Transcript Lecture 32- Metamorphic Minerals

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.
Representing Mineral Reactions
• albite  jadeite + quartz
From Hacker, B.R.,
Let’s put it all together…
• What if we had staurolite and andalusite
together? What conditions would that indicate?
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)
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 (6fold) chains.
• KYANITE: Triclinic: All the Al is octahedrally coordinated (6- and 6fold).
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.
Aluminosilicate Minerals
• 3 polymorphs of Al2SiO5 are important
metamorphic minerals
Andalusite
Kyanite
Sillimanite
Topaz
• Aluminosilicate mineral as well, one oxygen
substituted with OH, F
• Al2SiO4(F,OH)2
• Where do you think Topaz forms??
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)
Phyllosilicates
Yellow = (OH)
Serpentine: Mg3 [Si2O5] (OH)4
T-layers and triocathedral (Mg2+) layers
(OH) at center of T-rings and fill base of VI layer 
weak van der Waals bonds between T-O groups
T
O
T
O
T
O
vdw
vdw
Serpentine
Antigorite maintains a
sheet-like form by
alternating segments of
opposite curvature
Chrysotile does not do this
and tends to roll into tubes
Octahedra are a bit larger than tetrahedral
match, so they cause bending of the T-O
layers (after Klein and Hurlbut, 1999).
Serpentine
Nagby and Faust (1956) Am.
Mineralogist 41, 817-836.
Veblen and Busek, 1979,
Science 206, 1398-1400.
S = serpentine T = talc
The rolled tubes in chrysotile resolves the apparent
paradox of asbestosform sheet silicates
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
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
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
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
Epidote Group
• Sorosilicates (paired silicate tetrahedra)
• Include the mineral Epidote Ca2FeAl2Si3O12(OH),
Zoisite (Ca2Al3Si3O12(OH) and clinozoisite
(polymorph)
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
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
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
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?
Metamorphic Facies
• Table 25-1. The definitive mineral assemblages
thatTable
characterize
eachAssemblages
facies (for
mafic Facies
rocks).
25-1. Definitive Mineral
of Metamorphic
Facies
Zeolite
Definitive Mineral Assemblage in Mafic Rocks
zeolites: especially laumontite, wairakite, analcime
Prehnite-Pumpellyite
prehnite + pumpellyite (+ chlorite + albite)
Greenschist
chlorite + albite + epidote (or zoisite) + quartz ± actinolite
Amphibolite
hornblende + plagioclase (oligoclase-andesine) ± garnet
Granulite
orthopyroxene (+ clinopyrixene + plagioclase ± garnet ±
hornblende)
Blueschist
glaucophane + lawsonite or epidote (+albite ± chlorite)
Eclogite
pyrope garnet + omphacitic pyroxene (± kyanite)
Contact Facies
After Spear (1993)
Mineral assemblages in mafic rocks of the facies of contact metamorphism do not differ substantially from that of the corresponding
regional facies at higher pressure.
Let’s put it all together…
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)
Fig. 25-3.
Temperaturepressure 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.
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 ‘stauroliteout isograd’
Complicated by the fact that most of these
minerals are solid solutions
• Isograds for a
single shale unit
in southern
Vermont
• Which side
reflects a higher
grade, or higher
P/T environment?