Hornblende is a complex inosilicate mineral that belongs to the amphibole group, characterized by its dark color and elongated prismatic crystals. It plays an important role in metamorphic geology as it forms under specific pressure-temperature conditions, reflecting the metamorphic facies of the rocks in which it is found.
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Hornblende typically forms in high-grade metamorphic environments where temperatures range from 500°C to 800°C and pressures are substantial.
The presence of hornblende in a rock can indicate its formation in a subduction zone, where oceanic crust is subjected to high pressure and temperature conditions.
Hornblende is often found in igneous rocks as well, particularly in diorite and granite, where it contributes to the overall texture and mineral composition.
In addition to its role in metamorphic rocks, hornblende can also be an important indicator mineral for understanding geological history and tectonic settings.
Hornblende has a distinctive cleavage pattern, with two directions of cleavage that intersect at approximately 124° and 56°, which helps in identifying it in hand samples.
Review Questions
How does hornblende's formation relate to the pressure-temperature conditions found in specific metamorphic facies?
Hornblende forms primarily in high-grade metamorphic conditions, where temperatures typically range between 500°C to 800°C and pressures are elevated. This relationship is key to understanding metamorphic facies since hornblende's presence indicates specific geological environments such as subduction zones. By studying the mineral composition of rocks that contain hornblende, geologists can infer the metamorphic history and the environmental conditions during their formation.
Discuss the significance of hornblende in identifying geological processes, particularly in relation to subduction zones.
Hornblende serves as an important indicator of geological processes associated with subduction zones. In these areas, oceanic crust is pushed down into the mantle, creating high-pressure and high-temperature conditions conducive to hornblende formation. The occurrence of hornblende within metamorphic rocks provides valuable insights into the tectonic history of an area, including past subduction events and the evolution of the continental crust.
Evaluate the role of hornblende in both metamorphic and igneous contexts and how this dual role enhances our understanding of Earth's geological processes.
Hornblende plays a dual role in both metamorphic and igneous contexts, which enhances our understanding of Earth's geological processes. In metamorphic rocks, its formation signifies specific pressure-temperature conditions associated with high-grade metamorphism, particularly in environments like subduction zones. Conversely, its presence in igneous rocks such as diorite suggests cooling processes involving magma at varying depths. By studying hornblende across different rock types, geologists can piece together a more comprehensive picture of tectonic activities, magmatic processes, and thermal histories of the Earth’s crust.
A group of double-chain silicate minerals that includes hornblende, typically characterized by their dark colors and complex crystal structures.
Metamorphic Facies: A set of metamorphic mineral assemblages formed under specific pressure and temperature conditions that help to classify metamorphic rocks.
Schist: A medium-grade metamorphic rock that often contains large amounts of hornblende, along with other minerals like mica and quartz.