Amphiboles

Definition

Amphiboles are a group of double chain silicate minerals that are important in igneous and metamorphic rocks, characterized by their elongated crystals and complex chemical compositions. These minerals are significant for their role in rock classification and formation, as well as their distinct physical properties that influence how they appear in nature.

5 Must Know Facts For Your Next Test

1. Amphiboles have a general formula of $$A_{2}B_{5}Si_{8}O_{22}(OH)_{2}$$, where A and B represent various cations.
2. They are commonly found in metamorphic rocks like schist and gneiss, as well as in igneous rocks such as diorite and granite.
3. Amphiboles exhibit perfect cleavage in two directions at angles of about 60° and 120°, which is a key characteristic for identification.
4. These minerals can vary widely in color, ranging from green and blue to brown or black, depending on their specific composition.
5. Common amphibole species include hornblende and tremolite, each with unique properties that can affect the physical characteristics of the rocks they inhabit.

Review Questions

• Compare and contrast amphiboles with pyroxenes in terms of their structure and occurrence in rocks.
  • Amphiboles and pyroxenes are both silicate minerals, but they differ in their structural arrangements. Amphiboles have a double chain structure, while pyroxenes have a single chain structure. This structural difference leads to variations in their physical properties, such as cleavage angles and crystal habits. Additionally, amphiboles are commonly found in both igneous and metamorphic rocks, whereas pyroxenes are primarily associated with igneous environments.
• Discuss the significance of amphiboles in the classification of metamorphic rocks, highlighting their role as indicators of metamorphic conditions.
  • Amphiboles play a crucial role in the classification of metamorphic rocks due to their stability under specific temperature and pressure conditions. The presence of different amphibole species can indicate the metamorphic grade of a rock. For example, hornblende typically forms under medium-grade metamorphic conditions, whereas tremolite is associated with lower grades. By studying the amphibole content within a rock sample, geologists can infer the metamorphic history and environmental conditions that shaped the rock.
• Evaluate how amphiboles contribute to our understanding of the petrogenesis of igneous and metamorphic rocks through their chemical composition.
  • The chemical composition of amphiboles provides valuable insights into the processes involved in the formation of igneous and metamorphic rocks. Variations in their cation content can reflect changes in the source material or conditions during crystallization. By analyzing the amphibole composition within a rock, researchers can determine factors such as temperature, pressure, and the availability of fluids during formation. This understanding aids in reconstructing geological histories and informs broader theories about Earth's crustal evolution.

Related terms

Pyroxenes:

A group of single chain silicate minerals that are closely related to amphiboles, often found in similar geological environments.

Mafic Minerals: Minerals that are rich in magnesium and iron, including many types of amphiboles, and typically dark in color.

Silicate Structure: The arrangement of silicon-oxygen tetrahedra in silicate minerals, which determines their classification and properties.