Locate and examine the garnet mica schist specimen. It contains complete and und

Question

Locate and examine the garnet mica schist specimen. It contains complete and undistorted garnet crystals. Which sample is it? How is it possible for these crystals to exist in such perfect condition in a metamorphic rock?(1 point)

Using the tools for relative time discussed in class (included in the appendix), describe

the sequence of events that led to formation of this rock exposure? (15points)

Locate and examine the garnet mica schist specimen. It contains complete and undistorted garnet crystals. Which sample is it? How is it possible for these crystals to exist in such perfect condition in a metamorphic rock? Using the tools for relative time discussed in class (included in the appendix), describe the sequence of events that led to formation of this rock exposure?

Explanation / Answer

1) The rock has a purplish sheen because both dark mica (biotite) and light mica (muscovite) are present. It is just coarse enough to distinguish individual mica flakes, and some of them are reflecting the light from the illuminating lamps and appear as white specks. Also visible are large dull red-brown garnet crystals up to 1 cm across. Some of the garnets show a regular hexagonal outline. This texture, with large crystals growing in a finer matrix, is quite common in metamorphic rocks, and is called porphyroblastic texture. We have to make a distinction between porphyroblastic texture in metamorphic rocks and porphyritic texture in igneous rocks, since although they look similar they have very different origins. In porphyritic texture, big crystals grew first from molten material, and the fine matrix crystallized later. In porphyroblastic texture, the fine solid matrix of crystals was there first, and big new crystals grew in it.

2) Schist is medium grade metamorphic rock, formed by the metamorphosis of mudstone / shale, or some types of igneous rock, to a higher degree than slate, i.e. it has been subjected to higher temperatures and pressures. The resulting foliation is coarser and more distinct than that of slate due to the higher degree of crystallisation of mica minerals (biotite, chlorite, muscovite) forming larger crystals, and is often referred to as schistosity. These larger crystals reflect light so that schist often has a high lustre, i.e. it is shiny. Porphyroblasts are common in schist, and they provide information on the temperature and pressure conditions under which the rock formed. Due to the more extreme formation conditions, schist often shows complex folding patterns. There are many varieties of schist and they are named for the dominant mineral comprising the rock, e.g. mica schist, green schist (green because of high chlorite content), garnet schist etc.

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