Reaction rims

Definition

Reaction rims are concentric layers of minerals that form around a pre-existing mineral grain during metamorphism or contact metamorphism. These layers result from the interaction of different mineral phases, indicating a change in chemical conditions and the stability of the original minerals. They are essential for understanding paragenetic sequences and can provide insight into the metamorphic history of a rock.

5 Must Know Facts For Your Next Test

1. Reaction rims can indicate the presence of specific elements or conditions that were required for their formation, such as temperature and pressure changes.
2. These rims often exhibit unique textures and mineral assemblages that help geologists decipher the metamorphic history of a rock.
3. In many cases, reaction rims will develop around unstable minerals as they react with surrounding materials to form more stable mineral phases.
4. The thickness and composition of reaction rims can provide clues about the rate of metamorphism and the duration of exposure to changing conditions.
5. Reaction rims can also serve as markers for identifying contact metamorphic zones where temperature gradients are significant due to nearby igneous intrusions.

Review Questions

• How do reaction rims help in understanding the metamorphic history of a rock?
  • Reaction rims provide crucial information about the conditions under which a rock was formed. By examining the composition and layering of these rims, geologists can infer changes in temperature, pressure, and chemical environment that occurred during metamorphism. This understanding helps reconstruct the paragenetic sequence and can reveal how long the rock was subjected to metamorphic processes.
• Compare and contrast reaction rims with other types of mineral textures formed during metamorphism.
  • Reaction rims differ from other mineral textures such as foliation or granoblastic textures in that they specifically indicate chemical reactions occurring at boundaries between minerals. While foliation results from differential pressure and alignment of minerals, reaction rims reflect localized interactions between pre-existing minerals and new phases that emerge due to changing conditions. This distinction is vital for interpreting geological histories since it highlights different processes at play during metamorphism.
• Evaluate the significance of reaction rims in identifying contact metamorphic zones within a geological setting.
  • Reaction rims play a significant role in identifying contact metamorphic zones as they directly indicate areas where temperature gradients are altered due to nearby igneous activity. By analyzing these reaction rims, geologists can determine how far the effects of heat extended into surrounding rocks and identify the boundaries between different metamorphic environments. Understanding these zones is crucial for comprehending the broader geological context, including potential resource locations or hazards related to igneous intrusions.

Related terms

Metamorphism:

The process by which rocks undergo physical and chemical changes due to heat, pressure, and chemically active fluids, leading to the formation of new minerals and textures.

Paragenesis:

The study of the sequence and conditions under which minerals form in a rock, helping to reconstruct the history of geological events.

Contact Metamorphism: A type of metamorphism that occurs when rocks are heated by the intrusion of hot magma or lava, resulting in localized changes in mineralogy and texture.