Magnesio-hornblende

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

Magnesio-hornblende is a common, rock-forming mineral from the amphibole group, characterized by a dark green to black color and elongated, prismatic crystals.

## Characteristics Magnesio-hornblende is a silicate belonging to the species-rich amphibole group. It forms an isomorphic series with ferro-hornblende, meaning that magnesium and iron can substitute for each other in the crystal structure. Typical specimens appear as elongated, prismatic or acicular crystals, often terminated by characteristic, inclined faces. These crystals frequently form radial or tangled aggregates. This mineral is an important component of many igneous and metamorphic rocks, often imparting a dark coloration to them. ## Physical Properties The hardness of magnesio-hornblende on the Mohs scale ranges from 5 to 6, classifying it as a mineral with medium scratch resistance. The luster is typically vitreous, and on fracture surfaces, it can be dull. It is a relatively heavy mineral, with a density ranging from 3.07 to 3.28 g/cm³. Crystals are usually opaque, and only on thin edges may they be translucent. ## Colors and Varieties The color of magnesio-hornblende is its characteristic feature – most often it is dark green, greenish-black, or almost black. Brownish-green specimens are less common. Color variability is mainly a function of iron content. No distinct color or commercial varieties are distinguished; identification relies on chemical analysis to differentiate it from other hornblendes. ## History and Name The name "magnesio-hornblende" refers directly to its chemical composition – it is a magnesium-rich ("magnesio-") variety of hornblende. The name "hornblende" comes from old German mining terms: *Horn* ("horn," referring to its shape or luster) and *blenden* ("to deceive," "to mislead"), because this mineral resembled valuable metal ores but contained no useful metal. Magnesio-hornblende was formally recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 1978 as part of a revision of amphibole classification. ## Uses As a rock-forming mineral, magnesio-hornblende has no direct industrial application. However, it is a subject of scientific interest in petrology, helping to determine the pressure and temperature conditions under which rocks formed. For collectors, well-formed, large crystals from classic localities are particularly valuable.

Properties

Mohs hardness
5-6
Color
Medium green to dark green to green-black to black, brown
Luster
Vitreous
Streak
Pale grey-green, grey-white
Density
0
Cleavage
on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Magnesio-hornblende can be identified by its dark green to black color, vitreous luster, and characteristic, elongated crystal shape. A key diagnostic feature of amphiboles, including hornblende, is the angle between cleavage planes, approximately 124° and 56°, which is visible in cross-section of the crystals as a rhombic outline. ## Distinguishing from Similar Minerals Magnesio-hornblende is difficult to distinguish from other dark amphiboles (such as ferro-hornblende, pargasite) and pyroxenes (e.g., augite) without advanced analysis. It differs from augite by the aforementioned cleavage angle (in augite, it is close to 90°). From tourmaline (schorl), it is distinguished by lower hardness and a different crystal cross-section (in tourmaline, it is trigonal, often with rounded faces). ## Crystal Forms Crystals are typically prismatic, elongated along one axis, often with a hexagonal or rhombic cross-section. It also occurs in granular, fibrous, radial aggregates, or as embedded grains within rock.

Geological environment

## Genesis Magnesio-hornblende is a typical mineral of igneous and metamorphic rocks. It crystallizes from intermediate, water-rich magmas, forming rocks such as diorites, andesites, and granodiorites. It also forms during medium- to high-grade regional metamorphism, primarily in amphibolites, where it is a main component, and in gneisses. ## Mineral Associations In igneous rocks, magnesio-hornblende often coexists with plagioclase, biotite, quartz, and pyroxenes. In metamorphic rocks, its typical associated minerals are plagioclase, garnets (especially almandine), epidote, as well as staurolite and kyanite under higher pressure conditions. ## Localities As a common mineral, magnesio-hornblende occurs in many places worldwide. Well-formed crystals are known from Pargas, Finland (type locality), the Arendal area in Norway, the Bancroft region in Ontario (Canada), as well as numerous localities in the Czech Republic (e.g., Tešínsko area), Italy (including Val di Fassa valley), and the USA (New York and California states).

Rarity

Common

For collectors

## Quality Criteria For collectors, specimens with sharply terminated, well-formed, and lustrous crystals are most desirable. Large, single crystals with a distinct, hexagonal outline are highly valued, as are aesthetic groups of crystals grown on a contrasting rock matrix. A dark green, intense color is preferred over almost black, where details of the form are harder to discern. Specimens without mechanical damage, with clear terminations, achieve the highest value. ## Popular Localities Classic and most prized collector specimens come from historical localities in Europe, such as Pargas in Finland and Arendal in Norway. Large and well-formed crystals are also supplied by mines in the Bancroft region of Canada. Specimens from the Czech Republic and Italy are also popular in collections, often as examples of typical forms of this mineral.

Care and storage

## Cleaning Magnesio-hornblende specimens are best cleaned with a soft, dry brush to remove dust. For heavier dirt, distilled water and a delicate brush can be used. Ultrasonic cleaners should be avoided, as they can damage crystals, especially if the specimen has internal fractures. ## What to Avoid This mineral is sensitive to strong acids, which can etch it and cause surface dulling. Abrupt temperature changes should be avoided. Although stable in light, prolonged exposure to intense sunlight is not recommended for any collector's specimen. ## Storage Specimens should be stored in stable conditions, away from chemicals and sources of moisture. Due to its hardness of 5-6, it is susceptible to scratching by harder minerals such as quartz or corundum. It is recommended to store it in separate boxes or display cases where it will not come into contact with other, harder specimens.

External references

Sources

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