Amphibole

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Thin section microscopy Siilinjärvi R636 6335 amphibole

Amphibole 100

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Amphibole refers to an important group of silicate minerals that form in both igneous rocks and metamorphic rocks. The amphiboles are an essential part of the Earth's crust, contributing to a variety of geological processes and having significant implications for understanding the Earth's geology and the environmental aspects related to amphibole exposure.

Composition and Structure

Amphiboles are characterized by their complex chemical composition, which can be generalized by the formula (Ca,Na)_2-3(Mg,Fe,Al)_5(Si,Al)_8O_22(OH,F)_2. This formula reflects the presence of various cations, including calcium (Ca), sodium (Na), magnesium (Mg), iron (Fe), and aluminum (Al), in their structure. The structure of amphiboles is composed of double chains of silica tetrahedra linked together by metal ions, with hydroxyl groups or fluorine at the end of the chains. This structure is what distinguishes amphiboles from other silicate minerals, such as the pyroxenes, which have a single chain structure.

Types of Amphibole

There are many types of amphibole, each with its own unique chemical composition and physical properties. Some of the more common members include:

• Hornblende: The most common amphibole, found in many igneous rocks and metamorphic rocks.
• Tremolite: A white to light green mineral found in metamorphosed carbonate rocks.
• Actinolite: Similar to tremolite but with a higher iron content, giving it a darker color.
• Anthophyllite: Found in some metamorphic rocks, known for its fibrous and lamellar forms.
• Crocidolite: Also known as blue asbestos, a fibrous amphibole with significant health hazards.

Occurrence

Amphiboles are found in a variety of geological settings. In igneous rocks, they are typically associated with intermediate to felsic compositions, such as andesite, diorite, and granite. In metamorphic rocks, amphiboles are common in both regional and contact metamorphism, forming in rocks such as schist and gneiss. The presence and type of amphibole in a rock can provide valuable information about the conditions under which the rock formed.

Health Hazards

Certain types of amphibole, particularly those with a fibrous habit like crocidolite (blue asbestos), pose significant health hazards. Inhalation of asbestos fibers can lead to diseases such as asbestosis, lung cancer, and mesothelioma. Due to these health risks, the use of asbestos has been heavily regulated or banned in many countries.

Environmental Impact

Beyond health concerns, amphiboles can also impact the environment. The weathering of amphibole-bearing rocks can release ions into soils and water systems, affecting water chemistry and soil fertility. Additionally, the study of amphiboles can provide insights into environmental conditions such as pollution levels and the history of water-rock interactions.

Conclusion

Amphiboles play a crucial role in both the geology of the Earth and in environmental and health sciences. Their complex chemistry, diverse occurrences, and the implications of their presence in the environment make them a significant subject of study in the geosciences.

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