hornblend

Hornblende is a common rock-forming mineral in the amphibole group, a complex inosilicate. It typically forms dark-green to black, elongated prismatic crystals with a vitreous luster and two prominent cleavage directions at angles near 56 and 124 degrees. The mineral is usually opaque and displays a range of colors from green to brown to black, depending on its iron and magnesium content.

Chemical composition varies within the hornblende group. It is generally calcium- and sodium-rich and can include iron, magnesium, aluminum, and hydroxide. The group includes varieties such as magnesiohornblende and ferrohornblende, which reflect relative Mg and Fe content. Because of this variability, hornblende serves as a useful indicator of the chemical conditions during rock formation.

Occurrence and formation: Hornblende is widespread in both igneous and metamorphic rocks. It is common in intermediate to felsic rocks such as granodiorite, diorite, and andesite, and in metamorphic rocks like amphibolites, schists, and gneisses formed under regional metamorphism. It typically forms during the crystallization of hydrous magmas or during metamorphic hydration and alteration of aluminous rocks.

Properties and significance: With a Mohs hardness around 5–6 and a specific gravity roughly 2.9–3.5, hornblende is a key diagnostic mineral for petrographic analysis. While not a major ore mineral, its presence helps geologists infer pressure–temperature conditions and the evolution of igneous and metamorphic rocks.