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Phyllite

Phyllite is between slate and schist in the spectrum of metamorphic rocks. Geologists tell them

apart by their surfaces:slatehas flat cleavage faces and dull colors, phyllite has flat or crinkledcleavage faces and shiny colors, andschisthas intricately wavy cleavage (schistosity) and

glittering colors. Phyllite is "leaf-stone" in scientific Latin; the name may refer as much to

phyllite's color, which is often greenish, as to its ability to cleave into thin sheets. Phyllitegenerally is in the pelitic seriesrocks that are derived from clay sedimentsbut sometimes

other rock types can take on the characteristics of phyllite too. That is, phyllite is a textural rock

type, not a compositional one. The sheen of phyllite is from microscopic grains ofmica,graphite,chloriteand similar minerals that form under moderate pressure

http://geology.about.com/od/more_metrocks/ig/slate.-CLm/http://geology.about.com/od/more_metrocks/ig/slate.-CLm/http://geology.about.com/od/more_metrocks/ig/slate.-CLm/http://geology.about.com/library/bl/images/blschist.htmhttp://geology.about.com/library/bl/images/blschist.htmhttp://geology.about.com/library/bl/images/blschist.htmhttp://geology.about.com/library/bl/images/blmuscovite.htmhttp://geology.about.com/library/bl/images/blmuscovite.htmhttp://geology.about.com/library/bl/images/blmuscovite.htmhttp://geology.about.com/library/bl/images/blgraphite.htmhttp://geology.about.com/library/bl/images/blgraphite.htmhttp://geology.about.com/library/bl/images/blchlorite.htmhttp://geology.about.com/library/bl/images/blchlorite.htmhttp://geology.about.com/library/bl/images/blchlorite.htmhttp://geology.about.com/library/bl/images/blchlorite.htmhttp://geology.about.com/library/bl/images/blgraphite.htmhttp://geology.about.com/library/bl/images/blmuscovite.htmhttp://geology.about.com/library/bl/images/blschist.htmhttp://geology.about.com/od/more_metrocks/ig/slate.-CLm/

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http://geology.about.com/od/more_metrocks/ig/phyllite.-CN2/phyllitefracture.htm

A phyllite is a fine-grained, foliated, pelitic metamorphic rock with a pervasive cleavage produced by alignment of

micas. Pelites are metamorphosed argillaceous sediments, such as shale and mudstone, and are dominated by

muscovite, chlorite and biotite, often with graphite and iron oxides. They may also contain albite, quartz and zircon.

Phyllite may contain laminations in which quartz and feldspar are more abundant and represent original sedimentary

layering, they may also be interbedded with quartzite. Phyllites are typically grey to black in colour with a silky lustre

on their pervasive cleavage planes due to the alignment of mica minerals. Often phyllites exhibit a crenulation

cleavage which can be seen as a crenulation on the cleavage surface.

Phyllites are formed by regional metamorphism of argillaceous sediments since their cleavage arose due to deviatoric

stress. Phyllite is typical of greenschist grade metamorphism. With increasing metamorphic grade increases in mica

grain-size transforms phyllite into schist. However, some phyllites have experienced an overprint by contact

metamorphism resulting in the growth of biotite, andalusite or cordierite porphyroblasts, these are often randomly

orientated and grow across foliation.

https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=000000

00118

A type of metamorphic rock formed during low-grade metamorphism of clay-rich sediments called pelites.

Phyllites are very fine grained rocks with a grain size barely visible in a hand specimen. They have a well-

developed planar element called cleavage defined by alignment of mica grains and interlayering of quartz-

http://geology.about.com/od/more_metrocks/ig/phyllite.-CN2/phyllitefracture.htmhttp://geology.about.com/od/more_metrocks/ig/phyllite.-CN2/phyllitefracture.htmhttps://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000118https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000118https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000118https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000118https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000118http://geology.about.com/od/more_metrocks/ig/phyllite.-CN2/phyllitefracture.htm

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rich and mica-rich domains. Typically, mica grains show the greater alignment, although other mineral

components (quartz, carbonate, and feldspars) may show a preferred shape orientation. Where all minerals

of a particular type show the same degree of alignment and the fabric is well developed throughout the rock,

the fabric is termed a penetrative fabric. Cleavage surfaces in phyllites have a glittery, lustrous sheen due to

light reflecting off grains of chlorite and muscovite. The mineralogy of phyllites is dependent on chemical

composition; typical minerals in phyllites are chlorite, muscovite, and quartz. Other minerals that may bepresent in phyllites formed during low-grade metamorphism include chlorotoid, garnet (rarely), sodium-mica,

and sulfide minerals. These minerals are typically millimeter in scale and resolvable in a hand specimen by

using a 10 hand lens.See also:Chlorite;Muscovite;Quartz

Phyllite

Phyllite is an intermediate-grade, foliated metamorphic rocktype that resembles its sedimentary parent

rock, shale, and its lower-grade metamorphic counterpart, slate. Like slate, phyllite can be distinguished

from shale by its foliation, called slaty cleavage, and its brittleness, or fissility. Both slate and phyllite are

generally dark-colored; their most common color is dark gray-blue, but dark red and green varieties also

exist. Unlike slate, phyllite has a characteristic glossy sheen, its foliation is usually slightly contorted, and

it rarely retains traces of the original sedimentary bedding. Phyllite also lacks the large, visible mica

crystalsand high-grade index mineralsdiagnostic of schist, its higher-grade metamorphic cousin.

Heating and compression of clay-rich, bedded sedimentary rockscalled shales creates a series of rock

types of increasing metamorphic grade: slate, phyllite, schist, and gneiss. During metamorphismof

shales, and occasionally volcanic ash layers, metamorphism transforms platy clayminerals into small

sheets of mica. As the intensity of heating and compression, the so-called metamorphic grade, increases,

the mica sheets align themselves perpendicular to the direction of stress, and they grow larger. In phyllite,

the crystals of sheet-silicate minerals like chlorite, biotite, and muscovite are large enough to give the rock

its distinctive satin sheen and slaty cleavage, but not large enough to be visible to the unaided eye. Theamount of heat and pressure required to transform shale to phyllite is generally sufficient to destroy any

original sedimentary layering. Additional metamorphism transforms phyllite to schist; all the original clay

and small mica crystals transform into large mica crystals, any remaining organic material is destroyed,

and high-grade metamorphic index minerals like garnet and staurolite grow in the micaceous matrix.

http://www.encyclopedia.com/topic/Phyllite.aspx

http://www.accessscience.com/content/chlorite/131600http://www.accessscience.com/content/chlorite/131600http://www.accessscience.com/content/chlorite/131600http://www.accessscience.com/content/muscovite/440000http://www.accessscience.com/content/muscovite/440000http://www.accessscience.com/content/muscovite/440000http://www.accessscience.com/content/quartz/563500http://www.accessscience.com/content/quartz/563500http://www.accessscience.com/content/quartz/563500http://www.encyclopedia.com/topic/Phyllite.aspxhttp://www.encyclopedia.com/topic/Phyllite.aspxhttp://www.encyclopedia.com/topic/Phyllite.aspxhttp://www.accessscience.com/content/quartz/563500http://www.accessscience.com/content/muscovite/440000http://www.accessscience.com/content/chlorite/131600

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METAMORPHIC ROCKSFormed in the solid state in response to changes in

temperature, pressure and chemical conditions imposed at

depth below the level of weathering and lithification

Part I: Foliated Metamorphic Rocks

*Note: Phyllite has a texture that is intermediate between slate and schist. This results in

slaty cleavage and shiny cleavage surfaces but grains that are still microscopic.

SLATE

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Slate is a low-grade foliated metamorphic rock formed by regional metamorphism.

It is similar in appearance to mudstone and shale due to the low grade of

metamorphism but can be distinguished by its slaty cleavage and more dense,compact nature.

Grade: low

Parent Rock: clay-rich mudstone or shale

Nature of Metamorphism: regional with directed stress

Impt. Characteristics: planar alignment of microscopic clay grains cause the rock

to break in parallel layers (slaty cleavage).

PHYLLITE

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Phyllite is a low-grade foliated metamorphic rock that is intermediate in grade

between slate and schist. Aligned near-microscopic crystals of fine-grained

muscovite and chlorite give the rock a shiny surface or sheen along cleavage

surfaces.

Grade: low

Parent Rock: clay-rich mudstone or shale

Nature of Metamorphism: regional with directed stress

Impt. Characteristics: intermediate in texture between slate and schist with

planar alignment of near-microscopic platy minerals which give the rock a shiny

reflective appearance.

SCHIST

Schist is a medium-grade foliated metamorphic rock. Regional metamorphism of

clay-rich parent rock results in an increase in grain size and preferred planar

alignment of large visible platy grains.

Grade: medium

Parent Rock: clay-rich mudstone or shale

Nature of Metamorphism: regional with directed stress

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Impt. Characteristics: planar alignment of visible platy minerals (schistosity) such

as micas, talc and kyanite.

GNEISS

Gneiss is a high-grade foliated metamorphic rock which forms under extreme

temperatures and directed pressure. Gneiss often grades into migmatite

(intermixed igneous and metamorphic rock) reflecting the high temperatures of

formation of both rock types.

Grade: high

Parent Rock: variable

Nature of Metamorphism: regional with directed stress

Impt. Characteristics: distinct layers of dark- and light-colored minerals (gneissic

layering)

Click here to continue to Part II - Nonfoliated Metamorphic Rocks and Part

III - Common Metamorphic Minerals

Click here to return to Course Materials Homepage

http://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/crslnk.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htmhttp://itc.gsw.edu/faculty/tweiland/metarx2.htm

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http://itc.gsw.edu/faculty/tweiland/metarx.htm

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