Model Answer
0 min readIntroduction
Polymorphism, the ability of a chemical compound to exist in more than one crystal structure, is a fundamental concept in metamorphic petrology. This phenomenon arises due to variations in pressure and temperature conditions, leading to different stable mineral forms of the same chemical composition. The system Al2SiO5, encompassing minerals like kyanite, sillimanite, and andalusite, serves as an excellent illustration of polymorphism. Understanding the pressure-temperature (P-T) relationships governing the stability of these polymorphs is critical for deciphering the metamorphic history of rocks and interpreting regional metamorphic terrains.
Al2SiO5 Polymorphism: An Overview
Al2SiO5 exists in three main polymorphs: kyanite, sillimanite, and andalusite. Each polymorph is stable under a specific range of pressure and temperature conditions. These differences in stability are due to the varying arrangements of aluminum, silicon, and oxygen atoms within their crystal structures. Kyanite has a bladed habit, sillimanite is typically fibrous or prismatic, and andalusite often forms cruciform twins.
The Al2SiO5 P-T Phase Diagram
The P-T phase diagram for the Al2SiO5 system graphically represents the stability fields of each polymorph as a function of pressure and temperature. The diagram consists of three distinct fields, each corresponding to the stability of one polymorph. These fields are separated by invariant lines representing reactions where two polymorphs coexist in equilibrium.
(Note: Since I cannot directly render images, I have provided a link to a representative Al2SiO5 phase diagram. A hand-drawn, labelled diagram would be expected in an exam setting.)
Key Features of the Diagram:
- Low-Pressure/Low-Temperature Field: Andalusite is stable under relatively low pressure and low temperature conditions. This is typical of contact metamorphism near igneous intrusions.
- Intermediate-Pressure/Intermediate-Temperature Field: Kyanite is stable at intermediate to high pressures and moderate temperatures. This is commonly found in regional metamorphic rocks formed during orogenic events.
- High-Pressure/High-Temperature Field: Sillimanite is stable at high temperatures and pressures. It is often associated with high-grade regional metamorphism and sometimes with granulite facies rocks.
- Invariant Lines: The lines separating the stability fields represent reactions where two polymorphs coexist. For example:
- Andalusite + Kyanite = Sillimanite
- Kyanite + Sillimanite = Andalusite
- Andalusite + Sillimanite = Kyanite
Significance in Metamorphism
The Al2SiO5 P-T phase diagram is a powerful tool for understanding metamorphic processes. Its significance lies in the following:
- Metamorphic Grade Determination: The presence of a specific Al2SiO5 polymorph in a metamorphic rock can indicate the pressure and temperature conditions under which it formed, thus providing information about the metamorphic grade.
- P-T Path Reconstruction: By analyzing the mineral assemblages and textures in metamorphic rocks, geologists can reconstruct the P-T path followed during metamorphism. For example, the presence of andalusite included within kyanite suggests a decrease in pressure during metamorphism.
- Regional Metamorphic Mapping: The distribution of different Al2SiO5 polymorphs across a regional metamorphic terrain can help map out pressure-temperature gradients and identify areas of different metamorphic intensity.
- Geodynamic Interpretation: The metamorphic conditions indicated by the Al2SiO5 polymorphs can provide insights into the tectonic setting and geodynamic processes that led to the metamorphism. For instance, high-pressure metamorphism often indicates subduction zones or continental collision.
Furthermore, the phase diagram helps explain why different metamorphic rocks are found in different geological settings. For example, rocks containing andalusite are common in contact aureoles around granitic intrusions, while rocks containing kyanite and sillimanite are more typical of regional metamorphic belts.
Conclusion
The Al2SiO5 polymorph system and its associated P-T phase diagram are fundamental to understanding metamorphic processes. The stability of kyanite, sillimanite, and andalusite under varying pressure and temperature conditions allows geologists to decipher the metamorphic history of rocks, reconstruct P-T paths, and interpret the tectonic settings in which metamorphism occurred. This knowledge is crucial for understanding the evolution of the Earth’s crust and the dynamic processes that shape our planet.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.