Alumino-magnesiohornblende

Chemical formula: Ca₂(Mg₄Al)(Si₇Al)O₂₂(OH)₂

A widespread, rock-forming mineral from the amphibole group, characterized by a dark green or black color and prismatic crystals.

## Characteristics Alumino-magnesiohornblende is a silicate from the amphibole supergroup, belonging to the hornblende series. This name specifies the chemical composition within this complex group of minerals. It forms elongated, prismatic crystals, often with hexagonal or rhombic cross-sections. It usually occurs as grains embedded in rock or as granular, fibrous, and radial aggregates. Well-formed, isolated crystals are rare. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale. Its luster is most often vitreous, although it can be dull on some surfaces. It is a translucent mineral, becoming almost opaque in thicker sections. The density ranges from 3.1 to 3.3 g/cm³. ## Colors and Varieties Alumino-magnesiohornblende ranges in color from dark green, through brownish-green, to black. The name does not refer to a color variety, but to a specific chemical composition with a dominance of aluminum and magnesium, as defined by the International Mineralogical Association (IMA). ## History and Name The mineral's name reflects its chemical composition – it indicates the presence of aluminum (alumino) and magnesium (magnesio) and its belonging to the hornblende group. The name "hornblende" comes from the German words *Horn* (horn) and *blenden* (to deceive, to mislead), which referred to its appearance and occurrence in ore deposits where it was not an ore itself. The formal distinction of alumino-magnesiohornblende is a result of the 2012 revision of amphibole nomenclature. ## Uses As a common rock-forming mineral, alumino-magnesiohornblende is primarily important in petrology and geology for determining rock formation conditions. It has no direct industrial applications. It has the status of a mineral of interest to systematic collectors and rock researchers.

Properties

Mohs hardness: 5-6
Luster: Vitreous to dull
Streak: Grayish white, pale green
Density: 3.1-3.3
Cleavage: Perfect on {110}
Fracture: Uneven, Splintery
Transparency: Translucent to nearly opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature is excellent cleavage in two directions, intersecting at angles of approximately 56° and 124°. This is visible on cross-sections of crystals as a characteristic rhombic outline. Other features include dark color, prismatic habit, and vitreous luster. ## Distinguishing from Similar Minerals Alumino-magnesiohornblende is most often confused with pyroxenes (e.g., augite), which have a very similar appearance and hardness. The key to differentiation is the cleavage angles – in pyroxenes, they are approximately 90°. It can also be confused with tourmaline, which, however, does not exhibit cleavage, and its crystals often have a rounded triangular outline in cross-section.

Geological environment

## Genesis It is a very widespread rock-forming mineral. It forms over a wide range of temperatures and pressures. It crystallizes from magmas of intermediate composition and high water content, forming rocks such as diorites, andesites, and granodiorites. It is also a key component of metamorphic rocks, e.g., amphibolites, formed by regional metamorphism of igneous and sedimentary rocks. ## Mineral Associations In igneous rocks, it coexists mainly with plagioclase and biotite. In metamorphic rocks, it is most often accompanied by plagioclase, epidote, garnet, and chlorite. ## Localities As a rock-forming mineral, it is common worldwide. However, there are no famous typical collector localities from which specimens of this specific name would be obtained. Specimens sold as "hornblende" come from many places, including Ontario (Canada), Norway, the Czech Republic, and Italy.

Rarity

Common

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with large, well-formed, lustrous crystals with sharp edges. Crystals embedded in light matrix rock (e.g., marble or light feldspar) are also highly prized, creating an attractive contrast. Most specimens on the market are sold under the general name "hornblende." ## Popular Localities Collector-grade hornblende specimens come from many localities worldwide. Classic ones include the Bancroft area in Ontario (Canada), where large, black crystals were found, as well as Kragerø in Norway. However, it should be noted that they are rarely analyzed for the exact chemical composition corresponding to alumino-magnesiohornblende.

Care and storage

## Cleaning Specimens can be cleaned with a soft brush under running lukewarm water. For heavier soiling, distilled water can be used. Ultrasonic cleaners should be avoided, as they can damage the specimen along cleavage planes. ## What to Avoid The mineral is sensitive to acids, which can etch it. Avoid sudden temperature changes. Despite its relatively high hardness, it is brittle due to excellent cleavage, so it must be protected from impacts. ## Storage Store in separate, padded boxes or on stands to avoid scratches and chipping. It is not sensitive to sunlight. Standard room conditions are entirely sufficient.

Sources

IMA Mineral List (CNMNC)