What was gneiss before metamorphism

After metamorphism, the schist is very foliated the minerals of the rock are arranged in layers. It can be easily split into thin, flakey pieces. Schists often have a high luster they are very shiny due to large crystals within the rock. 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.

There is no evidence of foliation. A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them. A rock that is dominated by aligned crystals of amphibole. Previous: 7. Next: 7. Share This Book Share on Twitter. Gneiss displays distinct foliation , representing alternating layers composed of different minerals.

Because of the coarseness of the foliation, the layers are often sub-parallel, i. OpenLearn works with other organisations by providing free courses and resources that support our mission of opening up educational opportunities to more people in more places.

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Free courses. All content. Gneiss Updated Thursday, 28th September A brief description of the nature of gneiss. Copyright: The Open University How was it formed? Get closer to geology.