Mineral reactions and equilibria

1
Mineral reactions and equilibria

• Paragenesis mineral assemblages that are in
  equilibrium
• Paragenesis found through experimental studies.
• Criteria for evaluating equilibrium
• Absence of known incompatibles hematite and
  graphite, ol and Qtz
• All phases are in mutual contact
• No evidence of replacement
• Absence of domains showing deformation next to
  strain free grains
• Grain shapes indicative of minimum surface area
• No zoned grains

Metamorphic mineral reactions

• Solid-solid reactions
• Solid-fluid reactions (redox reactions,
  metasomatic reactions)
• Discontinuous reactions Univariant
• Polymorphic transition (calcite-aragonite)
• Net transfer (heterogeneous reactions) movement
  of matter between phases
• Contineous reaction continuously adjusting
  equilibrium solid solution

2
Polymorphic transitions
Example Quartz-coesite, graphite-diamond,
calcite-aragonite Independent of the bulk
composition Al2SiO5-system kyanite-sillimanite-an
dalusite V(J/bar) S(J/K) H(kJ) Andalusite 5.146
91.6 -2589.66 Sillimanite 4.984 95.08 -2586.37 Kya
nite 4.408 82.86 -2593.70
Difficulty achieving phase change. Al-diffusion
is slow. Sillimanite nucleation on muscovite
grains fibrolite
3
Al2SiO5
V(J/bar) S(J/K) H(kJ) Andalusite 5.146 91.6 -258
9.66 Sillimanite 4.984 95.08 -2586.37 Kyanite 4.4
08 82.86 -2593.70
Kyanite smallest molar volume high P
phase Sillimanite largest entropy high T phase
kyanite?andalusite
Clapeyron equation
Difficult to obtain equilibrium.
4
Solid-solid reactions
Reactions stability fields in P-T space with
reactions delineating the fields Pure solid
phases Ca3Al2Si3O12 SiO2?CaAl2Si2O82CaSiO3.
High T (greater entropy and molar volume) on
right V(J/bar) S(J/K) H(kJ) G(kJ) Anorthite 10.0
8 199.3 -4234 -4008 Grossular 12.53 260 -6640 -627
9 Wollastonite 3.99 81.7 -1635 -1549 Quartz 2.27
41.5 -911 -856 Coesite 2.06 38.5 -908 -853
Allows determination of the slope on P-T diagram
At equilibrium ?G0. ?G?H-T?S.
Or,
At equilibrium ?G0 and at Tref it simplifies to
Peq at 250C-8858.6 bar
5
Solid-solid reactions contd
Stability field of a phase, or phase assemblage,
shrinks in the presence of an additional reacting
phase.
Stick with te reaction Ca3Al2Si3O12
SiO2?CaAl2Si2O82CaSiO3.
Example 15kb-8000C. If kyanite present there
will be no gross and qtz. Boundary line
univariant equilibrium four phases, three
components (CaO-SiO2-Al2O3). Or anorthite or
wollasonite can exist at that P and T, but not
together
Reaction results in switching tielines
6
Basalt-granulite-eclogite
Olivine and plag not stable
6. 2Mg2SiO4?2MgSiO3MgO and CaAl2Si2O82MgO?CaMgSi
2O6MgAl2O4
S(2FoAn)387.5J/K S(2EnDiSp)364.0J/K V(2FoAn
)188.09cm3 V(2EnDiSp)168.42cm3
Next reaction 7. 3Mg2SiO44CaAl2Si2O8?2MgSiO3.Mg
Al(AlSi)O6 2CaMgSi2O6.CaAl(AlSi)O6SiO2 At
higher P Al in tetrahedral site Tschermak
component 8. NaAlSi3O8?NaAlSi2O6SiO2 9.
CaAl2Si2O82(Mg,Fe)SiO3? Ca(Mg,Fe)2Al2Si3O12SiO
2 10.3CaAl2Si2O8?Ca3Al2Si3O122Al2SiO5SiO2
Reaction 9 Plag-Opx tieline disappears, replaced
by Cpx-Gt
Gt
7
Basalt-granulite-eclogite contd
8
Contineous reactions
Solid solutions make many reactions
contineous Plagioclasejadeitic cpxquartz
albiteJadeite quartz Jadeite-diopside solid
solution Na-AlCa-Mg and Fe3Al. Exact
substitution hard to define Lower jadeite
fraction moves reaction to lower
P ?GGJdGQtz-GAb0. Which is for the pure
phases. However, for solid solutions
Remember ?GRTlnKeq.
Activities must be evaluated
9
Contineous reactions contd
Fe-Mg Solid solutions In pellitic systems The
Mg/Fe ratio between two minerals is constant over
large X-range Distribution coeficientKD is
constant for given P,T. For garnet-biotite
XMgMg/(MgFe2) always molar
10
Fe-Mg exchange contd
In Fe-endmembers 17. Fe-chloritequartzmuscovite
almandineannitewater Five components (Fe, Si,
Al, Ca, H-oxide), six phases univariant Add
MgO?divariant Continuous reaction 18.
Chloritequartzmuscovitegarnetbiotite
water Contains two exchange reactios 19.
Fe3Al2Si3O12KMg3AlSi3O10(OH)2Mg3Al2Si3O12KFe3Al
Si3O10(OH)2, and 20. Fe3Al2Si3O12Mg5AlSi3AlO10(OH
)8Mg3Al2Si3O12Fe5AlSi3AlO10(OH)8. KD for Fe/Mg
for garnet biotite is strongly dependent on T,
excellent thermometer
Heating of chlorite, quartz, muscovite rock. At
lower T Fe-rich minerals are formed, with
increasing T more Mg-rich
11
Fe-Mg exchange contd

• Result two contrasting paths of crystallization
• Equilibrium, every phase is homogeneous.
  Reaction complete when muscovite is consumed.
• Fractional crystallization Slow rates of
  diffusion in garnet?zoned garnets
• XMg increases outwards prograde zonation, XMg
  decreases outwards retrograde zonation.

12
Mineral-fluid reactions

• Three types of crustal fluids
• Hydrosphere
• Metamorphic
• Magmatic
• Sediment up to 30 pore aaspace/water, most
  released during burial
• With increasing to structurally bound water
  (5-10wt) can be driven ofdehydration
• Prograde sequence in pellitic rocks
• clay minerals?chlorites?micas?anhydrous silicates

13
Solid-fluid reactions
Entropy-volume relations
Volatile phase has larger partial molar volume
then when bound in a hydrous silicate. Separate
volatile phase always at the high T-side
positive ?T needs to be combined with positive ?S
for ?G-?S?T to be negative. ?V is positive so
positive slope on P-T diagram at low P. Effect
of increasing P larger on volatile phase results
in steepening univariant line. At high P fluid
can be compressed enough so ?Vlt0
Stability field shrinks in the presence of other
reactable phases Pure calcite CaCO3CaOCO2, at
gt12000C. 23. CaCO3SiO2CaSiO3CO2 at 6000C
14
Mixed fluids
PvolatileltPtotal
Either mixed fluids of not enough volatile phase
present. If PfluidPH2OPCO2, then aH2Olt1. The
composition of the fluid is an explicit variable
in mineral equilibria, in addition to P and
T. 23. CaCO3SiO2CaSiO3CO2 If PCO2ltPfluid,
XCO2lt1, then reaction 23 is forced to the
right Rule where the partial pressure of a
reacting volatile phase is ltPfluid the stability
field of the phase that contains the volatile
shrinks to lower T. Fluid is mobile and can
escape. Rule if there is insufficient volatile
to complete a reaction, the unreacted excess
solid remains stable with the volatile-bearing
product assemblage. 24.
?VS change in volume of solids
Dependence shown in fig b.
15
Other mixed fluid reactions

• Generalized mixed volatile reaction
• ABiH2OjCO2
• Solid-solid reaction grossularquartzwollastonit
  eanorthite, no volatiles in products or
  reactants, equi independent fluid composition
• Decarbonation reaction equilibrium T highest when
  XCO21.
• Dehydration reaction, as 2.
• Reaction releasing CO2 and H2O
  remolite3calcite2 quartz5diopsideH2O3CO2.
  Univariant reaction at highest T when H2O and CO2
  present in abundance of the reaction
  stoichiometry
• CO2 as reactant and H2O as product
  2zoisiteCO23anorthitecalciteH2O
• H2O as reactant, CO2 as product
  6dolomite8quartz2H2Otalc6calcite6CO2.

16
Buffered devolatilization
Closed system heating system has to follow the
univariant curve untill one of the phases s
depleted or Tmax is reached. At Tmax reaction
progresses until one of the phase is exhausted
Open system I fluid of same composition fluxes
through the system as rock is heated reaches
univariant line 3 components, 4 (solid phases),
isobaric, no freedom. System stays at 525C until
a phase is exhausted. II Fluid with XH2O1
fluxes through the system. At XCO20.3 system is
halted till one phase is exhausted. Fluid
composition drives the devolatization .
17
Fluid flow in continental crust

• Evidence
• Dischare from hydrothermal fields (Salton Sea)
• Veins in metamorphic terrains
• Stable isotope studies
• Fluid-induced mineral reactions in shear zones
  transform plagioclaseAl-rich pyroxenesgarnet to
  omphacite, garnet, kyanite, clinozoisite,
  phengite, amphibole and quartz
• Hydration reactions in contact aureoles

18
Contact aureole
19
Mechanics of fluid flow

• Porosity amount of pore space
• Permeability amount of interconnected pore space
• In most metamorphic rocks dihedral angle gt600,
  fluid will pool at multiple grain corners
• Reaction enhanced permeability products that are
  devolatilized have smaller volume then their
  volatile-bearing counterparts.
• 3 reduction for muscovitequartzsillimanite
  K-feldsparH2O
• 33 reduction for calcitequartzwollastoniteCO2.
  
• Fractures causes
• Intercrystalline mismatchesduring heating
• Non-hydrstatic pressure
• Fluid overpressure

Healed microcracks

• Driving force for fluid flow
• Flow of fluid through porous medium quantified by
  Darcys Law
• is viscosity (10-4 Pas for H2O-CO2 fluids), VD
  is volume flux volume of flow per unit area per
  unit time.
• Geothermal gradients gt200C/km density of water
  decreases with depth

20
Metasomatism

• Process where distance from source to sink is
  larger than grain scale.
• But is a local process, often associated with
  contact metamorphism
• Significant change in bulk composition.
• Three unknowns
• Character of protolith
• Gains and losses of elements
• Change in volume.
• Al2O3, TiO2 and HFSE assumed to be immobile
• Rock layers along strike can divulge protolith
• Solubility of components vary greatly, dependent
  on P,T, X
• Greater Cl concentrations enhances solubility,
  especially alkalies
• Silicates least soluble, chlorides, sulfates,
  carbonates higher solubility
• Ion-exchange reactions
• AX B BX A.
• solid fluid solid fluid

21
Ion exchange reactions
Common alkali exchange reactions 28.
KAl2AlSi3O10(OH)26SiO22K3KAlSi3O82H
muscovite fluid K-fsp
fluid 29. KAl2AlSi3O10(OH)22H3Al2SiO53SiO
23H2O2K muscovite
fluid fluid Pure phases activity1 for
29 High H stabilizes alumino silicates over
feldspars High H in granitic system results in
bleaching removal Of Na, K, Ca, Mg and Fe. In
initial stages sericitization white micas and
clay minerals stable (expense of feldspar) 31.
2KAlSi3O82HH2OAl2Si2O5(OH)44SiO22K
Biotite, amphibole and pyroxene replaced by
epidote and aluminous chlorite. Continued
alterationfeldspar and micas replaced by
pyrophyllite, andalusite. High S activity
pyrite High F activity topaz, High B activity
tourmaline
22
Isocon diagrams
Provide a frame of reference What is removed,
added, diluted and concentrated? Plot
concentration in metasomatized material versus
probable protolith. Use TiO2, Al2O3, MnO and Zr
as immobile elements Iso con connects these
elements of equal geochemical concentration
23
Redox equilibria
Oxygen fugacity. In the presence of free water,
at 2kbar 3000C 10-11bar, 10000C 10-3barr In crust
near NNO (nickel-nickel oxide) or QFM
(quartz-fayalite-magnetite) At the surface
hematite. Generalized reaction Fe2-rich
hydrous silicateO2Mg-rich hydrous
silicateFe3-oxides Carbon can buffer O
carbonate-graphite-methane Sulfur can buffer O
sulfate-sulfide Although metamorphic rocks tend
to keep the oxidation state of protolith
Also, increasing FeO/Fe2O3 with increasing
grade. Possible reaction 30 C2Fe2Fe23O4KAlSi
3O10(OH)23SiO2 magnetite
muscovite KAlFe62Al2Si6O10(OH)2Fe32Al2Si
3O12CO2 annite almandine
24
Kinetics

• General principle reaction cannot proceed unless
  there is a finite amount of disequilibrium
• Reaction affinity ?S(T-Teq) where (T-Teq) is
  called overstepping.
• Large ?S requires less overstepping.
• Dehydration reactions often require just a few
  degrees of overstepping
• Solid-solid tens of degrees (Al-polymorphs)
• Sometimes metastable intermediates speed up the
  reaction
• Example
• At 1 bar 8200C talc decomposes to anthophyllite
  which subsequently breaks down to
  enstatite-quartz-H2O.
• Three processes need for metamorphic eaction
• Breakdown of unstable phase
• Transport of ions
• Growth of new phase
• The slowest rate determines the overall reaction.
• Reactive surface area high- higher reaction rate.
  

25
Role of fluids in kinetics
Al2SiO5 polymorph transitions under dry
conditions extremely slow In most rocks
andalusite and sillimanite occur in different
domains Common texturefibrolitic sillimanite
intergrown with muscovite.
Domain I sillimanite XX Domain II Kyanite
breakdown
In general terms (Carmichael) given a choice
between a number of possible reaction paths, a
natural metamorphic reaction will proceed to
completion by means of that path for which the
activation energy is lowest, provided that
overstepping of the equilibrium T is too small to
activate other paths.
26
Petrological implications
Isograds mapable zones of stability fields and
their boundaries. Simplest isograd polymorph
independent of bulk composition
Increasing T garnet more Mg-rich
garnetchloritemuscovite staurolitebiotitequar
tzwater
27
P-T estimates

• Calibration with experiments
• Thermobarometers
• Fe-Ti-oxides magnetite-ilmenite (fO2 and T)
  magmatic and metamorphic
• Ternary feldspars and pyroxenes high grade
  metamophic-magmatic
• Jadeite endmember in cpx coexisting with plag and
  qtz
• For amphibolite facies rocks coexisting
  garnetbiotiteplagioclaseAl2SiO5quartz
• For pure endmembers
• Garnet-biotite Fe-Mg exchange
• 19. Fe3Al2Si3O12KMg3AlSi3O10(OH)2Mg3Al2Si3O12KF
  e3AlSi3O10(OH)2
• almandine phlogopite
  pyrope annite
• Small ?V makes it insensitive to P and large
  ?S/?VdP/dT
• 37. 3CaAl2Si2O8Ca3Al2Si3O122Al2SiO5SiO2 GASP
• anorthite grossular
• Heterogeneous reaction large ?V, small
  ?S/?VdP/dT insensitive to T.
• At equilibrium RTlnKeq-(?H-T?S)?V(P-Pref) and
• In ideal solutions activities can be cast in
  terms of mole fraction

and
28
P-T estimates contd
Or,
42. 52,112-19.51T0.238P3RTlnKD
For 37. 3CaAl2Si2O8Ca3Al2Si3O122Al2SiO5SiO2
44. -48,357150.66T(P-1)(-6.608)RTlnKeq0