amfibolites

Amfibolites are a group of metamorphic rocks. They are characterized by the abundance of amphibole minerals, such as hornblende and actinolite. Plagioclase feldspar is another common mineral found in amfibolites. The specific mineral composition can vary, leading to different types of amfibolites, but amphibole is always a dominant component.

These rocks are typically medium to coarse-grained and can exhibit a visible layering or foliation due to the alignment of platy or elongated minerals. The color of amfibolites often ranges from dark green to black, depending on the type of amphibole present and the presence of other minerals like biotite or garnet.

Amfibolites form under medium to high-grade metamorphic conditions, meaning they are subjected to elevated temperatures and pressures. They commonly originate from the metamorphism of mafic igneous rocks, such as basalts and gabbros. However, they can also form from the metamorphism of sedimentary rocks rich in calcium and magnesium, like impure dolomites or magnesium-rich shales.

The presence of amfibolites in a geological setting indicates past high-temperature and high-pressure events, often associated with mountain building processes or deep crustal metamorphism. They are found in many ancient continental crust regions and are important indicators of the thermal and pressure history of the Earth's crust.