Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated, and their texture is described as massive.
In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels.
Marble is metamorphosed limestone. When it forms, the calcite crystals recrystallize and tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white as in Figure 6. Quartzite is metamorphosed sandstone Figure 6. 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. See Appendix 2 for Practice Exercise 6. As conditions change with increasing metamorphism, certain minerals become unstable and undergo solid-state changes to form new, stable minerals. Note that while garnet, for example, is a common mineral in schist, it is not present in all schists! The new minerals that form in a metamorphic rock are dependent upon the composition of the protolith and a wide variety of minerals are possible.
Not all minerals in a metamorphic rock are indicative of a particular metamorphic grade. Quartz, feldspar, and calcite not shown , for example, are stable over the entire range of temperatures shown in Figure 6. Type of foliation defined by closely spaced, flat surfaces along which a slate splits. Formed by the growth of microscopic mica minerals. Type of foliation defined by scaly layers of visible mica minerals or other platy or elongate minerals.
Rocks with this texture appear shiny or sparkly, as the light glints off cleavage planes of the aligned minerals. A purely physical process no change in composition that occurs in a solid-state during metamorphism. Atoms in a mineral are re-organized and typically grain size increases. Skip to content Increase Font Size. In the original rock left there is no alignment of minerals.
In the squeezed rock right the minerals have been elongated in the direction perpendicular to the squeezing. However, the classifiction of metamorphic rocks is more difficult to get into than with igneous and sedimentary rocks, for a couple of reasons. First, is the fact that metamorphism takes place through heat, pressure, and chemically active fluids, but there are many combinations of heat, pressure, and chemically active fluids and so there are many different responses. Or, to put it another way, there is not a direct relationship between the processes of metamorphism and the rocks that result - you have to understand each case on its own.
Third, totally different parent rocks undergoing the same kind of metamorphism can end up looking virtually the same. For example, greenschist shale parent and greenstone mafic igneous parent. Fourth, the texture terminology we use for metamorphic rocks is used to describe rocks that may have totally different parents, or may be metamorphosed under totally different conditions.
The most common example is the term schist. Schist is a type of layering or foliation found of metamorphic rocks where minerals large enough to be seen by eye tend to line up all in the same direction.
But schistose rocks include Greenschist chlorite dominated , Blueschist glaucophane dominated, plus many other minerals , and just plain Schist biotite or muscovite, quartz, and feldspar dominated , and each of these form under different tectonic conditions. To add confusion some rocks with a schist-like texture do not include the term in the name, like amphibolite. Another example is the texture term gneiss.
This series is listed below: Rutile, sphene, magnetite Tourmaline kyanite, staurolite, garnet, andalusite Epidote, zoisite, lawsonite, forsterite Pyroxenes, amphiboles, wollastonite Micas, chlorites, talc, stilpnomelane, prehnite Dolomite, calcite Scapolite, cordierite, feldspars Quartz This series can, in a rather general way, enable us to determine the origin of a given rock. For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase.
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