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. Quartzite is metamorphosed sandstone Figure 7. 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. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.
An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock. Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7. In some cases, hornfels has visible crystals of minerals like biotite or andalusite.
If the hornfels formed in a situation without directed pressure, then these minerals would be randomly orientated, not foliated as they would be if formed with directed pressure. Skip to content Chapter 7 Metamorphism and Metamorphic Rocks. Exercise 7. The mica crystals are consistently parallel to one another. A very hard rock with a granular appearance and a glassy lustre. Gneiss is an old German word meaning bright or sparkling.
Other specimens - Click the thumbnails to enlarge. In the dimension stone trade, any rock with visible, interlocking grains of feldspar is considered to be "granite" in that industry. Seeing gneiss, gabbro , labradorite , diorite , and other types of rock marketed as "granite" disturbs many geologists.
However, this long-time practice of the dimension stone trade simplifies discussions with customers since not everyone knows the technical names of unusual igneous and metamorphic rocks.
The best way to learn about rocks is to have specimens available for testing and examination. Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch.
If samples of these rock units are broken into one-inch pieces, many of them will be too small to exhibit the banding features of gneiss. This will confuse many students and cause others to incorrectly identify the rock. Teachers can avoid these problems by collecting specimens that clearly display a banded structure. Teachers who purchase specimens must examine them carefully before they are presented to students. After students have learned to identify gneiss and many other rock types, presenting specimens of gneiss that do not exhibit banding can be a challenging way to have students: A consider possibilities that are not obvious, and, B realize that a single rock specimen may not adequately represent a rock unit.
Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. What Is Gneiss? How Does Gneiss Form? Hardness Picks. Rock, Mineral and Fossil Collections.
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. Marble that forms during regional metamorphism—and in fact that includes most marble—may or may not develop a foliated texture, but foliation is typically not easy to see in marble. Quartzite is metamorphosed sandstone Figure 7.
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. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.
An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock. Since these micas are very small, this rock would not appear to be foliated to the naked eye. Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7. In some cases, hornfels has visible crystals of minerals like biotite or andalusite.
0コメント