Alumino-ferrohornblende

Chemical formula: Ca₂(Fe₂+₄Al)(Si₇Al)O₂₂(OH)₂

Alumino-ferrohornblende is a common, rock-forming mineral of the amphibole group, characterized by its dark color and elongated, prismatic crystals.

## Characteristics Alumino-ferrohornblende is a silicate belonging to the amphibole group, constituting an important rock-forming mineral. It is a specific member of the hornblende series, defined by high aluminum and iron content. It forms elongated, prismatic or columnar crystals, often with a characteristic, hexagonal cross-section resembling a rhomb. It also occurs as granular aggregates, fibrous, or compact masses. Its color is typically dark green, brownish-green, and most commonly black. The luster is typically vitreous, although on weathered surfaces it can be dull. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale, making it relatively scratch-resistant. Its density is approximately 3.2-3.3 g/cm³, typical for iron-rich silicates. It is a brittle mineral, and on thin edges or in small fragments, it can be translucent, although it is usually opaque. ## Colors and Varieties Alumino-ferrohornblende exhibits colors ranging from dark green, through brownish-green, to completely black. The name "hornblende" is essentially a term encompassing an entire series of minerals with complex chemical compositions, and alumino-ferrohornblende is its specific, formally defined member. It does not have separate commercial or color varieties. ## History and Name The mineral's name is descriptive and directly derived from its chemical composition: "alumino" indicates the presence of aluminum, "ferro" indicates the dominance of iron, and "hornblende" specifies its belonging to this well-known amphibole subgroup. The name hornblende itself has historical roots in German, where the words *Horn* (horn) and *blenden* (to deceive, to mislead) described a mineral that visually resembled metal ores but could not be smelted. ## Uses As a common mineral, alumino-ferrohornblende has no direct industrial application. However, it has scientific significance as an indicator of the formation conditions of igneous and metamorphic rocks. Rocks rich in hornblende, such as amphibolites, are sometimes used locally as building or decorative stone.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Gray to greenish-gray
Density: 3.2-3.3
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Key features for identifying alumino-ferrohornblende are its dark (usually black) color, vitreous luster, and elongated, prismatic crystal habit. The most important diagnostic feature, often visible to the naked eye or under a magnifying glass, is cleavage in two directions intersecting at angles of approximately 56° and 124°. This gives the crystals a characteristic, rhombic cross-section. ## Distinguishing from Similar Minerals Alumino-ferrohornblende can be easily confused with minerals from the pyroxene group (e.g., augite). The main difference lies in the cleavage angle: pyroxenes have cleavage in two directions at an angle close to 90°, which gives them a more "square" cross-section. It is distinguished from black tourmaline (schorl) by the presence of cleavage (tourmaline does not have it) and a different crystal cross-section (a rounded triangle for tourmaline). Biotite, although also dark, has perfect cleavage in one direction, breaking into thin, flexible sheets. ## Crystal Forms Most often, it forms long, hexagonal prismatic and columnar crystals. It also occurs in the form of granular aggregates, fibrous (amphibole asbestos), and compact, amorphous rock masses.

Geological environment

## Genesis It is a very widespread rock-forming mineral. It forms under a wide range of temperature and pressure conditions. It crystallizes from magma as one of the main components of intermediate intrusive igneous rocks (such as diorite, granodiorite, syenite) and extrusive rocks (andesite). It is also a key mineral in metamorphic rocks, especially amphibolites, which form during regional medium- to high-grade metamorphism from basaltic igneous rocks or from impure carbonate rocks. ## Mineral Associations Alumino-ferrohornblende often co-occurs with plagioclase, biotite, quartz, garnet, epidote, and also with pyroxenes (mainly augite). ## Localities As a common mineral, it occurs worldwide in appropriate rock types. Well-formed, large crystals, prized by collectors, come from classic hornblende localities such as Bancroft in Ontario (Canada), Kragerø and Arendal (Norway), Pargas (Finland), as well as numerous sites in the Alps.

Rarity

Common

For collectors

## Quality Criteria The most sought-after by collectors are specimens with well-formed, sharp, and lustrous crystals of significant size. Single, terminated crystals or their groups (druses) set on a contrasting matrix, for example, white feldspar or calcite, are highly valued. Surface cleanliness and the absence of mechanical damage significantly increase the value of the specimen. ## Popular Localities Localities from which the most famous collector's specimens of hornblende originate include primarily the Bancroft area in Canada, where very large, black crystals have been found. Equally classic are specimens from Norway (Kragerø, Arendal) and Finland (Pargas).

Care and storage

## Cleaning Alumino-ferrohornblende specimens are relatively durable and can be safely cleaned with lukewarm water and a soft brush. Ultrasonic cleaners should be avoided, as they can damage specimens with internal fractures. ## What to Avoid The mineral is sensitive to strong acids, which can etch it. However, it is resistant to temperature changes and sunlight, so it does not require special protection against fading. ## Storage Store under standard conditions, protecting specimens from impacts and abrasions that could damage crystal edges or polished surfaces. Well-formed crystals are worth displaying in enclosed display cases or specimen boxes.

Sources

IMA Mineral List (CNMNC)