Prograde metamorphism, in general, is the term used to describe changes in mineral assemblage and mineral composition that take place during burial and heating, whereas retrograde metamorphism refers to changes that take place during rock uplift and cooling. The maintenance of thermodynamic equilibrium would require that one…
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What exactly does retrograde metamorphism mean?
The process of altering the mineral composition of relatively high-grade metamorphic rocks at temperatures lower than those of their initial metamorphism is known as retrograde metamorphism (also known as diaphthoresis or retrogressive metamorphism).
What kinds of rocks undergo retrograde metamorphism?
The following rocks have undergone metamorphic foliation: slate, schist, migmatite, phyllite, and gneiss. The metamorphic rocks marble and quartzite are nonfoliated. We refer to the temperature at which metamorphism takes place generally as the metamorphic grade.
How is metamorphism recognized?
The two classifications of metamorphic rocks are foliates and non-foliates. Micas and chlorites make up a sizable portion of foliates. The cleavage of these minerals is quite clear. Foliated metamorphic rocks will fracture along cleavage lines parallel to the rock’s constituent minerals.
How can I tell if a rock has undergone metamorphism?
Students will investigate how to group and identify rocks using their physical qualities in this research.
Materials Needed
For each group of students:
- rocks with stick-on numbers ranging from 1 to 6 (at least two sedimentary, igneous and metamorphic rocks; see the Digging Deeper section at the end of the investigation for specific examples)
- magnifying glasses for each pupil or pair of pupils
- pens for jotting down observations
- Data Table for Rock (handout)
- Sheet for Rock Identification (handout)
Safety
It is generally thought that conducting this experiment with students is safe. Review the investigation for your particular setting, resources, students, and standard safety procedures as well.
Setting the Scene
Remind the class of the rocks they looked at in their initial examination. What about the rocks was the same? What had changed? What traits did they believe would help others recognize their rocks? Create a list of their suggestions so you have it handy for the research.
Presenting the Investigation Question
Set the scene before introducing your students to the inquiry: “How can you distinguish between rocks?
Inform your students that they will be researching this subject and that they will be able to offer trustworthy answers once they have completed their research.
Have your pupils come up with different possible approaches to tackling this research subject.
- Create a testable experiment that might be utilized to answer the research topic.
- What supplies would be required?
- What would be required of you?
- What will be evaluated?
- How much time would the test require?
Assessing What Your Students Already Know
These opening inquiries can be discussed by your students individually, in groups, and as a class:
- What traits can you see in samples of rocks?
- How can you distinguish between various rocks?
Make a note of the thoughts your kids have and have them report them out. Include a list of questions regarding rocks in your lesson. More of these queries will likely have their answers by the time the investigation is finished.
Exploring the Concept
- Divide your class into groups of roughly four pupils, and if you haven’t already, have them seat around their respective tables or workstations.
- Tell your students how long they will have to finish their inquiry before they start. (Group learning methodologies frequently need the selection of a timekeeper to keep the group on schedule.)
- Give your pupils the following resources for research:
- mobile lenses (enough for everyone)
- samples of sedimentary, metamorphic, and igneous rocks
- Pencils for jotting down observations
- Copy Master 1 Word Document (38 KB) | Copy Master 1 Adobe PDF for the rock data table (10 KB)
- Inform your children that they will be looking at their rocks and noting their findings in their Rock Data Tables. While working, go around to check on progress and respond to inquiries.
- When they’re done, instruct the kids to create three groups of rocks using the information from their tables. Each set of rocks ought to share comparable qualities. Once this has been accomplished, invite groups to volunteer to discuss their groupings and the rationale behind them.
- Distribute copies of the Rock Identification Sheet to the pupils. Request that they utilize the sheets and their rock data tables to classify their rocks into the appropriate groupings. They can attempt to determine the precise names of the rock samples if they are confident in their ability to do so.
- Hold a class discussion about how it is possible to classify rocks using their characteristics after the pupils have finished. Make a flipchart list of their suggestions for subsequent use.
Applying Students’ Understanding
Ask the students to sort the rocks into groups according to their qualities after providing each group with a fresh batch of rocks (you can alternate the rocks between groups). Ask them to explain the rationale for the grouping of each rock.
Revisiting Investigation Question 2
Asking your pupils to consider this question and how their responses might have altered as a result of this inquiry will allow you to wrap up this investigation. For instance, igneous rocks do not often have visible grains, whereas sedimentary rocks do. Bands of crystals are a common arrangement in metamorphic rocks.
Digging Deeper
You can opt to explain to your students the more comprehensive information about this inquiry that is provided in the passage that follows.
Rocks can be divided into three basic types based on their formation.
Layers of sedimentary rock are common. Checking to see if a rock sample is formed of grains might help determine whether it is sedimentary. The sedimentary rocks limestone, sandstone, coal, and shale are some examples of.
When magma from deep inside the Earth rises to the surface or is pushed upward by a volcano as lava and ash, igneous rocks are created. It crystallizes into rock here when it cools. In igneous rocks, look for crystals. The igneous rocks gabbro, granite, pumice, and obsidian are a few examples.
Rocks that have undergone significant heat or pressure changes during formation are known as metamorphic rocks. If a rock sample’s crystals are arranged in bands, for example, that is a sign of metamorphism. Rocks that have undergone metamorphism include slate, gneiss, schist, and marble.
Retrograde metamorphism has an origin.
- Fluid Phase. Any open area between the mineral grains of a rock may be home to a fluid.
While primarily water, this liquid also has dissolved ions in it. The fluid phase is crucial because the movement of dissolved ions through the fluid can significantly speed up chemical reactions that entail converting one solid mineral into another. The process is known as metasomatism if the rock is chemically altered as a result of these fluids.
- Time – Metamorphic transformation is a long process since it entails altering the rock while it is still solid. There are various processes active during metamorphism. The size and shape of minerals change as a result of recrystallization. In addition, new minerals are created as a result of polymorphic phase transformations, which are reactions between the minerals that result in new mineral sets that are more stable under the pressure and temperature of the environment. Polymorphs are compounds with the same chemical formula but different crystal structures.
According to laboratory research, the size of the mineral grains formed during metamorphism grows over time.
Thus, the metamorphism of rocks with coarse grains took place over a lengthy period of time.
According to experiments, it took tens of millions of years.
The temperature range for low-grade metamorphism is between 200 to 320oC.
and low pressure, respectively. Low grade metamorphic rocks exhibit an increased
a profusion of hydrous minerals (water-containing minerals, H2O,
in their crystalline composition).
- Some illustrations of hydrous minerals found in low-grade metamorphic rocks include:
- The process of high-grade metamorphism occurs at temperatures above 320C and
quite a bit of pressure Hydrous minerals appear when metamorphism grade increases.
become less hydrous as a result of losing water, and non-hydrous minerals are more prevalent.
Examples of non-hydrous and less hydrated minerals that make up high grade
metamorphic rocks
At the greatest grade of muscovite, a hydrous mineral that gradually dissolves
metamorphism
- A hydrous mineral with exceptionally high metamorphism stability is biotite.
- A non-hydrous mineral is pyroxene.
- A non-hydrous mineral is garnet.
As temperatures and pressure drop as a result of tectonic or overlaying rock erosion
uplift, one may anticipate metamorphism to take the opposite course and finally bring back the
restoring rocks to their untainted state.
Retrograde refers to such a procedure.
metamorphism.
Retrograde metamorphism would not occur frequently if it were common.
Observe metamorphic rocks on the Earth’s surface.
Since metamorphic rocks are present,
Retrograde metamorphism does not seem to be widespread near the Earth’s surface.
These are some of the causes:
- The slower the temperature drops, the slower the chemical processes are.
- Prograde metamorphism is a process in which fluids like CO2 and H2O are
the fluids are required to create the hydrous minerals that are stable at
the outside of Earth.
- In the presence of fluids, chemical reactions proceed more quickly, however if the fluids
are ejected during prograde metamorphosis, they won’t be able to accelerate
retrograde metamorphism responses
- Foliated These were created under differential stress and feature a planar foliation due to the preferred orientation (alignment) of the minerals.
They are categorized by composition, grain size, and foliation type and contain a considerable quantity of sheet silicate (platy minerals).
- Non-foliated These are made up of of equal minerals and lack any visible planar fabric or foliation. They crystallized under non-differential stress conditions. These are mostly categorized according to the minerals present or the protolith’s chemical makeup.
- Slate – The development of fine-grained rocks causes slates to form at low metamorphic grades.
clay minerals and chlorite. These sheet silicates’ preferred orientation
causes the rock to break easily along the planes perpendicular to the sheet silicates, resulting
a slate-colored cleavage
You should be aware that in this instance, the maximum
the original bedding planes are stressed at an angle, causing the slatey cleavage
has grown away from the initial bedding at an angle.
Slates are useful for blackboards and shingles due to the almost flawless fracture along planes.
- Fine, mica-rich rock called phyllite is the result of low- to medium-grade metamorphism. The clay minerals have re-crystallized into microscopic micas (biotite and muscovite) that have a satiny sheen in phyllites.
Between slate and schist is phyllite.
– As the grade increases, the size of the mineral grains tends to increase.
of transformation.
The rock eventually forms a near-planar foliation as a result of the
Silicate sheet orientation that is desired (mainly biotite and muscovite).
Agate and
However, feldspar granules do not exhibit a preferential orientation.
The planar irregularity
Schistosity refers to the foliation at this level.
Micas are not the only minerals found in schist.
Among them are minerals like –
Quartz,
Feldspars,
Kyanite,
Garnet,
both Staurolite and
Sillimanite.
Pophyroblasts are these non-mica minerals that have a grain size larger than the remainder of the rock.
Gneiss The stability of the sheet silicates decreases with metamorphic grade.
and the growth of minerals with dark colors, such as hornblende and pyroxene, begins. These gloomy
Throughout the rock, colorful minerals frequently segregate into discrete bands, providing the
gneissic banding in rock.
Because the minerals with dark colors tend to
Despite the fact that they create elongated crystals as opposed to sheet-like crystals,
ideal positioning with their long axes parallel to the largest differential
stress.
Granulite – During the most advanced metamorphic stages, all of the hydrous
There are few minerals exist that remain stable as minerals and sheet silicates do.
would demonstrate an inclination.
The final rock will have a granulitic composition.
akin to the phaneritic texture found in igneous rocks.
- MigmatitesThe rock may start to melt and start to mix with the solids if the temperature hits the solidus temperature, which is the initial melting point.
These melts typically contain felsic material, leaving the mafic material metamorphic.
Retrograde metamorphism occurs where?
Retrograde metamorphism, on the other hand, refers to changes that take place when a rock is uplifted and cooled. The reactions that take place during prograde metamorphism should all be expected to be reversed during the subsequent elevation of the rocks and reexposure at the Earth’s surface if thermodynamic equilibrium were always maintained; in this scenario, metamorphic rocks would never be found in outcrop. However, two conditions prevent all metamorphic rocks from regressing when they reach the Earth’s surface. First, by moving fluid upward along grain boundaries and through cracks, water and carbon dioxide generated during prograde devolatilization events are effectively removed. The reaction cannot be reversed during cooling unless water is subsequently added to the rock because almost all of the water released during heating by reactions such as when chlorite (Fe9Al6Si5O20(OH)16) reacts with quartz (4SiO2) to yield garnet (3Fe3Al2Si3O12) and water (8H2O) is removed from the site of reaction. Thus, garnet is thermodynamically unstable at such low temperatures and pressures, but it can still be kept near the Earth’s surface. The fact that reaction rates are accelerated by rising temperatures is the second factor that prevents metamorphic reactions from commonly occurring in reverse during cooling. Metastable mineral assemblages and compositions can be preserved well outside of their typical stability zones after cooling because reaction kinetics become slow. Prograde rather than retrograde reactions are therefore typically more effective, and metamorphic assemblages indicating even extremely high temperatures or pressures or both are found exposed all over the planet.
A retrograde mineral is what?
- the original rock’s overall chemical makeup.
- the level of pressure that metamorphism attained.
- the degree of heat that metamorphism reaches.
- any fluid phase’s make-up that existed during metamorphism.
When a rock is subjected to greater pressure and temperature, the mineral assemblage changes.
assuming the circumstances are maintained for, should indicate stable chemical equilibrium.
a sufficient amount of time for equilibrium to be reached.
after metamorphism
Most metamorphic rocks indicate an era of geologic time that typically spans considerable distances.
mineral assemblage that is in balance.
Consider this: In metamorphic rocks, when pressure and temperature
can both change during metamorphism, the presence of a divariant (F=2) would be the most plausible scenario.
a combination of phases.
It would be less probable for a monovariant assemblage (F=1) to occur.
and a stable equilibrium would be represented by an invariant assemblage (F=0).
It would be even less likely to occur given the difference in temperature and pressure.
Therefore, for F=2, C=P, the more phases there are in a rock, the more
The number of components will be equal to the common divariant assemblage.
If
P is
larger than C, one of three mineral assemblage scenarios is possible.
The assemblage is a non-equilibrium assemblage (perhaps as a result of
due to incomplete chemical processes or the development of retrograde minerals
while the metamorphic rock is cooling, being lifted, or being unroofed).
The assemblage depicts equilibrium that is either univariant or invariant, as
mentioned earlier.
If scenario (1) explains why there isn’t a correspondence with the
phase rule, it is typically possible to identify by closely examining the rock.
Reaction
Rock textures could be a sign of an incomplete reaction.
Popular retrograde
minerals that can withstand pressures and temperatures that are lower than the majority of the
The rock’s minerals may be recognized.
Then, these retrograde periods could
phase rule could be added to the number of phases being thought about, then
applicable exclusively to the known equilibrium phases.
(For instance, the existence
Occurrence chlorite in rocks with the amphibolite and granulite facies would suggest that the
Chlorite is a retrograde mineral that forms as a result of weathering; as such, it would not
be taken into account when implementing the phase rule.)
The second scenario is always conceivable, particularly if there are
This may be the case if the minerals are properly picked and retrograde minerals are not taken into account.
The phase rule specifies the number of components that must be picked.
so as to depict the very minimum required to create every potential rock phase.
Remember that the number of components is not just the number of components that are oxide.
or the number of elements listed in the rock’s chemical analysis.
Simply said,
Take a look at the primary metamorphic rock phases and remember that some ions
if the components can readily swap places in stable solutions, the number of components
frequently be lowered to 7 or 8.
For instance:
Based on the frequent substitution of Ti into (Si,Ti)O2,
most silicates include tetrahedral sites)
MgO is typically required since the compositions of Fe-Mg solid solutions are both
dependent on temperature and pressure (although these two factors can be mixed,
It would shave off one from the overall number of components.
Based on the generally used formula (Al,Fe+3)2O3,
Minerals have been found to substitute Fe+3 for Al+3Al.
H2O, which is often found in a fluid phase but is also a vital
element found in hydrous minerals.
Typically present in a fluid phase, CO2 is also an
significant carbonate mineral component.
If it is assumed that H2O and CO2 are always present and can be combined to generate hydrous
the number of components can be lowered to 5 or 6 and carbonate minerals.
So for a
two-variant assemblage There should be 5 or 6 separate mineral phases present, as expected.
if the assemblage is invariant, there could be up to 8 phases in a metamorphic rock.
This serves as the foundation for creating the AKF and ACF diagrams.
where the number of components has been decreased to four as previously described, by
Alkali feldspar and quartz are always possible assumptions.
However, you are
advised that the methodology above is not always universally applicable and that each rock
be taken into account on a case-by-case basis.
either prograde or progressive metamorphism
happens on the rock as the temperature and pressure rise.
pressure increases
A rock with a specific chemical composition and rising temperature is anticipated to go through
between its constituent minerals and any fluid, in a constant cycle of chemical reactions
phase to create several novel mineral assemblages that are stable at the higher temperatures
temperatures and pressures.
Here, we provide an example of how the mineral assemblages may
a hypothetical group of rocks undergo alteration, beginning with a low grade mineral assemblage as
represented in the following ACF diagram.
Because of the emergence of sillimanite and the absence of andalusite,
Simple reaction:
Muscovite and quartz could have interacted with chlorite to create biotite, cordierite, and
also fluid
Anthophyllite, quartz, and fluid would result from the breakdown of talc:
and hornblende would result from the reaction of quartz, actinolite, epidote (zoisite), and chlorite.
also fluid
Next, let’s raise the temperature and pressure to allow for the formation of a fresh batch of minerals.
for each rock, grows.
At these novel pressure and temperature levels,
Plagioclase, sillimanite, cordierite, and quartz would be found in pelitic rocks.
k-spar, too.
Plagioclase would be present in quartzo-feldspathic rocks,
cordierite, quartz, k-spar, and almandine/pyrope garnet.
In reference to the disappearance of Moscowite, we can say:
We can record the following for the appearance of wollastonite and the elimination of calcite:
Hyperstne and quartz would result from the breakdown of anthophyllite.
The following reaction must occur in order for the breakdown of biotite to produce hypersthene and k-spar:
have taken place:
Lastly, in regards to the synthesis of almandine/pyrope from cordierite and anthophyllite,
the following response might have taken place:
Retrograde metamorphism would be a frequent occurrence upon elevation and
Eventually, unroofing metamorphic rocks would recover to stable mineral compositions.
lower temperatures and pressures.
However, high-grade metamorphic rocks are widespread at
the Earth’s surface and typically only display modest retrograde minerals.
Three
Two of the factors that prevent retrograde metamorphosis involve the fluid phase.
Higher temperatures hasten chemical processes.
During
retrograde transformation The response thresholds are often breached to higher levels
temperature.
Diffusion rates increase and become more molecular at higher temperatures.
It takes more vibration to break chemical bonds.
So, when in prograde
Reaction times during metamorphosis are quicker.
On a rock, when the temperature drops, the
reaction boundaries are crossed at low temperatures, which causes the reaction to occur.
The pace is substantially slower.
Of course, how quickly the temperature is dropped will determine this.
It is undoubtedly
temperatures might possibly drop so slowly during uplift and unroofing,
that time would allow for the formation of retrograde mineral assemblages.
Moreover, another episode
prograde metamorphism, during which temperatures may rise once more.
Minerals that are stable at the increased temperature could begin to expand and cover the existing minerals.
that during the initial metamorphic phase formed at a higher temperature.
As we’ve just seen, prograde metamorphism involves a fluid phase.
pushed away by the devolatilization reactions.
As the pressure builds,
Rocks’ porosity likewise declines, therefore this fluid phase will probably be forced out of
the body of rock.
The formation of hydrous compounds is not possible without the fluid phase.
minerals and carbonates, as water and carbon dioxide are two essential
There could not be all the necessary ingredients for such reactions.
Chemical processes can also be catalyzed by the fluid phase.
Despite the fact that the net reactions can seem to be solid-solid reactions, there
possibly more complex.
For instance, the fluid phase might completely dissolve a mineral
a new mineral to precipitate in a different area of the rock, just as
occurs when sedimentary rocks are forming.
When the fluid phase is removed
Consequently, it won’t be available to catalyze the processes to during prograde metamorphism.
As pressure and temperature are reduced, the retrograde mineral assemblage is produced.
All metamorphic rocks will eventually undergo a transformation to an
a collection of minerals that can withstand the conditions found close to the Earth’s surface.
However, this procedure, known as weathering, takes place close to
the surface of the earth.
The following time, we’ll delve deeper into some of the elements that influence
chemical processes that take place during metamorphosis.