hornblendite

Hornblendite is an ultramafic igneous rock composed almost entirely of hornblende amphibole. It is a coarse-grained rock that typically appears dark in color, often black or dark green, due to the abundance of hornblende. While hornblende is the dominant mineral, minor amounts of other mafic minerals such as pyroxene or olivine may occasionally be present, though in hornblendite they are subordinate. Accessory minerals like magnetite or apatite can also occur in small quantities.

Hornblendite is formed through magmatic processes, typically as a cumulate rock where hornblende crystals settle out of a cooling magma. It is often found in association with other ultramafic or mafic intrusive rocks, such as gabbros or peridotites, within larger igneous intrusions. These rocks can also form as dikes or sills. Due to its composition, hornblendite is generally dense and resistant to weathering. It is not a common rock type but can be found in various geological settings worldwide. Its economic significance is generally low, though it may be explored for potential mineral deposits associated with its formation environment.