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Phyllite Rock

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Phyllite is a foliated metamorphic rock that is primarily composed of quartz, mica, chlorite, and sericite. The Phyllite rock is formed by the slate that is further metamorphosed so that the coarse-grained mica attain a preferred accommodation. The word Phyllite is derived from the Greek phyllon meaning “ leaf”. 


Phyllite metamorphic rock has an excellent tendency to split into the sheets and is generally black to grey or light greenish-grey. The appearance of the rock is commonly crinkled or wavy.  The Phyllite rock is commonly found in Dalradian metasediments of northwest Arran. The Tretorn phyllites rock and Woolgarden phyllites rock are found in North Cornwall.

Phyllite Properties

Name/Origin

The term phyllite is derived from the Greek Phlon means “ leaf”.

Colour

Shiny grey, black to grey, or light greenish-grey in colour.

Parent Rock

Shale or Pelite, or slate which is derived from a shale protolith is a parent rock for phyllite

Metamorphic Type

Regional

Group

Metamorphic rock

Metamorphic Grade

Low grade ( higher than slate)

Metamorphism Degree

Between slate and schist

Dominant Minerals

Mica minerals

Metamorphic Environment 

Inferior regional metamorphism along a convergent plate boundary

Texture

Fine-grained, foliated, and well - developed schistosity

Hardness

1-2 on Mohs scale, also has a good fissility ( tendency to split into sheets)

Minerals 

Graphite, sericite, chlorite, muscovite, or translucent coarse grain white mica, quartz, and plagioclase.


The Table Given Below Shows the Phyllite Mineral Composition

Phyllite Mineral Composition

Mineral

Content (%)


WP

CP

Quartz

50.8

49.4

Muscovite

6.1

6.8

Microline

2.0

1.2

Muscovite ₂MI

37.4

31.2

Kaolinite

3.7

8.0

Rutile


1.0

Goethite


2.4

Total

100.0

100.0


Phyllite Composition

According to the Phyllite Composition Table Given Above, We Can Say:

  • Phyllite has a coarse grain of mica such as Sericite and Muscovite.

  • Phyllite includes almost half the amount of quartz mineral in its composition.

  • Quartz and feldspar are found in large quantities in phyllite.

  • Crystals of ratile, cordierite, andalusite, staurolite, and garnet are also found in phyllite.

  • Large crystals or porphyroblast are found in the horizontal axis in phyllite.

  • Organic minerals are metamorphosed into graphite and give phyllite black to dark colour shades and submetallic luster.


Phyllite Uses

Phyllite is a soft and durable metamorphic rock. It is often used as a floor tile, decorative aggregates, and decorative stone in counterparts. It is also used as an exterior building, or facing stone, and garden decoration. The other phyllite uses include commemorative tablets, cemetery markers, writing tablets, etc.  The slabs of Phyllite are often trimmed and used as a landscape, paving, or sidewalk stone. 


Phyllite Parent Rock

The phyllite parent rocks are shale, pelite, or slate which further comes from a shale protolith. Shale can be transformed into schist, slate or genesis depending on the degree of heat and temperature. 

Similar to slate, phyllite has a typical texture known as phyllitic sheen and has the feasibility or tendency to split into sheets like slate rocks.


Phyllite Rock Formation

The phyllite rock forms when Slate rock further transformation occurs, and very coarse grain mica attains perfect orientation.

  • The slate rock consists of clay minerals in a semi-random orientation.

  • When the slate rocks are buried further and attain a high temperature, the extremely fine clay transforms into mica and the flacks of clay minerals gain a horizontal positioning.

  • The chemical reactions and heat transform clay mineral grain into chlorine or mica minerals. They are enlarged forms of coarse-grain clay flakes.

  • Hence, they form phyllite rock. 

  • Further metamorphosis and sedimentation process transform Phyllite into Schist, and then Geiss by enlarging mica flakes.


Where is Phyllite Rock Found?

Phyllite rock is found throughout the world  in many regions such as:

  • Scottish Islands

  • The Alps in Europe

  • The Appalachians in North America


Did You Know?

  • Large crystals or porphyroblasts are found in phyllite rock in parallel orientation.

  • Organic minerals metamorphosed into graphite and give phyllite a black to dark grey shades that give it a submetallic luster.

  • Phyllites are most commonly found in the Dalradian metasediments of northwest Arran.

  • Quartz and feldspar are found in large quantities in phyllite.

  • Phyllite has a good tendency to split into sheets.

  • Fine grains of mica minerals such as Muscovite and sericite are also found in phyllite

  • Tredorn Phyllite and Woolgarden phyllites are found in North Cornwall.

FAQ (Frequently Asked Questions)

1. What Is the Primary Difference between Phyllite, Schist, and Slate Rock from a Geological Point of View?

  • Phyllite has coarse-grained mica flakes in a preferred orientation

  • Slate has extremely fine-grained in a preferred orientation.

  • Schist has extremely large flakes in a preferred orientation.

2. What Colour is Phyllite Rock?

  • Phyllite rock is found in black to shiny grey and greenish-grey colours, which are lighter in shades and tones in comparison to slate rock.

  • Often phyllite transforms into tan or brown colour over time.

  • The reflective sheen of phyllite generally gives a silver or non-metallic appearance. 

3. What are the Commonly Metamorphic Rocks?

The common metamorphic rock includes phyllite schist, gneiss, marble, and quartzite.