Pyroxene

Definition

Pyroxene is a group of important rock-forming minerals that are characterized by their monoclinic or orthorhombic crystal systems and chains of silicate tetrahedra. These minerals are typically found in igneous and metamorphic rocks, playing a crucial role in understanding the mineral composition and evolution of these rocks due to their abundance and variety.

5 Must Know Facts For Your Next Test

1. Pyroxenes can be categorized into two main types: clinopyroxene and orthopyroxene, based on their crystal structures.
2. These minerals commonly occur in mafic and ultramafic rocks, such as basalt and peridotite, and are essential for understanding the petrology of these rocks.
3. Pyroxenes often exhibit a characteristic cleavage at approximately 90 degrees, which helps in their identification during mineralogical studies.
4. They are also involved in the process of solid solution, where different cations can substitute for one another within the mineral structure, leading to a range of compositions.
5. The presence of pyroxene can indicate the temperature and pressure conditions during rock formation, making them significant indicators of geological history.

Review Questions

• How do the crystal structures of clinopyroxene and orthopyroxene differ, and why is this distinction important in mineralogy?
  • Clinopyroxene and orthopyroxene differ primarily in their crystal structures; clinopyroxenes have a monoclinic system while orthopyroxenes crystallize in an orthorhombic system. This distinction affects their physical properties, such as cleavage angles and specific gravity, which are crucial for accurate identification. Understanding these differences helps geologists classify rocks more precisely based on their mineral content.
• Discuss the role of pyroxenes in the classification of igneous rocks, particularly in relation to mafic compositions.
  • Pyroxenes play a key role in classifying igneous rocks, especially those with mafic compositions. These rocks typically contain high amounts of iron and magnesium-rich minerals like pyroxene, which help define their texture and mineralogy. By analyzing the types and proportions of pyroxenes present, geologists can infer the conditions under which the rock formed, aiding in the understanding of its geological history.
• Evaluate the importance of pyroxenes in understanding metamorphic processes and their implications for tectonic activity.
  • Pyroxenes are vital for understanding metamorphic processes because their stability under varying temperature and pressure conditions provides insights into the metamorphic environment. The transformation of original igneous or sedimentary rocks into metamorphic rocks can be traced through changes in pyroxene composition. Analyzing these changes allows geologists to assess tectonic activity, as variations in pyroxene indicate shifts in geothermal gradients and pressures associated with tectonic movements.

Related terms

Silicate:

A class of minerals that contain silicon and oxygen as their primary components, forming the building blocks of most of the Earth's crust.

Amphibole: A group of inosilicate minerals that are often found alongside pyroxenes, with distinct double-chain silicate structures and similar occurrences in geological environments.

Mafic: Describes igneous rocks that are rich in magnesium and iron, typically containing a significant amount of pyroxene among other minerals.