Augite

Definition

Augite is a common pyroxene mineral that is typically dark green to black in color and characterized by its two distinct cleavage planes at nearly 90 degrees. This mineral is primarily composed of calcium, magnesium, iron, and aluminum and is frequently found in igneous rocks, particularly basalt and gabbro. Augite plays a significant role in the formation of these rocks and helps to provide insights into the cooling and crystallization processes of magma.

5 Must Know Facts For Your Next Test

1. Augite is commonly found in mafic and intermediate igneous rocks, reflecting its formation from relatively high-temperature magma.
2. It has a Mohs hardness of around 5.5 to 6, making it moderately hard compared to other minerals.
3. The crystal structure of augite is monoclinic, which contributes to its distinctive cleavage angles and overall physical appearance.
4. Augite can form as a result of both volcanic and plutonic processes, meaning it can be found in both extrusive and intrusive igneous rocks.
5. In addition to its role in geology, augite can also be used as an indicator mineral for understanding the conditions under which certain igneous rocks formed.

Review Questions

• How does the presence of augite in igneous rocks influence our understanding of their formation process?
  • The presence of augite in igneous rocks helps geologists understand the temperature and composition of the magma from which these rocks crystallized. Augite typically forms from higher temperature magmas, so its occurrence in a rock like basalt indicates that it cooled relatively quickly after eruption. Additionally, analyzing the proportion of augite compared to other minerals can provide insights into the specific cooling history and crystallization sequence of the rock.
• Compare and contrast the role of augite in basalt versus gabbro, including differences in their formation conditions.
  • In basalt, augite typically forms from rapidly cooling lava during volcanic eruptions, leading to its fine-grained texture. Conversely, in gabbro, which cools more slowly beneath the Earth's surface, augite crystals are larger due to prolonged crystallization. This difference highlights how cooling rates influence crystal size and mineral assemblages; thus, while both rock types contain augite, the conditions under which they form significantly affect its characteristics.
• Evaluate the significance of augite as an indicator mineral in geological studies of igneous rocks and their environments.
  • Augite serves as a crucial indicator mineral in geological studies because its presence provides valuable information about the thermal and compositional conditions of magma. By analyzing augite within rock samples, scientists can infer the tectonic settings—such as subduction zones or rift environments—where these rocks formed. Furthermore, studying augite's chemical variations can help researchers understand past volcanic activity and predict future eruptions based on the historical behavior of magma systems.

Related terms

Pyroxene: A group of important rock-forming silicate minerals that are typically characterized by their chain silicate structures and two cleavages at nearly 90 degrees.

Basalt: A fine-grained igneous rock that is rich in iron and magnesium, commonly associated with volcanic activity and often contains augite as a major mineral component.

Magma: Molten rock located beneath the Earth's surface that can crystallize into igneous rocks upon cooling, with augite forming as one of the minerals during this process.