How does garnet form in schist

The major minerals in a schist are quartz and mica. During the compression of the rock metamorphism the mica grains, which are thin and flat, are squeezed into sheet-like layers and give a silky, shiny appearance to the rock. If schist rocks are broken, they will bread into wavy surfaces. Get your sample of Garnet Schist today! It is also hard to understand how garnet can grow into nice euhedral crystals under these conditions.

Photo by Jackdann88, used here under a Creative Commons license. Schist is not a rock with numerous industrial uses. Its abundant mica grains and its schistosity make it a rock of low physical strength, usually unsuitable for use as a construction aggregate , building stone, or decorative stone. The only exception is for its use as a fill when the physical properties of the material are not critical. Schist is often the host rock for a variety of gemstones that form in metamorphic rocks.

Gem-quality garnet , kyanite , tanzanite , emerald , andalusite , sphene , sapphire , ruby , scapolite , iolite , chrysoberyl and many other gem materials are found in schist.

Gem materials found in schist are often highly included. This is because their mineral crystals grow within the rock matrix, often including mineral grains of the host rock instead of replacing them or pushing them aside. The best metamorphic host rock for gem materials is usually limestone , which is easily dissolved or replaced when the gem materials are formed.

Home » Rocks » Metamorphic Rocks » Schist Schist A foliated metamorphic rock that contains abundant platy mineral grains. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Hardness Picks. Rock, Mineral and Fossil Collections. Flint, Chert, and Jasper. A sample of muscovite-garnet-staurolite schist that contains porphyroblasts of garnet red, equant , kyanite blue and staurolite dark, elongated. Width of sample 7 cm. Foliation surfaces are commonly wavy which reflects the presence and growth of porphyroblasts.

Microscopically, schists commonly show a crenulation fabric which indicates the presence of older foliation that may represent an earlier episode of deformation 4. Common minerals in schistose rocks indicate that these rocks formed at low- to intermediate grade conditions subgreenschist, greenschist, blueschist, and amphibolite facies. This means that not only composition but the formation conditions too are highly variable. Schist is rich in flaky and soft sheet silicate minerals which makes it structurally weaker than gneiss or granite.

That is the reason why this rock type is used less frequently as a building stone. However, some varieties have an attractive appearance which makes them useful as a facing or decorative stone. Schist may be worth mining if it contains useful minerals in large concentration. Common minerals extracted from schistose metamorphic rocks are garnet, kyanite, talc and graphite.

This rock is mined because of its very high graphite content. Width of specimen 27 cm. Quartz schist with perfectly parallel cleavage surfaces. This rock is quarried as a dimension stone. Alta, Norway. Width of view 50 cm. A heap of quartz schist slabs demonstrating platy habit which is caused by the abundance of platy minerals.

Talc schist is a source of talc. This rock type is also known as a soapstone because it is slippery when touched. Talc is a very soft mineral which makes it possible to cut this rock with a knife.

Lipasvaara, Finland. Riebeckite amphibole group mineral schist with a lineated fabric. Width of sample from Germany is 14 cm. Andalusite brown porphyroblasts in a sericite schist sample. Kapteeninautio, Finland. Width of sample 12 cm. Tonalite dike cutting through graphite-rich schistose rock. Width of the dike is approximately cm. Small lithic fragments of mica schist in a river sand from Canada. Width of view 20 mm. The surrounding matrix is often a grayish and mica-rich.

The garnet-muscovite schist shown above is early Late Cretaceous in age 89 to 92 million years. Rocks from this locality contain near-gem-quality almandine garnets. Chlorite schist 8. Chlorite schist is a moderately common metamorphic rock.

It is principally composed principally of chlorite , a silicate mineral with a greenish to grayish-green color. Talc schist is a metamorphic rock dominated by the mineral talc. Unlike soapstone and steatite both talcose, crystalline-textured metamorphic rocks , talc schist has a foliated texture in the sample shown below, the foliation is only seen when viewing the rock on edge.

Some talc schists formed by hydrothermal metamorphism of dolomitic marbles.