A metamorphic facies is a set of
metamorphic mineral assemblages that
were formed under similar pressures and
temperatures. The assemblage is typical
of what is formed in conditions
corresponding to an area on the two
dimensional graph of temperature vs.
pressure. Rocks which contain certain
minerals can therefore be linked to
certain tectonic settings, times and
places in the geological history of the
area. The boundaries between facies are
wide because they are gradational and
approximate. The area on the graph
corresponding to rock formation at the
lowest values of temperature and
pressure is the range of formation of
sedimentary rocks, as opposed to
metamorphic rocks, in a process called
diagenesis.
Historic definition 
The name facies was first used for
specific sedimentary environments in
sedimentary rocks by Swiss geologist
Amanz Gressly in 1838. Analogous with
these sedimentary facies a number of
metamorphic facies were proposed in 1920
by Finnish petrologist Pentti Eskola.
Eskola's classification was refined by
New-Zealand geologist Francis John
Turner throughout his career. A classic
work of Turner's was the book he
published in 1948 titled Mineralogical
and Structural Evolution of Metamorphic
Rocks. Turner continued to work in the
field, refining the metamorphic facies
classifications through the end of his
career in the early 1970s.
Underlying principles 
The different metamorphic facies are
defined by the mineralogical composition
of a rock. When the temperature or
pressure in a rock body change, the rock
can cross into a different facies and
some minerals become stable while others
become unstable or metastable. Whether
minerals really react depends on the
reaction kinetics, the activation energy
of the reaction and how much fluid is
present in the rock.
The minerals in a metamorphic rock and
their age relations can be studied by
optical microscopy or Scanning Electron
Microscopy of thin sections of the rock.
Apart from the metamorphic facies of a
rock, a whole terrane can be described
by the abbreviations LT, MT, HT, LP, MP,
HP. Since the 1980s the term UHP is used
for rocks that saw extreme pressures.
Which minerals grow in a rock is also
dependent of the original composition of
the protolith. Carbonate rocks have a
different composition from say a basalt
lava, the minerals that can grow in them
are different too. Therefore, a
metapsammite and a metapelite will have
different mineralogical compositions
even though they were in the same
metamorphic facies.
Index minerals 
Every metamorphic facies has some index
minerals by which it can be recognized.
That does not mean these minerals will
necessarily be visible with the naked
eye, or even exist in the rock; when the
rock did not have the right chemical
composition they will not grow.
Very typical index minerals are the
polymorphs of aluminosilicate.
Andalusite is stable at low pressure,
kyanite is stable at high pressure but
relatively low temperature and
sillimanite is stable at high
temperature.
Metamorphic facies and their mineral
assemblages 
= Zeolite facies =
The zeolite facies is the metamorphic
facies with the lowest metamorphic
grade. At lower temperature and pressure
processes in the rock are called
diagenesis. The facies is named for
zeolites, strongly hydrated
tectosilicates. It can have the
following mineral assemblages:
In meta-igneous rocks and greywackes:
heulandite + analcite + quartz ± clay
minerals
laumontite + albite + quartz ± chlorite
In metapelites:
muscovite + chlorite + albite + quartz
= Prehnite-pumpellyite-facies =
The prehnite-pumpellyite facies is a
little higher in pressure and
temperature than the zeolite facies. It
is named for the minerals prehnite and
pumpellyite. The prehnite-pumpellyite is
characterized by the mineral
assemblages:
In meta-igneous rocks and greywackes:
prehnite + pumpellyite + chlorite +
albite + quartz
pumpellyite + chlorite + epidote +
albite + quartz
pumpellyite + epidote + stilpnomelane +
muscovite + albite + quartz
In metapelites:
muscovite + chlorite + albite + quartz
= Greenschist facies =
The greenschist facies is at medium
pressure and temperature. The facies is
named for the typical schistose texture
of the rocks and green colour of the
minerals chlorite, epidote and
actinolite. Characteristic mineral
assemblages are:
In metabasites:
chlorite + albite + epidote ±
actinolite, quartz
In metagreywackes:
albite + quartz + epidote + muscovite ±
stilpnomelane
In metapelites:
muscovite + chlorite + albite + quartz
chloritoid + chlorite + muscovite +
quartz ± paragonite
biotite + muscovite + chlorite + albite
+ quartz + Mn-garnet
In Si-rich dolostones:
dolomite + quartz
= Amphibolite-facies =
The amphibolite facies is a facies of
medium pressure and average to high
temperature. It is named after
amphiboles that form under such
circumstances. It has the following
mineral assemblages:
In metabasites:
hornblende + plagioclase ± epidote,
garnet, cummingtonite, diopside, biotite
In metapelites:
muscovite + biotite + quartz +
plagioclase ± garnet, staurolite,
kyanite/sillimanite
In Si-dolostones:
dolomite + calcite + tremolite ± talc
dolomite + calcite + diopside ±
forsterite
= Granulite facies =
The granulite facies is the highest
grade of metamorphism at medium
pressure. The depth at which it occurs
is not constant. A characteristic
mineral for this facies and the
pyroxene-hornblende facies is
orthopyroxene. The granulite facies is
characterized by the following mineral
assemblages:
In metabasites:
orthopyroxene + clinopyroxene +
hornblende + plagioclase ± biotite
orthopyroxene + clinopyroxene +
plagioclase ± quartz
clinopyroxene + plagioclase + garnet ±
orthopyroxene
In metapelites:
garnet + cordierite + sillimanite +
K-feldspar + quartz ± biotite
sapphirine + orthopyroxene + K-feldspar
+ quartz ± osumilite
= Blueschist facies =
The blueschist facies is at relatively
low temperature but high pressure, such
as occurs in rocks in a subduction zone.
The facies is named after the schistose
character of the rocks and the blue
minerals glaucophane and lawsonite. The
blueschist facies forms the following
mineral assemblages:
In metabasites:
glaucophane + lawsonite + chlorite +
sphene ± epidote ± phengite ±
paragonite, omphacite
In metagreywackes:
quartz + jadeite + lawsonite ± phengite,
glaucophane, chlorite
In metapelites:
phengite + paragonite + carpholite +
chlorite + quartz
In carbonate-rocks:
aragonite
= Eclogite facies =
The eclogite facies is the facies at the
highest pressure and high temperature.
It is named for the metabasic rock
eclogite. The eclogite facies has the
mineral assemblages:
In metabasites:
omphacite + garnet ± kyanite, quartz,
hornblende, zoisite
In metagranodiorite:
quartz + phengite + jadeite/omphacite +
garnet
In metapelites:
phengite + garnet + kyanite + chloritoid
+ quartz
phengite + kyanite + talc + quartz ±
jadeite
= Albite-epidote-hornfels facies =
The albite-epidote-hornfels facies is a
facies at low pressure and relatively
low temperatures. It is named for the
two minerals albite and epidote, though
they are also stable in other facies.
Hornfels is a rock formed by contact
metamorphism, a process that
characteristically involves high
temperatures but low pressures/depths.
This facies is characterized by the
following minerals:
In metabasites:
albite + epidote + actinolite + chlorite
+ quartz
In metapelites:
muscovite + biotite + chlorite + quartz
= Hornblende-hornfels facies =
The hornblende-hornfels facies is a
facies with the same low pressures but
slightly higher temperatures as the
albite-epidote facies. Though it is
named for the mineral hornblende, the
appearance of that mineral is not
constrained to this facies. The
hornblende-hornfels facies has the
following mineral assemblages:
In metabasites:
hornblende + plagioclase ± diopside,
anthophyllite/cummingtonite, quartz
In metapelites:
muscovite + biotite + andalusite +
cordierite + quartz + plagioclase
In K2O-poor sediments or meta-igneous
rocks:
cordierite + anthophyllite + biotite +
plagioclase + quartz
In Si-rich dolostones:
dolomite + calcite + tremolite ± talc
= Pyroxene-hornfels facies =
The pyroxene-hornfels facies is the
contact-metamorphic facies with the
highest temperatures and is, like the
granulite facies, characterized by the
mineral orthopyroxene. It is
characterized by the following mineral
assemblages:
In metabasites:
orthopyroxene + clinopyroxene +
plagioclase ± olivine or quartz
In metapelites:
cordierite + quartz + sillimanite +
K-feldspar ± biotite ± garnet
(If the temperature is below 750 °C
there will be andalusite instead of
sillimanite)
cordierite + orthopyroxene + plagioclase
± garnet, spinel
In carbonate rocks:
calcite + forsterite ± diopside,
periclase
diopside + grossular + wollastonite ±
vesuvianite
= Sanidinite facies =
The sanidinite facies is a rare facies
of extremely high temperatures and low
pressure. It can only be reached under
certain contact-metamorphic
circumstances. Due to the high
temperature the rock experiences partial
melting and glass is formed. This facies
is named for the mineral sanidine. It is
characterized by the following mineral
assemblages:
In metapelites:
cordierite + mullite + sanidine +
tridymite + glass
In carbonates:
wollastonite + anorthite + diopside
monticellite + melilite ± calcite,
diopside.
References 
Eskola, Pentti Eelis, 1920: "The mineral
facies of rocks"
Phillpots, Anthony R., 1990: Principles
of Igneous and Metamorphic Petrology
Duff, P. McL. D., 1996; Holmes'
Principles of Physical Geology
Visser, W.A., 1980; Geological
Nomenclature
Metamorphic facies by Dave Waters
See also 
metamorphic rock
metamorphism
