1. 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 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.

2. Representing Mineral Reactions
albite  jadeite + quartz

3. From Hacker, B.R.,

4. Let’s put it all together…

5. What if we had staurolite and andalusite together
What if we had staurolite and andalusite together? What conditions would that indicate?

6. 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)

8. 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.

9. Aluminosilicate Minerals
3 polymorphs of Al2SiO5 are important metamorphic minerals
Andalusite
Kyanite
Sillimanite

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

11. 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 world’s asbestos production  MUCH LESS DANGEROUS than crocidolite)

12. Phyllosilicates T O - T O - T O Serpentine: Mg3 [Si2O5] (OH)4
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

13. Chrysotile does not do this and tends to roll into tubes
Serpentine
Octahedra are a bit larger than tetrahedral match, so they cause bending of the T-O layers (after Klein and Hurlbut, 1999).
Antigorite maintains a sheet-like form by alternating segments of opposite curvature
Chrysotile does not do this and tends to roll into tubes

14. Serpentine
Veblen and Busek, 1979, Science 206,
S = serpentine T = talc
Nagby and Faust (1956) Am. Mineralogist 41,
The rolled tubes in chrysotile resolves the apparent paradox of asbestosform sheet silicates

15. 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

16. 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

17. 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

18. 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

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

20. Garnets Garnet: A2+3 B3+2 [SiO4]3 “Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
Garnet (001) view blue = Si purple = A turquoise = B

21. 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 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

22. 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

23. 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?
The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed
They are thus separated by mineral reaction isograds
The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase
The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

24. Metamorphic Facies
Table The definitive mineral assemblages that characterize each facies (for mafic rocks).
The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table (details and variations later)

25. Let’s put it all together…

26. 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)
The contrast between Miyashiro’s higher-T-lower-P Ryoke-Abukuma belt (Section ) and the classical Barrovian sequence also led him to suggest that more than one type of facies series is probable
Metamorphic field gradients are highly variable (even in the same orogenic belt) and transitional series abound
Miyashiro later chose to limit the number instead to the three broad major types of facies series (also called “baric types”) illustrated in Fig. 25-3
Although the boundaries and gaps between them are somewhat artificial, the three series represent common types of tectonic environments

27. Fig 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.
The high P/T series, for example, typically occurs in subduction zones where “normal” isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermally
The facies sequence here is (zeolite facies) - (prehnite- pumpellyite facies) - blueschist facies - eclogite facies.
The medium P/T series is characteristic of common orogenic belts (Barrovian type)
The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies)
Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2)
The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid
The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan or Ryoke-Abukuma type), rift areas, or contact metamorphism
The sequence of facies may be a low-pressure version of the medium P/T series described above (but with cordierite and/or andalusite), or the sequence (zeolite facies) - albite-epidote hornfels facies - hornblende hornfels facies - pyroxene hornfels facies
Sanidinite facies rocks are rare, requiring the transport of great heat to shallow levels

28. Isograds
Lines (on a map) or Surfaces (in the 3D world) marking the appearance or disappearance of the Index minerals in rocks of appropriate composition e.g. the ‘garnet-in isograd’; the ‘staurolite-out isograd’ Complicated by the fact that most of these minerals are solid solutions

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