The%20Phase%20Rule%20in%20Metamorphic%20Systems

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The Phase Rule in Metamorphic Systems

• Consider the following three scenarios

• C 1 (Al2SiO5)
• F 1 common
• F 2 rare
• F 3 only at the specific P-T conditions of the
  invariant point
• ( 0.37 GPa and 500oC)

Figure 21-9. The P-T phase diagram for the system
Al2SiO5 calculated using the program TWQ
(Berman, 1988, 1990, 1991). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Representing Mineral Reactions

• albite ? jadeite quartz

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From Hacker, B.R.,
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Lets put it all together
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• What if we had staurolite and andalusite
  together? What conditions would that indicate?

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Metamorphic facies

• P-T conditions, presence of fluids induces
  different metamorphic mineral assemblages
  (governed by thermodynamics/ kinetics)
• These assemblages are lumped into metamorphic
  facies (or grades)

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Aluminosilicate Minerals

• SILLIMANITE Orthorhombic Octahedral Al chains
  (6-fold) are crosslinked by both Si and Al
  tetrahedra (4-fold).
• ANDALUSITE Orthorhombic 5-coordinated Al Same
  octahedral (6-fold) chains.
• KYANITE Triclinic All the Al is octahedrally
  coordinated (6- and 6-fold).

Andalusite
Kyanite
Sillimanite

• Clearly, changes in structure are in response to
  changing P and T. Result is changes in Al
  coordination.
• Phase transformations require rebonding of Al.
  Reconstructive polymorphism requires more energy
  than do displacive transformations. Metastability
  of these 3 are therefore important (Kinetic
  factors limit equilibrium attainment).
• All 3 are VERY important metamorphic index
  minerals.

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Aluminosilicate Minerals

• 3 polymorphs of Al2SiO5 are important metamorphic
  minerals

Andalusite
Kyanite
Sillimanite
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Topaz

• Aluminosilicate mineral as well, one oxygen
  substituted with OH, F
• Al2SiO4(F,OH)2
• Where do you think Topaz forms??

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Serpentine Minerals

• Mg3Si2O5(OH)4 minerals (principally as
  antigorite, lizardite, chrysotile polymorphs)
• Forms from hydration reaction of magnesium
  silicates
• Mg2SiO4 3 H2O ? Mg3Si2O5(OH)4 Mg(OH)2
• forsterite serpentine brucite
• Asbestosform variety is chrysotile (accounts for
  95 of worlds asbestos production ? MUCH LESS
  DANGEROUS than crocidolite)

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Phyllosilicates
T O - T O - T O
Yellow (OH)
vdw
Serpentine Mg3 Si2O5 (OH)4 T-layers and
triocathedral (Mg2) layers (OH) at center of
T-rings and fill base of VI layer ?
vdw
weak van der Waals bonds between T-O groups
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Serpentine
Antigorite maintains a sheet-like form by
alternating segments of opposite curvature
Chrysotile does not do this and tends to roll
into tubes
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Serpentine
Veblen and Busek, 1979, Science 206, 1398-1400.
S serpentine T talc
Nagby and Faust (1956) Am. Mineralogist 41,
817-836.
The rolled tubes in chrysotile resolves the
apparent paradox of asbestosform sheet silicates
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Chlorite

• Another phyllosilicate, a group of difficult to
  distinguish minerals
• Typically green, and the dominant and
  characteristic mineral of greenschist facies
  rocks
• Forms from the alteration of Mg-Fe silicates
  (pyroxenes, amphiboles, biotite, garnets)
• Clinochlore, chamosite,
• pennantite, nimmite end members
• Chloritoid - Similar in appearance to chlorite,
  but different 2V and relief

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Prehnite-Pumpellyite

• Low-grade metamorphic minerals
• Minerals related to chlorite, form at slightly
  lower P-T conditions
• Prehnite is also green, pumpellyite green too,
  varies based on Fe content
• Prehnite chlorite ? pumpellyite quartz

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Micas

• Biotite and Muscovite are also important
  metamorphic minerals (muscovite often the
  principle component of schists)
• Phlogopite similar to biotite, but has little
  iron, forms from Mg-rich carbonate deposits and a
  common mineral in kimberlites (diamond-bearing
  material)
• Sericite white mica (similar to muscovite)
  common product of plagioclase feldspar alteration
  at low grades

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Zeolites

• Diverse group of minerals forming at lower
  metamorphic grades
• Framework silicas, but characteristically
  containing large voids and highly variable
  amounts of H2O
• Name is from the greek meaning to boil stone as
  the water can de driven off with heat
• Voids can acts as molecular sieves and traps for
  many molecules
• Diversity of minerals in this group makes a for a
  wide variety of sieve and trapping properties
  selective for different molecules

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Epidote Group

• Sorosilicates (paired silicate tetrahedra)
• Include the mineral Epidote Ca2FeAl2Si3O12(OH),
  Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
  (polymorph)

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• Garnets

Garnet A23 B32 SiO43 Pyralspites - B
Al Pyrope Mg3 Al2 SiO43 Almandine Fe3 Al2
SiO43 Spessartine Mn3 Al2 SiO43
Ugrandites - A Ca Uvarovite Ca3 Cr2
SiO43 Grossularite Ca3 Al2 SiO43
Andradite Ca3 Fe2 SiO43 Occurrence Mostly
metamorphic Some high-Al igneous Also in some
mantle peridotites
Garnet (001) view blue Si purple A
turquoise B
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Staurolite

• Aluminosilicate - Fe2Al9Si4O22(OH)2
• Similar structure to kyanite with tetrahedrally
  coordinated Fe2 easily replaced by Zn2 and Mg2
• Medium-grade metamorphic mineral, typically
  forms around 400-500 C
• chloritoid quartz staurolite garnet
• chloritoid chlorite muscovite staurolite
  biotite quartz water
• Degrades to almandine (garnet at higher T)
• staurolite muscovite quartz almandine
  aluminosilicate biotite water

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Metamorphic chain silicates

• Actinolite and tremolite are chain silicates
  derived from dolomite and quartz and common in
  low-mid grade metamorphic rocks
• Riebeckite and Glaucophane are also chain
  silicates higher grade minerals, often a blue
  color
• These minerals usually lower P, higher T
  conditions

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Metamorphic Facies

• Where do we find these regimes of P-T off of
  the typical continental isotherms??
• How is the environment that forms a blueschist
  facies rock different from one forming a hornfels?

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Metamorphic Facies

• Table 25-1. The definitive mineral assemblages
  that characterize each facies (for mafic rocks).

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Lets put it all together
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Facies Series

• Miyashiro (1961) initially proposed five facies
  series, most of them named for a specific
  representative type locality The series were
• 1. Contact Facies Series (very low-P)
• 2. Buchan or Abukuma Facies Series (low-P
  regional)
• 3. Barrovian Facies Series (medium-P regional)
• 4. Sanbagawa Facies Series (high-P, moderate-T)
• 5. Franciscan Facies Series (high-P, low T)

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Fig. 25-3. Temperature-pressure diagram showing
the three major types of metamorphic facies
series proposed by Miyashiro (1973, 1994). Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Isograds

• Lines (on a map) or Surfaces (in the 3D world)
  marking the appearance or disappearance of the
  Index minerals in rocks of appropriate
  compositione.g. the garnet-in isograd the
  staurolite-out isogradComplicated by the fact
  that most of these minerals are solid solutions

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• Isograds for a single shale unit in southern
  Vermont
• Which side reflects a higher grade, or higher P/T
  environment?