact metamorphism results in the formation of gneissic subgroups such as the andalusite-, staurolite-, kyanite– and sillimanite-schists. Schists rich in quartz originate from sandstones. Hematite-schists are known as schistose ironstones.

Other schists of igneous origin include foliated serpentines (based on masses rich in olivine, a magnesium iron silicate), quartz-porphyries (mainly feldspar) and felsic tuffs (formed from volcanic ash). iv

In names consisting of two or more minerals, the more abundant mineral is named second.

Main mineral content of schist and gneiss compared:

Schist

*Alusite

*Amphibole

*Biotite

*Chlorite

*Epidote

*Feldspar

*Garnet

*Graphite

*Hornblade

*Kyanit

*Micas

*Muscovite

*Porphyroblasts

*Quartz

*Sillimanite­­­

*Staurolite

*Talc

Gneiss

*Biotite

*Chlorite

*Feldspar

*Garnet

*Graphite

*Hornblade

*Micas

*Muscovite

*Quartz

*Quartzite

*Silica

*Zircon

Main compound content of schist and gneiss compared:

Schist

*Calcium Oxide

*Carbon Dioxide

*Magnesium Oxide

Gneiss

*Aluminium Oxide

*Sodium Chloride

*Calcium Oxide

*Iron (III) Oxide

*Iron Oxide

*Potassium Oxide

*Magnesium Carbonate

*Magnesium Oxide

*Manganese Oxide

*Phosphorus Pentoxide

*Silicon Dioxide

*Titanium Dioxide

Locations of deposits of schist and gneiss compared

Summary

Schist

• Formed mainly from sedimentary rock e.g. slate
• Formed by medium-grade metamorphosis
• Finer-grained
• Black, blue, brown, grey, green and silver
• Schistocity due to large plate-shaped crystals
• Not as hard or strong
• Resistant to impact, pressure and water
• Used as decorative stone, construction fill and host rock for gems
• Types are named after main mineral content, such as the common Mica schist
• Main mineral composition includes chlorite, muscovite and biotite
• Found in a wider selection of Asian countries, as well as Africa and Australia

Gneiss

• Formed from sedimentary rock or igneous rock e.g. granite
• Formed by high-grade metamorphosis
• Coarser-grained
• Black, brown, pink, red and white
• Gneissic banding due to distinct foliation
• Harder, stronger
• Resistant to pressure, heat, wear and scratching
• Used in construction, as dimension stone and in architectural projects
• Broadly categorized as Augen gneiss, Henderson gneiss, Lewisian gneiss, Archean gneiss and Proterozoic gneiss
• Variety of minerals, some of which are specific to igneous rocks

Found in a wider selection of African countries, as well as Asia and Australia.

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