Hornblende

Chemical formula: AnCa₂(Z²⁺₅-mZ³⁺m)(Si₈-(n+m)Al(n+m))(OH,F,Cl)₂

Hornblende is an important group of amphibole silicates with a complex composition, forming dark, prismatic crystals in many igneous and metamorphic rocks.

## Characteristics Hornblende is a name for a series of minerals from the amphibole group, not a single mineral with a constant chemical composition. It is a key component of many igneous rocks (e.g., diorites, tonalites, andesites) and metamorphic rocks (e.g., amphibolites). Typical specimens form elongated, prismatic, or columnar crystals, often with a hexagonal cross-section. It also occurs as granular, radial, or fibrous aggregates. Its color is usually dark – from dark green, through brown, to black. ## Physical Properties This mineral is characterized by a hardness ranging from 5-6 on the Mohs scale and a density from 3.0 to 3.4 g/cm³, depending on its exact chemical composition. The luster is typically vitreous, and on cleavage planes, it can be pearly. It is an opaque mineral or translucent only on thin edges. ## Colors and Varieties Dark green, greenish-black, brown, and black colors dominate. The variability in color is a result of different iron, magnesium, and titanium content. Some titanium-rich varieties, known as oxyhornblende (formerly basaltic hornblende), are brown to black and form under volcanic conditions. Pargasite and hastingsite are examples of specific minerals within the hornblende series. ## History and Name The name "hornblende" comes from old German mining terms: *Horn* ("horn", referring to the shape of the crystals or their luster) and *blenden* ("to deceive", "to shine"), because this mineral resembled valuable metal ores but contained no useful metal. The name was formally introduced by Abraham Gottlob Werner in 1789. ## Uses Hornblende has no direct industrial applications. However, it is an important rock-forming mineral, and its presence and composition provide geologists with information about the conditions of rock formation. It is primarily of scientific and collectible significance.

Properties

Mohs hardness: 5-6
Luster: Vitreous, pearly on cleavage planes
Streak: White to light gray
Density: 3.0-3.4
Cleavage: Perfect, in two directions, intersecting at angles of ~56° and 124°
Fracture: Uneven, stepped
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of hornblende is its habit – elongated, prismatic crystals with a hexagonal cross-section. Crucial is also its perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°. Its dark color (green, brown, black) and vitreous luster are also characteristic. ## Distinguishing from Similar Minerals Hornblende is most often confused with augite (a mineral from the pyroxene group). Augite has a similar color and hardness, but its crystals are usually shorter, stockier, and the cross-section is almost square. The cleavage angles in augite are approximately 87° and 93° (i.e., close to 90°), which is the primary distinguishing feature from hornblende. It is distinguished from tourmaline (schorl) by the presence of distinct cleavage, which tourmaline lacks. ## Crystal Forms Hornblende forms columnar, acicular, or prismatic crystals. It also occurs as granular, fibrous, felted (as in nephrite, which is an amphibole variety), and radial aggregates.

Geological environment

## Genesis Hornblende is a widely distributed rock-forming mineral. It crystallizes from magma over a wide range of temperatures and pressures, being a typical component of intermediate intrusive igneous rocks (diorite, granodiorite, syenite) and volcanic rocks (andesite). It also forms during regional metamorphism of medium and high grade, creating rocks such as amphibolites, which result from the alteration of basalts or marls. ## Mineral Associations In igneous rocks, hornblende often coexists with plagioclase, biotite, quartz, and pyroxenes (augite). In metamorphic rocks, its typical associated minerals are plagioclase, garnets (especially almandine), and epidote. ## Localities Well-formed hornblende crystals come from many places around the world. Classic localities include the Bancroft area in Ontario (Canada), where large, well-formed crystals are found in syenites. In Europe, they are known from Kragerø and Arendal (Norway), Pargas (Finland), and from the Czech Republic (near Teplice). It commonly occurs in Alpine rocks in Italy and Austria. In Poland, it is a common component of granitoids and amphibolites in Lower Silesia.

Rarity

Common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with sharply terminated, well-formed, lustrous crystals with a distinct prismatic habit. Large, single crystals or attractive groups grown on a contrasting rock matrix (e.g., on white calcite or plagioclase) are particularly prized. Specimens from classic, historical localities also have higher value. ## Popular Localities The most valued collectible specimens come from Ontario province in Canada (especially from the Bancroft and Wilberforce areas), where crystals several tens of centimeters long have been found. Other important sources include New York and New Jersey in the USA, Norway (Arendal), the Czech Republic, and Alpine fissures in Italy.

Care and storage

## Cleaning Hornblende specimens can be safely cleaned with lukewarm water and a mild soap, using a soft brush. Strong detergents should be avoided. After washing, the specimen should be thoroughly rinsed, preferably in distilled water, and left to dry completely. ## What to Avoid Hornblende is sensitive to strong acids, which can damage its surface. Avoid sudden temperature changes. Although relatively hard, it is brittle and susceptible to mechanical damage along cleavage planes. ## Storage Specimens should be stored in separate boxes or on a soft surface to prevent scratching by harder minerals (e.g., quartz, corundum) and to protect them from impacts. It does not require special protection from light or humidity, but stable conditions are always recommended.