Dark horizontal seams are largely graphite aligned in the shear foliation. Large black blobs are graphite as well. This graphite was introduced via hot water but has since been recrystallized. Narrow ductile shear zones with deformed quartz veins cut across the foliation in the semischist fault slice, downstream of Fiddlers Flat. The margins of these ductile shears have been brecciated in a brittle manner, and the breccias have been partially reactivated to form soft gouge that is part of the major nearby Cretaceous normal fault zone.
The ductile foliation is strongly defined by graphite-rich seams, with grains that range in size from micron-scale dusty material to 0. Large graphite grains occur in the recrystallised vein quartz, where they are up to 1 mm across. Brecciated margins of graphitic quartz veins have been cemented with finer grained graphite, and graphite cement protrudes beyond the breccias as delicate encrustations. All the graphite in these shear zones was ultimately derived from fragments of carbonaceous material that accumulated on the sea floor with the greywacke and argillite sediments, probably in the Triassic million years ago.
The fragmental organic carbon was dissolved and transported by hot fluids during metamorphism of the sediments to schist, in the Jurassic million years ago. The carbon was transported as methane and carbon dioxide, and graphite deposition results from mixing these, and interaction with the rocks, probably during lowering temperature.
The carbon was redeposited as coarser grained graphite, especially in the shear zones as in above photographs along which fluid flowed most readily. These processes resulted in disseminated gold at Macraes that initially developed in pyrite and arsenopyrite grains hosted by a weakly graphitic foliation. On-going deformation, both ductile and brittle, resulted in remobilisation of gold, sulphides, and graphite.
COVID information. Find out more Future student? Current Students Close. For Otago Staff Close. About University of Otago Close. Research and Enterprise at Otago Close. Muscovite schist, biotite schist, and chlorite schist often called "greenstone" are commonly used names. Other names based upon obvious metamorphic minerals are garnet schist, kyanite schist, staurolite schist, hornblende schist, and graphite schist.
Some names used for schist often consist of three words, such as garnet graphite schist. Garnet graphite schist is a schist that contains graphite as its dominant mineral, but abundant garnet is visible and present. Garnet mica schist in thin section: This is a microscopic view of a garnet grain that has grown in schist.
The large black grain is the garnet, the red elongate grains are mica flakes. The black, gray, and white grains are mostly silt or smaller size grains of quartz and feldspar.
The garnet has grown by replacing, displacing, and including the mineral grains of the surrounding rock. You can see many of these grains as inclusions within the garnet.
From this photo it is easy to understand why clean, gem-quality garnets with no inclusions are very hard to find. 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. The name of a particular schistose rock depends on the dominant minerals present — muscovite-garnet-staurolite schist, for example. Several less known names have been given to a specific varieties: staurotile contains staurolite porphyroblasts , prasinite metamorphosed mafic rock with epidote, chlorite and hornblende in equal proportions , sismondinite chloritoid is the dominant mineral phase.
Characteristically twinned staurolite porphyroblast in an Al-rich metapelite staurolite schist or staurotile. Width of sample 19 cm. Aluminous varieties often contain large crystals in a finer matrix.
These crystals formed as the metamorphism progressed and they can convert to each-other as conditions change. Such large and often euhedral crystals are known as porphyroblasts. Common minerals that form porphyroblasts are garnet, staurolite, kyanite and andalusite.
Porphyroblasts somewhat resemble phenocrysts in igneous rocks — both are larger crystals in a fine r matrix but the mineralogy is distinctly different. Quartz is a common phenocryst in igneous rocks, but it never occurs as a porphyroblast in metamorphic rocks.
Feldspar, micas, olivine , pyroxenes and amphiboles are all common phenocrysts but uncommon as porphyroblasts 2. 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.
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