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Fig. 40 Grade and facies describe metamorphism
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Many of the metamorphic rocks consist of flaky materials such as mica and chlorite. These minerals cause the rock to split into thin sheets and rocks become foliated. Slate, schist and gneiss belong to the group of foliated metamorphic rocks. Marble and quartzite are non-foliated metamorphic rocks.
The structure of metamorphic rocks is of importance because it shows the nature of pre-existing rocks and the mechanism of metamorphic deformation. Every trace of original structure is of great importance to geologists. It gives an opportunity of analyzing the causes of its metamorphism. Metamorphic rocks represent the oldest portion of the Earth’s crust. They are mostly found in the regions of mountain belts where great dislocations on the Earth took place.
(М.Я. Баракова, Р. И. Журавлева, Английский язык для горных инженеров, М. 2001)
| TEXTURE | DIAGNOSTIC FEATURES | ROCK NAME |
| FOLIATED | Granulitic texture, medium to coarse, even grained. Foliation present in light-colored quartzo-feldspathic rocks, but virtually lacking in pyroxene-plagioclase-bearing rocks. | GRANULITE |
| Gneissic texture. Coarse grained. Foliation present as macroscopic grains arranged in alternating light and dark bands. Abundant quartz and feldspar in light-colored bands. Dark bands may contain hornblende, augite, garnet, or biotite. | GNEISS | |
| Schistose texture. Medium to fine grained. Common minerals are chlorite, biotite, muscovite, garnet, and dark elongate silicate minerals. Feldspars commonly absent. Recognizable minerals used as part of rock name. Porphyroblasts common. | SCHIST | |
| Phyllitic texture. Fine grained to dense. Micaceous minerals are dominant. Has a sparkling appearance. | PHYLLITE | |
| Slaty texture (slaty cleavage apparent). Dense, microscopic grains. Color variable; black, and dark gray common. Also occurs in green, dark red, and dark purple colors. | SLATE | |
| NONFOLIATED | Crystalline. Hard (scratches glass). Breaks across grains as easily as around them. Color variable; white, pink, buff, brown, red, purple. | QUARTZITE |
| Dense, dark-colored; various shades of gray, gray-green, to nearly black. | HORNFELS | |
| Crystalline. Composed of calcite or dolomite. Color variable; white, pink, gray, among others. Fossils in some varieties. | MARBLE | |
| Black, shiny luster. Conchoidal fracture. | ANTHRACITE COAL |
Дата публикования: 2014-11-03; Прочитано: 503 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!
