Which metamorphic rocks are foliated

They still are not visible as individual crystals, but the larger size leads to a satiny sheen on the surface. The cleavage of phyllite is slightly wavy compared to that of slate. Schist Figure Individual crystal faces may flash when the sample is turned in the light, making the rock appear to sparkle.

Other minerals such as garnet might also be visible, but it is not unusual to find that schist consists predominantly of a single mineral. Gneiss Figure Gneiss features minerals that have separated into bands of different colours. The bands of colours are what define foliation within gneiss. Sometimes the bands are very obvious and continuous Figure Dark bands are largely amphibole while the light-coloured bands are feldspar and quartz.

Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. While slate and phyllite typically form only from mudrock protoliths, schist and especially gneiss can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks. Schist and gneiss can be named on the basis of important minerals that are present: a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist.

One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or amphibolite Figure Metamorphic rocks that form under low-pressure conditions or under the effects confining pressure, which is equal in all directions, do not become foliated.

In most cases, this is because they are not buried deeply enough, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. Metamorphism that happens because of proximity to magma is called contact metamorphism.

Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels. Marble Figure When it forms, the calcite crystals recrystallize re-form into larger blocky calcite crystals , and any sedimentary textures and fossils that might have been present are destroyed.

If the original limestone is pure calcite, then the marble will be white. Quartzite Figure It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.

Even if formed under directed pressure, quartzite is generally not foliated because quartz crystals do not normally align with the directional pressure. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.

Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals e. NPS photo by Dan Kish. Introduction Metamorphic rocks form when high temperatures and pressure act on a rock to alter its physical and chemical properties metamorphism means 'to change form'. Parent Rock: Rock from which soil, sediment, or other rock is derived. Foliated metamorphic gneiss.

NPS photo by Tim Connors Foliated Metamorphic Rocks As pressure squeezes on a parent rock during recrystallization it causes the platy or elongated minerals within the rock to become aligned, or foliated. Non-foliated metamorphic soapstone. Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled. Schists are usually named by the main minerals that they are formed from.

Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this. Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. Gneiss is coarser than schist and has distinct banding.

This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz.

Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneouse rock grantite. Gneiss can be used by man as paving and building stone. Non-Foliates are metamorphic rocks that have no cleavage at all. Quartzite and marble are two examples of non-foliates that we are going to study.

Quartzite is composed of sandstone that has been metamorphosed. Quartzite is much harder than the parent rock sandstone. It forms from sandstone that has come into contact with deeply buried magmas. Quartzite looks similar to its parent rock. The best way to tell quartzite from sandstone is to break the rocks. Sandstone will shatter into many individual grains of sand while quartzite will break across the grains. Marble is metamorphosed limestone or dolomite.

Both limestone and dolomite have a large concentration of calcium carbonate CaCO3. Marble has many different sizes of crystals.

Marble has many color variances due to the impurities present at formation. Some of the different colors of marble are white, red, black, mottled and banded, gray, pink, and green. Marble is much harder than its parent rock. This allows it to take a polish which makes it a good material for use as a building material, making sink tops, bathtubs, and a carving stone for artists. Today, headstones are made from marble and granite because both of these rocks weather very slowly and carve well with sharp edges.

It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.

An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock. Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7. In some cases, hornfels has visible crystals of minerals like biotite or andalusite.

If the hornfels formed in a situation without directed pressure, then these minerals would be randomly orientated, not foliated as they would be if formed with directed pressure. Skip to content Chapter 7 Metamorphism and Metamorphic Rocks. Exercise 7. The mica crystals are consistently parallel to one another. A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.

A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them.