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Phyllite is a fine-grained, foliated metamorphic rock that is intermediate in metamorphic grade between slate and schist. It is characterized by a distinctive silky or pearly luster, known as 'phyllitic luster,' which is caused by the parallel alignment of very fine-grained mica (sericite) and chlorite crystals. The foliation is typically wavy or crenulated, and while it can be split along planes, its cleavage is generally less perfect than that of slate. Its color is typically grayish, greenish, or brownish.
How to Identify
- Color
- Typically gray, greenish-gray, or brownish-gray. Can also be dark gray to black.
- Luster
- Distinctive silky, pearly, or satiny sheen (phyllitic luster) on cleavage surfaces, due to fine-grained mica.
- Texture
- Fine-grained, but individual mineral grains (micas) are just barely visible to the naked eye or with a hand lens, unlike slate where grains are microscopic. Strongly foliated, often with a wavy or crenulated cleavage.
- Crystal Form
- Individual crystals are too small to be well-formed and are typically platy (micas, chlorite) and aligned.
- Cleavage
- Well-developed slaty cleavage, but often wavy or crenulated, and less perfect than slate. It splits into thin, somewhat irregular sheets.
- Geological Environment
- Forms in regional metamorphic terrains, typically in the greenschist facies, where shales or mudstones have been subjected to moderate temperature and pressure during orogenic events.
Key Facts
- Hardness: 2-3 on Mohs scale (due to mica content)
- Specific Gravity: 2.7-2.8 g/cm
- Crystal System: Monoclinic (for constituent micas and chlorite)
- Color: Gray, greenish-gray, brownish-gray, dark gray
- Luster: Silky, pearly, satiny
- Transparency: Opaque
- Fracture: Irregular to subconchoidal, often controlled by cleavage
- Cleavage: Excellent slaty cleavage, often wavy or crenulated
- Composition: Dominated by fine-grained muscovite (sericite), chlorite, quartz, and often albite. May contain minor amounts of graphite, tourmaline, and garnet.
Quick Check
- Color: Gray, greenish-gray, brownish-gray
- Luster: Silky, pearly, satiny (phyllitic luster)
- Streak: White to light gray
Physical Characteristics
- Crystal Habit: Microscopic to very fine-grained platy minerals (micas, chlorite) aligned in parallel.
- Cleavage Type: Slaty cleavage, often crenulated or wavy, allowing splitting into thin sheets.
- Fracture Type: Irregular to subconchoidal, often controlled by the foliation planes.
- Tenacity: Brittle
- Luster Type: Silky, pearly, or satiny due to the reflection of light off aligned mica flakes.
Formation
Phyllite forms through regional metamorphism of fine-grained sedimentary rocks (like shale or mudstone) or volcanic rocks (like tuff) under conditions of low to medium temperature (approximately 300-500 C) and moderate pressure. It represents an intermediate metamorphic grade between slate and schist. The key process is the recrystallization of clay minerals into fine-grained micas (muscovite, chlorite) and other platy minerals, which align perpendicularly to the maximum stress direction, imparting a characteristic foliation.
Usage
Historically, phyllite has been used locally for roofing tiles, flagging stones, and decorative building material, similar to slate but generally less durable due to its coarser grain size and less perfect cleavage. Its attractive sheen can make it desirable for architectural facings. It is also used as aggregate in construction.
Age Distribution
Phyllites can form from sedimentary rocks of any age that have undergone appropriate metamorphic conditions. They are common in ancient orogenic belts (e.g., Appalachian Mountains, Caledonian Orogeny, Hercynian Orogeny) ranging from Proterozoic to Mesozoic.
Where to Find
Appalachian Mountains, USA
Extensive occurrences throughout the Appalachian orogenic belt, particularly in the Piedmont and Blue Ridge provinces.
Caledonian Orogeny regions (e.g., Scotland, Norway)
Common in areas affected by the Caledonian orogeny, where ancient sedimentary sequences were metamorphosed.
Hercynian Orogeny regions (e.g., Central Europe)
Found in metamorphic belts across Central Europe associated with the Hercynian (Variscan) orogeny.
Alpine Orogeny regions (e.g., Alps)
Present in some lower-grade metamorphic zones within the Alps.
New England, USA
Abundant in states like Vermont, New Hampshire, and Massachusetts.
Finding Tips
Look for Orogenic Belts
Phyllite is characteristic of mountain-building regions where sedimentary rocks have undergone regional metamorphism.
Examine Road Cuts and Quarries
These exposures often reveal large sections of bedrock, making it easier to identify foliated metamorphic rocks.
Check for Silky Luster
The most distinguishing feature is the characteristic silky or pearly sheen on cleavage surfaces, which differentiates it from duller slate.
Assess Grain Size
While fine-grained, phyllite's mica flakes are often just visible, unlike the microscopic grains of slate but smaller than the easily visible grains of schist.
Observe Cleavage and Foliaion
Note the wavy or crenulated nature of the foliation, which is often more pronounced than in slate.
Similar Rocks
Slate
Slate
Also known as: Roofing slate
Schist
Schist
Also known as: Mica schist
Shale
Shale
Also known as: Mudstone
Scientific Classification
- Mineral Class
- Metamorphic Rock
- Group
- Foliated Metamorphic Rocks
- Crystal System
- Not applicable for the rock as a whole; constituent minerals like mica are monoclinic.
- Chemical Formula
- Variable, primarily (K,Na)(Al,Mg,Fe)(Si,Al)O(OH) (for micas) and (Mg,Fe)AlSiO(OH) (for chlorite), plus SiO (quartz).
- Composition
- Predominantly quartz, sericite (fine-grained muscovite), and chlorite. Minor constituents can include albite, graphite, tourmaline, and garnet.
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