Phyllite

Definition

Phyllite is a fine-grained metamorphic rock that is characterized by its shiny appearance and foliation, which results from the alignment of platy minerals, primarily mica. It forms under moderate temperature and pressure conditions, typically from the metamorphism of shale or mudstone, and exhibits a distinct sheen due to the presence of fine-grained mica. Phyllite represents an important stage in the metamorphic process, bridging the gap between slate and schist.

5 Must Know Facts For Your Next Test

1. Phyllite typically forms at temperatures between 300°C and 500°C and at depths of 5 to 10 kilometers within the Earth's crust.
2. The foliation in phyllite is less pronounced than in schist, but more developed than in slate, indicating its intermediate metamorphic grade.
3. Phyllite often has a silvery or shiny luster due to the presence of fine mica flakes that reflect light.
4. It can vary in color from light gray to dark green or brown, depending on its mineral composition and the parent rock it originated from.
5. Phyllite is often used as a decorative stone in architecture and landscaping due to its attractive appearance and ability to be polished.

Review Questions

• Compare and contrast phyllite with slate and schist regarding their formation conditions and physical characteristics.
  • Phyllite lies between slate and schist on the metamorphic spectrum. Slate forms under lower temperatures and pressures, making it finer-grained and with less pronounced foliation than phyllite. In contrast, schist forms under higher temperatures and pressures, resulting in coarser grains and more noticeable foliation. Thus, while all three are products of metamorphism, phyllite serves as a transitional rock exhibiting properties that are intermediate between slate's smooth texture and schist's prominent layering.
• Discuss the significance of foliation in phyllite and how it influences its formation and characteristics.
  • Foliation in phyllite plays a critical role in defining its structure and appearance. The alignment of platy minerals under directed pressure leads to the characteristic layering seen in phyllite. This process not only contributes to its sheen but also impacts how the rock breaks, as it tends to split along these planes. Understanding foliation is key to recognizing phyllite among other metamorphic rocks, highlighting how the conditions of formation affect its physical attributes.
• Evaluate the role of temperature and pressure in determining the classification of phyllite within the metamorphic rock series.
  • Temperature and pressure are essential factors that influence the classification of phyllite within the metamorphic rock series. Phyllite forms under moderate conditions, specifically at temperatures between 300°C and 500°C and depths ranging from 5 to 10 kilometers. These conditions differentiate phyllite from both lower-grade rocks like slate, which forms under cooler conditions, and higher-grade rocks like schist, which forms at even greater temperatures and pressures. Thus, understanding these factors not only aids in classifying phyllite but also illustrates the broader context of metamorphic processes.

Related terms

metamorphism: The process by which rocks change their mineralogy, texture, or chemical composition due to changes in temperature, pressure, or chemically active fluids.

foliation:

The repetitive layering in metamorphic rocks caused by the alignment of mineral grains under directed pressure.

schist:

A medium to coarse-grained metamorphic rock that has a well-developed foliation and is richer in larger micas compared to phyllite.