Cordierite

Definition

Cordierite is a mineral that typically forms in metamorphic rocks and is known for its distinctive pleochroism and high-temperature stability. It often appears in the context of regional metamorphism, particularly in areas where granitic or sedimentary rocks are subjected to heat and pressure, leading to the development of schist or gneiss that can contain this mineral. Cordierite is significant in understanding the conditions under which metamorphic processes occur, as its presence indicates specific temperature and pressure ranges during rock formation.

5 Must Know Facts For Your Next Test

1. Cordierite is typically blue to gray in color and exhibits a unique pleochroism, which means it can show different colors when viewed from different angles.
2. It is stable at high temperatures, making it a common indicator of high-grade metamorphic environments.
3. Cordierite can form as a result of the metamorphism of pelitic rocks and can also indicate the presence of specific minerals such as biotite and garnet in the rock.
4. The presence of cordierite in metamorphic rocks can provide insights into the metamorphic history of an area, particularly the temperature and pressure conditions during rock formation.
5. Cordierite is also used as a gemstone due to its aesthetic qualities and is sometimes referred to as 'iolite' when used for jewelry.

Review Questions

• How does the presence of cordierite in metamorphic rocks indicate specific conditions of temperature and pressure during metamorphism?
  • The presence of cordierite in metamorphic rocks indicates that these rocks underwent specific temperature and pressure conditions during their formation. Cordierite typically forms at high temperatures ranging from approximately 600°C to 800°C under moderate to high-pressure conditions. Its occurrence suggests that the surrounding environment had enough heat to facilitate the transformation of parent rocks while preventing further alterations that would destabilize cordierite.
• Discuss the role of cordierite in understanding the metamorphic history of pelitic rocks.
  • Cordierite plays a crucial role in piecing together the metamorphic history of pelitic rocks. When these fine-grained sedimentary rocks undergo metamorphism, cordierite often forms along with other minerals like biotite and garnet. Analyzing the presence and abundance of cordierite helps geologists determine the peak metamorphic conditions that affected these rocks, thereby reconstructing their geological history and the environmental conditions they experienced during transformation.
• Evaluate how the properties of cordierite can be used to infer the tectonic settings where regional metamorphism occurs.
  • The properties of cordierite provide valuable insights into the tectonic settings where regional metamorphism occurs. For instance, its high-temperature stability suggests formation within tectonically active regions such as convergent plate boundaries where crustal thickening occurs. By studying cordierite-rich rocks alongside other associated minerals, geologists can infer the specific tectonic processes at play, including subduction or continental collision, allowing for a better understanding of regional geology and tectonics.

Related terms

metamorphism: The process by which existing rocks undergo changes in mineralogy, texture, and chemical composition due to changes in temperature, pressure, or the presence of chemically active fluids.

pelitic rocks: A category of metamorphic rocks derived from fine-grained sedimentary rocks, rich in clay minerals, which often contain cordierite when subjected to specific metamorphic conditions.

migmatite: A complex metamorphic rock that exhibits both igneous and sedimentary characteristics, often containing cordierite and forming under extreme temperature and pressure conditions.