Hornblende

Definition

Hornblende is a complex, dark-colored amphibole mineral that is commonly found in igneous and metamorphic rocks. It is characterized by its elongated prismatic crystals and distinctive cleavage, often exhibiting a variety of colors including green, black, and brown. Hornblende plays an important role in the classification and formation of various types of igneous rocks, particularly those that are intermediate to mafic in composition.

5 Must Know Facts For Your Next Test

1. Hornblende is a significant component of many intermediate to mafic igneous rocks like diorite and basalt.
2. It has a high density and can be identified by its distinct cleavage at angles of about 60 and 120 degrees.
3. Hornblende can also form under metamorphic conditions, contributing to the mineral assemblages in schists and gneisses.
4. The presence of hornblende can indicate the cooling history of a rock, as it typically crystallizes at higher temperatures than many other minerals.
5. Its complex chemical formula allows for a variety of substitutions among elements, which can result in a range of colors and physical properties.

Review Questions

• How does the presence of hornblende influence the classification of igneous rocks?
  • Hornblende significantly influences the classification of igneous rocks due to its association with intermediate to mafic compositions. Rocks containing hornblende, such as diorite or basalt, typically have a higher concentration of iron and magnesium compared to felsic rocks. Its presence indicates specific crystallization conditions and temperatures during rock formation, providing insights into the geological processes at play.
• Discuss the role of hornblende in understanding metamorphic processes.
  • Hornblende plays a crucial role in understanding metamorphic processes as it often forms under high-pressure and high-temperature conditions typical of regional metamorphism. Its presence in metamorphic rocks like schists can indicate the original rock's mineral composition and the conditions during metamorphism. Additionally, studying hornblende can help geologists reconstruct metamorphic pathways and pressure-temperature histories.
• Evaluate the importance of hornblende's chemical composition in determining its properties and occurrences in various rock types.
  • The chemical composition of hornblende is vital for determining its physical properties and occurrences across different rock types. The complex formula allows for substitutions among elements such as calcium, sodium, magnesium, iron, and aluminum, leading to diverse colors and densities. This variability can affect its stability and reaction with other minerals during the formation of both igneous and metamorphic rocks, highlighting its role as an indicator of geological processes and environments.

Related terms

Amphibole: A group of silicate minerals that share similar structural characteristics, often forming elongated crystals and commonly found in both igneous and metamorphic rocks.

Igneous Rock: Rock formed from the cooling and solidification of molten magma or lava, classified as either intrusive or extrusive based on its formation process.

Mafic: Describes igneous rocks that are rich in magnesium and iron, typically darker in color and having lower silica content.