hornblendebearing

Hornblendebearing is a geological descriptive term for rocks or mineral assemblages that contain hornblende, the common dark amphibole mineral. Hornblende is a member of the amphibole group and typically forms as elongated, prismatic crystals. Its composition is highly variable, commonly written as Ca2(Mg,Fe,Al)4Si8O22(OH)2, with variations that include ferrohornblende and magnesiohornblende depending on the relative amounts of iron, magnesium, calcium, and aluminum.

Occurrence and rock types

Hornblende occurs widely in igneous rocks of intermediate to felsic composition, such as diorite, granodiorite, and andesite, where it may be a major dark constituent. It is also common in granitoids and can be abundant in certain rhyolites and rhyodacites. In metamorphic rocks, hornblende is a characteristic mineral of amphibolites and related schists and granulites that form under hydrous conditions. The presence of hornblende in a rock often reflects relatively water-rich conditions during its formation.

Identification and properties

Hornblende crystals are typically dark green to black with a vitreous to pearly luster. They show two directions of cleavage at approximately 56 and 124 degrees. The mineral is usually relatively hard, with a hardness around 5–6, and has a specific gravity in the low to mid 3s. Amphiboles, including hornblende, can exhibit weak pleochroism, appearing slightly differently when viewed from different angles under polarized light.

Significance

Hornblende-bearing rocks provide clues about the pressure–temperature and fluid conditions during formation. The mineral assemblage supports hydrous melting and metamorphic histories and can be used in geothermobarometry and, in some cases, radiometric dating (for example, by Ar-Ar methods on hornblende or related minerals) to constrain ages of rocks and metamorphic events.

Notes

The term is descriptive and non-specific to a single mineral species, encompassing a range of hornblende-group compositions within igneous and metamorphic contexts.