Magnesio-ferri-hornblende

Chemical formula: &#₉₇₄₄;Ca₂(Mg₄Fe₃+)[(Si₇Al)O₂₂](OH)₂

Magnesio-ferri-hornblende is a mineral from the amphibole group, an important component of igneous and metamorphic rocks, typically dark green to black in color.

## Characteristics Magnesio-ferri-hornblende is a silicate belonging to the calcic amphibole group. It is an important rock-forming mineral, being a key component of many igneous and metamorphic rocks. It forms prismatic crystals, ranging from short and stout to elongated, acicular. It usually occurs as embedded grains or compact, granular, or fibrous aggregates. Its characteristic feature is a dark, most often black, dark green, or dark brown color. ## Physical Properties The mineral's hardness on the Mohs scale is 5-6, making it relatively scratch-resistant. The luster is typically vitreous, and on fracture surfaces, it can be somewhat duller. It is an opaque mineral, becoming translucent only on very thin edges or in thin fragments. The density ranges from 3.2-3.3 g/cm³. ## Colors and Varieties The color palette is limited to dark shades of green, brown, and black. This mineral is part of the complex hornblende solid solution series, and its precise identification and differentiation from other members of the group (such as ferro-hornblende or magnesio-hornblende) based on visual characteristics is impossible – it requires advanced chemical analyses. No commercial varieties are distinguished for it. ## History and Name The mineral's name, approved by the International Mineralogical Association (IMA) as part of the new amphibole classification in 2012, directly reflects its chemical composition. The prefix "Magnesio-" indicates the dominance of magnesium, "ferri-" indicates the presence of ferric iron (Fe³⁺), and "hornblende" refers to its belonging to this broad and long-known group of minerals. The name "hornblende" comes from old German mining terms "Horn" (horn) and "blenden" (to deceive, to mislead), which referred to the mineral's appearance and the lack of useful metal ore. ## Uses Magnesio-ferri-hornblende has no direct industrial application. Its significance is primarily scientific, as an indicator of the pressure and temperature conditions under which rocks formed (petrology). Rocks containing hornblende, such as diorites or amphibolites, are used as building and decorative stone.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Pale green to grayish green
Density: 3.2-3.3
Cleavage: Good on {110}
Fracture: Uneven to subconchoidal
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature for identifying the mineral as a member of the amphibole group is its cleavage. It has two perfect cleavage planes intersecting at angles of approximately 56° and 124°. In cross-section of the crystal, this creates a characteristic rhombic outline. Dark color and vitreous luster are also helpful in identification. ## Differentiation from Similar Minerals Magnesio-ferri-hornblende is most often confused with pyroxenes (e.g., augite), which have a similar color and are also rock-forming minerals. The key distinction lies in the cleavage angle – for pyroxenes, it is approximately 90°. Differentiation from other dark amphiboles (e.g., ferro-hornblende, pargasite) is practically impossible without chemical analysis. ## Crystal Forms Crystals are monoclinic, columnar or prismatic in habit, often terminated by simple faces. The cross-section of crystals is hexagonal or rhombic. It also occurs as granular, fibrous (asbestiform), or radial aggregates.

Geological environment

## Genesis Magnesio-ferri-hornblende is a product of crystallization from intermediate (e.g., andesitic, dioritic) magmas rich in water. It also commonly forms in regional metamorphic processes, especially under amphibolite facies conditions, as a result of the transformation of igneous rocks (basalts, gabbros) or impure sedimentary rocks (marls, dolomites). ## Mineral Associations In metamorphic rocks (amphibolites), it most often coexists with plagioclase, garnet (almandine), epidote, and biotite. In igneous rocks (diorites, granodiorites, syenites), it is accompanied by plagioclase, orthoclase, quartz, biotite, as well as titanite and magnetite. ## Localities As a rock-forming mineral, it is widely distributed. Well-formed crystals are known from many classic localities. The type locality is in the vicinity of Kragerø, Norway. Other known occurrences include the Zillertal valley in Austria, the Bancroft area in Ontario (Canada), as well as numerous localities in Italy, the Czech Republic, and the USA.

Rarity

Not very common

For collectors

## Quality Criteria The collector's value of magnesio-ferri-hornblende specimens (most often sold under the general name "hornblende") depends on the degree of crystal development. Sharp, well-terminated, lustrous crystals of large size are most valued. Their attractiveness is enhanced by their placement on a light, contrasting rock matrix, for example, on white feldspar or calcite. ## Popular Localities Localities providing the best collector specimens include those where the mineral crystallized in fissures or rock cavities. Classic examples include Kragerø (Norway), Pargas (Finland), Bancroft (Ontario, Canada), and some Alpine localities in Austria and Italy.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. Due to the presence of cleavage planes, ultrasonic cleaners should be avoided, as they can cause cracking or delamination of crystals. ## What to Avoid The mineral is sensitive to strong acids, especially hydrofluoric acid, which dissolves it. Avoid sudden temperature changes and impacts, as it is brittle and can easily cleave along cleavage directions. ## Storage Storage does not require special conditions. The mineral is insensitive to light. To avoid scratches, it should be stored separately from harder minerals, such as quartz or corundum. It is best to keep it in separate boxes or on soft pads.

Sources

IMA Mineral List (CNMNC)