Metaminerals PDF
Metaminerals PDF
Metaminerals PDF
The mineral assemblages that occur in metamorphic rocks depend on four factors:
z The composition of any fluid phase that was present during metamorphism.
If a rock is taken to some higher pressure and temperature then the mineral assemblage that
develops should represent stable chemical equilibrium if the conditions are held for a long
enough period of time that equilibrium can be achieved. Since metamorphism usually involves
long periods of geologic time, most metamorphic rocks represent an equilibrium mineral
assemblage.
F =C+2-P
If you think about it, in metamorphic rocks where temperature and pressure can both vary
during metamorphism, the most likely case would be to find a divariant (F=2) assemblage of
phases. A univariant assemblage (F=1) would be less likely to occur, and an invariant
assemblage (F=0) would represent equilibrium at a fixed point in temperature and pressure, and
would thus be even less likely to occur.
So, for F=2, C=P, the number of phases present in a rock for the more common divariant
assemblage will be equal to the number of components. If P is greater than C, then one of three
possibilities exist for the mineral assemblage.
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If possibility (1) is the reason for the lack of correspondence with the phase rule, it can usually
be determined by close inspection of the rock. Reaction textures present in the rock might
indicate incomplete reaction. Known retrograde minerals, i.e. those stable at lower pressures
and temperatures than the rest of the minerals in the rock, could be identified. These retrograde
phases could then be subtracted from the number of phases being considered and the phase rule
could be reapplied to only the phases known to be in equilibrium. (For example, the presence
of chlorite in amphibolite and granulite facies rocks would be indicative that the chlorite is a
retrograde mineral or mineral produced during weathering, and thus would not be considered in
the application of the phase rule.)
Possibility 2 could always occur, and if the number of components is chosen correctly and
retrograde minerals are not considered, then this may be the case.
The number of components, as stated in the phase rule, must be chosen so as to represent the
minimum number necessary to form all phases possible in the rock. Recall that the number of
components is not strictly the number of oxide components or the number of elements as
reported in a chemical analysis of the rock. If we just consider the major phases that make up
metamorphic rocks and consider that some ions freely substitute for one another in solid
solutions, then the number of components can often be reduced to 7 or 8. For example:
5. MgO - usually needed because Fe-Mg solid solution compositions are both temperature
and pressure dependent. (although sometimes these two are combined, which would
reduce the total number of components by 1).
7. H2O - usually present in a fluid phase, but also an important component of hydrous
minerals.
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8. CO2 - also usually present in a fluid phase, but also an important component in carbonate
minerals.
If H2O and CO2 are assumed to be always present and available to form hydrous and carbonate
minerals, then the number of components can be reduced to 5 or 6. Thus for a divariant
assemblage (F=2) we would expect to find 5 or 6 different mineral phases present in a
metamorphic rock, or up to 8 phases if the assemblage is invariant.
This is the basis for the construction of the AKF and ACF diagrams discussed previously,
where the number of components have been reduced to 4, by making assumptions like quartz
and alkali feldspar can always be present. Still, you are cautioned that the above analysis is not
always generally applicable, and each rock must be considered on a case-by case scenario.
Progressive or prograde metamorphism occurs as the temperature and pressure are increased
on the rock. As the pressure and temperature increase, a rock of a given chemical composition
is expected to undergo a continuous series of chemical reactions between its constituent
minerals and any fluid phase present to produce a series of new mineral assemblages that are
stable at the higher pressures and temperatures. We here illustrate how the mineral
assemblages might change in a hypothetical set of rocks, starting with a low grade mineral
assemblage as shown in the ACF diagram below.
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Next, let's try to write the chemical reactions that must have occurred between the two sets of
pressure/temperature conditions that would explain the new mineral assemblages.
For the disappearance of andalusite and appearance of sillimanite the reaction is simple:
Epidote (zoisite) would break down to produce the anorthite component of plagioclase and
grossularite, resulting in the expulsion of water as a fluid phase:
Chlorite could have reacted with muscovite and quartz to produce biotite, cordierite, and fluid:
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and chlorite, actinolite, epidote (zoisite), and quartz would react to produce hornblende and
fluid:
7(Mg,Fe)5Al2Si3O10(OH)2+13Ca2(Mg,Fe)5Si8O22(OH)2+12Ca2Al3Si3O12(OH)+14SiO2 =>
Chlorite Actinolite epidote (Zoisite) Qtz
Next, let's increase the temperature and pressure so that a new set of minerals develops for each
rock.
Again, we can make a list of the phases that disappeared and those that appeared at some point
between the two pressure/temperature conditions.
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Exploring the reactions that must have taken place to explain the new mineral assemblage, we
proceed as follows:
For the disappearance of calcite and the appearance of wollastonite we can write:
For the breakdown of biotite to form hypersthene and k-spar the following reaction must have
occurred:
And finally, for the formation of almandine/pyrope from cordierite and anthophyllite, the
following reaction could have occurred:
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Retrograde Metamorphism
If retrograde metamorphism were a common process then upon uplift and unroofing
metamorphic rocks would progressively return to mineral assemblages stable at lower pressures
and temperatures. Yet, high grade metamorphic rocks are common at the surface of the Earth
and usually show only minor retrograde minerals. Three factors inhibit retrograde
metamorphism, two of which involve the fluid phase.
2. During prograde metamorphism, as we have just seen, a fluid phase is driven off as a
result of the devolatilization reactions. As pressure increases, porosity of rocks also
decreases, and thus this fluid phase will likely be driven out of the rock body. In the
absence of the fluid phase it is impossible to form hydrous minerals and carbonates, since
H2O and CO2, two of the key components needed in such reactions, may not be present.
3. The fluid phase also helps to catalyze chemical reactions. Although the net reactions may
appear to be solid-solid reactions, in reality there may be more involved. For example
the fluid phase could dissolve a mineral in one part of the rock and precipitate a new
mineral in another part of the rock, just as happens during diagenesis of sedimentary
rocks. If the fluid phase is driven off during prograde metamorphism, then it will not be
available to catalyze the reactions to produce the retrograde mineral assemblage as
pressure and temperature are lowered.
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Given enough time, all metamorphic rocks will eventually change to an assemblage of minerals
stable under conditions present near the surface of the Earth.
This process, however, is called weathering, and occurs near the earth's surface..
Next time we will explore in more detail some the factors that govern the chemical reactions
that occur during metamorphism.
1. What faactors are responsible for the mineral assemblages that develop in metamorphic
rocks?
2. Given a sequence of triangular diagrams, be able to deduce and write the chemical
reactions that must have occurred as the pressure and temperature conditions changed
during progressinve metamorphism.
4. What is the ultimate fate of metamorphic rocks that have reached the surface of the earth
and why does this occur?
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