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Chapter 8: Problem 5

How does slate differ from a phyllite? How does phyllite differ from a schist? How does schist differ from a gneiss?

Short Answer

Expert verified
Slate is a fine-grained metamorphic rock with a dull appearance and slaty cleavage, formed at a low metamorphic grade. Phyllite is intermediate between slate and schist, with larger mineral grains and a lustrous sheen, formed at a higher metamorphic grade than slate. Schist is coarse-grained with prominent foliation due to the alignment of mineral grains, formed at higher metamorphic grades than phyllite. Gneiss is characterized by distinct light and dark bands of minerals, formed at even higher metamorphic grades than schist, with larger mineral grains and pronounced mineral segregation.

Step by step solution

01

1. Identifying characteristics of metamorphic rocks

Metamorphic rocks are formed when pre-existing rocks (either igneous, sedimentary, or previously existing metamorphic rocks) undergo changes due to heat, pressure or mineral exchange. These transformations cause changes in the mineral composition, texture, and overall appearance of the rocks. The four specific metamorphic rocks in this exercise are slate, phyllite, schist, and gneiss.
02

2. Describing the properties of slate

Slate is a fine-grained metamorphic rock formed from the metamorphism of shale or mudstone. It is composed of parallel layers of clay minerals, which have been compressed and reorientated under low-grade metamorphic conditions. Slate exhibits a distinct foliation, also known as slaty cleavage, which allows it to easily split into flat sheets.
03

3. Describing the properties of phyllite

Phyllite is a metamorphic rock that is intermediate in grade between slate and schist. It is formed as a result of further metamorphism of slate. The clay minerals in phyllite recrystallize and grow larger, giving the rock a lustrous sheen in comparison to the dull appearance of slate. Phyllite has a distinct foliation with a wavy, undulating texture.
04

4. Comparing slate and phyllite

Both slate and phyllite are fine-grained metamorphic rocks formed from the metamorphism of shale or mudstone. However, phyllite is formed at a higher metamorphic grade than slate. As a result, the minerals in phyllite are larger and it has a more lustrous appearance than slate. Both rocks have foliation, but the foliation in phyllite is characterized by a wavy texture, while in slate it is defined by its slaty cleavage.
05

5. Describing the properties of schist

Schist is a medium- to coarse-grained metamorphic rock formed at a higher grade of metamorphism than phyllite. The minerals in schist are larger than those in both slate and phyllite, often visible to the naked eye. Schist is characterized by a distinct planar alignment of its mineral grains, which can include mica, feldspar, and quartz. This alignment causes the rock to easily split along its planes, giving it a strong foliation.
06

6. Comparing phyllite and schist

Phyllite and schist are both formed under higher-grade metamorphic conditions than slate. However, schist is formed at a higher metamorphic grade than phyllite, resulting in larger minerals and a more pronounced foliation. While both rocks have foliation, schist is more coarse-grained and has a higher concentration of large mica flakes compared to the finer grained phyllite.
07

7. Describing the properties of gneiss

Gneiss is a coarse-grained metamorphic rock formed at even higher metamorphic grade than schist. Gneiss is characterized by the segregation of light (mainly quartz and feldspar) and dark (biotite and amphibole) mineral bands, referred to as gneissic banding. The minerals in gneiss are typically larger than those in schist.
08

8. Comparing schist and gneiss

Schist and gneiss are both formed at higher metamorphic grades than slate and phyllite. Schist is characterized by a strong foliation due to the alignment of its mineral grains, while gneiss is identified by its distinct banding of light and dark minerals. Gneiss forms at a higher metamorphic grade than schist, resulting in larger mineral grains and a more pronounced segregation of minerals into visible bands.

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Most popular questions from this chapter

Do you think that you would be likely to find a broad region (hundreds of kilometers across and hundreds of kilometers long) in which the outcrop consists of highgrade hornfels? Why or why not? (Hint: Think about the causes of metamorphism and the conditions under which a hornfels forms.)

What phenomena can cause metamorphism?

Where would you go if you wanted to find exposed metamorphic rocks, and how did such rocks return to the surface of the Earth after being at depth in the crust?

How are metamorphic rocks different from igneous and sedimentary rocks?

What is metamorphic foliation, and how does it form?
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