Pyroxenes

Definition

Pyroxenes are a group of important rock-forming silicate minerals characterized by their monoclinic or orthorhombic crystal structures and defined by the presence of chains of silicate tetrahedra. They are primarily composed of silicon, oxygen, and various metal cations, often forming in igneous and metamorphic rocks, which makes them crucial for understanding Earth materials and their classifications.

5 Must Know Facts For Your Next Test

1. Pyroxenes typically exhibit a two-directional cleavage at angles of approximately 90 degrees, which is a distinguishing feature used to identify them.
2. They are divided into two main groups: clinopyroxenes and orthopyroxenes, based on their crystal structure and composition.
3. Common examples of pyroxenes include augite, diopside, and enstatite, each varying in their chemical composition and properties.
4. Pyroxenes are significant indicators of the temperature and pressure conditions during rock formation, providing insights into the geological history of the area.
5. They often contain varying amounts of iron, magnesium, calcium, and aluminum, which can affect their color and density.

Review Questions

• How do the structural characteristics of pyroxenes relate to their classification within Earth materials?
  • The structural characteristics of pyroxenes, including their distinct chain-like arrangement of silicate tetrahedra and unique cleavage angles, are essential for classifying them as silicate minerals. Their monoclinic or orthorhombic crystal systems further differentiate them from other mineral groups. Understanding these structural features helps geologists identify pyroxenes in rock samples and determine their role in various geological processes.
• Discuss the differences between clinopyroxenes and orthopyroxenes in terms of their chemical composition and crystal structure.
  • Clinopyroxenes have a more complex crystal structure with a monoclinic symmetry, while orthopyroxenes possess an orthorhombic symmetry. Chemically, clinopyroxenes typically contain more calcium along with iron and magnesium, while orthopyroxenes usually have a higher concentration of magnesium and iron. These differences influence their physical properties, such as color and cleavage angles, which are critical for identifying these minerals in geological studies.
• Evaluate the role of pyroxenes in understanding the formation conditions of igneous rocks and how they can inform our knowledge about Earth's geological history.
  • Pyroxenes play a vital role in revealing the formation conditions of igneous rocks due to their sensitivity to temperature and pressure during crystallization. By analyzing the types and compositions of pyroxenes present in a rock sample, geologists can infer the geothermal gradients and magmatic processes that occurred. This evaluation contributes to our understanding of Earth's geological history, as variations in pyroxene composition can indicate changes in tectonic activity or environmental conditions over time.

Related terms

Silicate Tetrahedron: A fundamental building block of silicate minerals consisting of a silicon atom surrounded by four oxygen atoms, arranged in a tetrahedral shape.

Feldspar: A group of silicate minerals that are among the most abundant in Earth's crust, often found alongside pyroxenes in igneous rocks.

Olivine: A magnesium iron silicate mineral that is commonly found in mafic and ultramafic igneous rocks, known for its high-temperature stability.