Pyroxene

Definition

Pyroxene is a group of important rock-forming silicate minerals commonly found in igneous and metamorphic rocks. They typically exhibit a single chain structure of silicon-oxygen tetrahedra and are characterized by their dark color, two directions of cleavage at nearly 90 degrees, and high density. This mineral group plays a vital role in the classification of igneous rocks and understanding metamorphic conditions, as it provides insights into the mineral composition and the processes that formed these rocks.

5 Must Know Facts For Your Next Test

1. Pyroxenes are commonly divided into two main groups: orthopyroxenes (with a monoclinic structure) and clinopyroxenes (with an orthorhombic structure).
2. These minerals often crystallize from molten magma during the cooling process, contributing to the texture and composition of igneous rocks like basalt and gabbro.
3. In metamorphic rocks, pyroxenes can form under high-temperature and high-pressure conditions, indicating specific metamorphic facies that reflect the environmental conditions during rock formation.
4. The presence of different types of pyroxenes can help geologists determine the history of a rock, including its formation environment and the temperature-pressure conditions it experienced.
5. Pyroxenes are important in petrology as they help classify igneous rocks based on their mineral content, especially in mafic and ultramafic classifications.

Review Questions

• How does the presence of pyroxene influence the classification of igneous rocks?
  • Pyroxene plays a significant role in classifying igneous rocks because its mineral composition helps define whether a rock is considered mafic or felsic. Mafic rocks like basalt contain significant amounts of pyroxene along with olivine, while felsic rocks generally lack pyroxene. The type and abundance of pyroxene can also indicate the cooling rate of the magma and the specific conditions under which the rock formed.
• What are the key differences between orthopyroxenes and clinopyroxenes in terms of structure and formation?
  • Orthopyroxenes have a monoclinic crystal system while clinopyroxenes have an orthorhombic system. This structural difference affects their physical properties, such as cleavage angles and density. In terms of formation, orthopyroxenes tend to crystallize at higher temperatures compared to clinopyroxenes, which are often associated with more evolved magmas and can form under varying pressure-temperature conditions in metamorphic settings.
• Evaluate the significance of pyroxenes in understanding metamorphic facies and their associated pressure-temperature conditions.
  • Pyroxenes are crucial for interpreting metamorphic facies because they provide information about the temperature and pressure conditions during rock metamorphism. For instance, specific pyroxene compositions can indicate whether a rock underwent low-grade or high-grade metamorphism. By analyzing pyroxene minerals within a sample, geologists can deduce the metamorphic environment, such as whether it was formed under contact or regional metamorphism, allowing for a deeper understanding of Earth's geological processes.

Related terms

Olivine:

A common silicate mineral often found in mafic and ultramafic igneous rocks, recognized for its green color and high magnesium and iron content.

Amphibole:

A group of double chain silicate minerals that are important constituents of metamorphic rocks, typically forming under higher pressure and temperature conditions compared to pyroxenes.

Basalt:

A fine-grained igneous rock that is rich in iron and magnesium, often containing significant amounts of pyroxene, especially in its mafic composition.