The term texture refers to the size, shape, and boundary relationships of the minerals, particles, and other substances that make up a rock. There are two major textural groups in metamorphic rocks: Foliated and Non-Foliated. In this texture, the mineral crystals in the rock are aligned with each other. This alignment may be displayed as parallel planes along which the rock splits, by overlapping sheets of platy minerals such as micas, by the parallel alignment of elongate minerals such as amphiboles, or by alternating layers of light and dark minerals.
Foliated texture is further subdivided based on the presence or absence of pronounced color banding in the rock. Rocks without distinct alternating bands of light and dark minerals are described a nonlayered, whereas rocks with alternating bands of dark and light minerals are described as layered. Layered is also referred to as gneissic foliation. Foliated textures are further described on the basis of the grain crystal size in the rock.
Examples of complete descriptions of foliated metamorphic rocks include: foliated, nonlayered, very fine grained for slate, foliated, layered, coarse grained for gneiss, and foliated, nonlayered, fine grained for phyllite. Foliated textures produced by shearing and breaking, such as in a fault zone or a meteor impact crater, are referred to as mylonitic.
Clay microscopic Slate Very, very thin, irregular layers of mica. Feldspar Schist Thin, irregular layers of mica and platy minerals Bluish color; mica, quartz Blueschist Thin, irregular layers of mica and platy minerals Greenish color; mica, quartz, serpentine Greenschist Thick bands, wavy, semi-continuous layers of white quartz, feldspar, and mica. Garnet Gneiss Non-Foliated Metamorphic Rocks In this texture the mineral crystals in the rock have grown in many directions and do not show alignment.
As a result, nonfoliated rocks commonly appear massive and structureless, with only a few lines of impurities through the rock. These rocks may break across, rather than around, mineral grains to produce a scaly surface on the specimen. For example 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, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite. If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it will partially melt.
The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite Figure 7. JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions.
The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. 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.
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 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 7. When minerals lie on a plane AND point in a common direction; this is both foliation and lineation.
Foliated metamorphic rocks are named based on the style of their foliations. Each rock name has a specific texture that defines and distinguishes it, with their descriptions listed below.
Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of stress. The minerals in slate are too small to see with the unaided eye. The thin layers in slate may resemble sedimentary bedding, but they are a result of directed stress and may lie at angles to the original strata. In fact, original sedimentary layering may be partially or completely obscured by the foliation.
Thin slabs of slate are often used as a building material for roofs and tiles. Phyllite is a foliated metamorphic rock in which platy minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains, perhaps even a wavy appearance, called crenulations. Similar to phyllite but with even larger grains is the foliated metamorphic rock schist , which has large platy grains visible as individual crystals.
Common minerals are muscovite, biotite, and porphyroblasts of garnets. A porphyroblast is a large crystal of a particular mineral surrounded by small grains. Schistosity is a textural description of foliation created by the parallel alignment of platy visible grains. Some schists are named for their minerals such as mica schist mostly micas , garnet schist mica schist with garnets , and staurolite schist mica schists with staurolite.
Bottom- Gneiss in outcrop at Belteviga Bay, Norway. Notice the light and dark stripes on the rock. Click the image for more attributions. Green crystals are the amphibole hornblende, and colourless crystals are plagioclase feldspar. Note horizontal crystal alignment. Source: D. Click the image for original figure caption and terms of use.
Types of Non-foliated Metamorphic Rocks Metamorphic rocks that form under low-pressure conditions or under the effects confining pressure, which is equal in all directions, do not become foliated. Marble Marble Figure Left- Marble made of pure calcite is white.
Upper right- microscope view of calcite crystals within marble that are blocky and not aligned. Quartzite Quartzite Figure Left- Quartzite from the Baraboo Range, Wisconsin. Right- Photomicrograph showing quartz grains in quartzite from the Southern Appalachians. In the upper left half of the image, blocky quartz crystals show some evidence of alignment running from the upper right to the lower left.
Hornfels Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Left- Hornfels from the Novosibirsk region of Russia from a sedimentary protolith. Dark and light bands preserve the bedding of the original sedimentary rock. The rock has been recrystallized during contact metamorphism and does not display foliation. Right- Hornfels in thin section from a sedimentary protolith.
Note that the brown mica crystals are not aligned. The dark band at the top reflects the layering within the sedimentary parent rock, similar to the way those layers are preserved in the sample on the left.
Click the image for terms of use. Click the table for a text version. Migmatite: Both Metamorphic and Igneous If a metamorphic rock is heated enough, it can begin to undergo partial melting in the same way that igneous rocks do. Ptygmatic folding happens when a stiff layer within a rock is surrounded by weaker layers. Folding causes the stiff layer to crinkle while the weaker layers deform around it. Which metamorphic rock is described in each of the following? A rock with visible minerals of mica and with small crystals of andalusite.
0コメント