Orthopyroxene

Definition

Orthopyroxene is a type of pyroxene mineral that typically contains magnesium and iron, characterized by a monoclinic crystal structure. This mineral is a significant component in many igneous rocks, particularly those with a high magnesium content, and it plays an essential role in the classification of these rocks. Its properties and formation processes are closely related to the geochemical environment in which it crystallizes.

5 Must Know Facts For Your Next Test

1. Orthopyroxene is usually found in high-temperature metamorphic rocks and mafic igneous rocks, such as basalt and gabbro.
2. It has a distinct cleavage at 87° and 93°, which helps in identifying the mineral in hand specimens.
3. Common varieties of orthopyroxene include enstatite and ferrosilite, which differ based on their iron and magnesium content.
4. The presence of orthopyroxene in a rock can indicate the conditions under which the rock formed, particularly the temperature and pressure during crystallization.
5. Orthopyroxene can also undergo exsolution, where it separates into two distinct phases at lower temperatures, affecting its physical properties.

Review Questions

• How does the presence of orthopyroxene influence the classification of igneous rocks?
  • The presence of orthopyroxene in igneous rocks helps determine their classification because it indicates a higher magnesium content and typically associates with mafic rock types. When identifying rocks, geologists look at mineral assemblages, and finding orthopyroxene suggests specific formation conditions related to temperature and pressure. This connection aids in understanding the geological history of the area where the rock was found.
• Compare and contrast orthopyroxene and clinopyroxene in terms of their structural differences and implications for rock types.
  • Orthopyroxene and clinopyroxene differ primarily in their crystal structures; orthopyroxene has a monoclinic structure while clinopyroxene has a more complex structure with a single chain of tetrahedra. These structural differences affect their stability at varying temperatures and pressures, influencing the types of igneous rocks they are found in. For instance, clinopyroxene tends to be more abundant in intermediate to felsic rocks compared to orthopyroxene's predominance in mafic rocks.
• Evaluate the significance of orthopyroxene's exsolution process on our understanding of metamorphic conditions.
  • The exsolution process of orthopyroxene is significant because it provides insights into the thermal history of metamorphic rocks. When orthopyroxene crystallizes at high temperatures, it can later exsolve into distinct phases as temperatures drop, indicating changes in the environmental conditions. Studying these exsolved phases helps geologists reconstruct the metamorphic path that the rock underwent, revealing important information about the tectonic processes and thermal gradients present during its formation.

Related terms

Clinopyroxene: A type of pyroxene mineral with a similar composition to orthopyroxene but differing in crystal structure, typically found in mafic and ultramafic igneous rocks.

Mafic: Refers to igneous rocks that are rich in magnesium and iron, where orthopyroxene commonly occurs as a significant mineral.

Silicate: A group of minerals that contain silicon and oxygen, which includes both orthopyroxene and clinopyroxene, making them important in understanding rock formation.