Hornblende

Hornblende is a common dark amphibole, a complex calcium-magnesium-iron silicate of the inosilicate (double-chain) class rather than a single mineral with a fixed formula. Hornblende crystallizes from intermediate to mafic magmas at temperatures between 700 and 1000 degrees Celsius. It is one of the key minerals in Bowen's reaction series, forming in the continuous reaction series after olivine and pyroxene have crystallized from the cooling melt. Hornblende requires water to form, meaning it only crystallizes from hydrous magmas containing at least 2 to 4 percent dissolved water. This is why it is abundant in continental volcanic arc rocks (like andesites and diorites) where subducting ocean plates release water into the overlying mantle wedge.

The crystal structure consists of double chains of silicon-oxygen tetrahedra running parallel to the c-axis of the crystal, with calcium, magnesium, iron, and aluminum ions occupying octahedral sites between the chains. Hydroxyl groups (OH) sit at the center of each ring of tetrahedra. This double-chain structure controls the crystal's habit (elongated, prismatic) and cleavage (two directions intersecting at 56 and 124 degrees), which are diagnostic features for identification.

In metamorphic rocks, hornblende forms during amphibolite-facies metamorphism at temperatures between 500 and 700 degrees Celsius and pressures of 2 to 10 kilobars. The transition from greenschist facies (dominated by chlorite and actinolite) to amphibolite facies (dominated by hornblende) is one of the most important boundaries in metamorphic petrology. Rocks composed primarily of hornblende and plagioclase feldspar are called amphibolites and represent metamorphosed basalts and gabbros.

Hornblende forms elongated, prismatic to columnar crystals with a roughly diamond-shaped cross-section. The most diagnostic feature is its cleavage: two sets of cleavage planes intersecting at approximately 56 and 124 degrees, visible on broken cross-sections. This distinguishes it from pyroxene, which has two cleavage planes intersecting at nearly 90 degrees. At hardness 5 to 6, hornblende scratches glass and cannot be scratched with a knife.

Distinguish it from augite (pyroxene), which has a blockier crystal habit and 90-degree cleavage angles. Unlike biotite, hornblende does not split into thin flexible sheets. It differs from tourmaline in having cleavage (tourmaline has no cleavage and shows conchoidal fracture). In hand specimens, hornblende is typically dark green to black with a vitreous luster on fresh surfaces. In thin section under polarized light, it shows strong pleochroism from pale green to dark green or brown, which is a key identifier for petrographers.