Alumino-ferrotschermakite

Chemical formula: &#₉₇₄₄;Ca₂(Fe₂+₃Al₂)(Si₆Al₂)O₂₂(OH)₂

Alumino-ferrotschermakite is an amphibole group mineral, an important constituent of some metamorphic and igneous rocks.

## Characteristics Alumino-ferrotschermakite is a mineral belonging to the amphibole supergroup, specifically to the calcic amphibole group. It is an important rock-forming mineral, although its identification requires advanced analytical techniques. Macroscopically, it is indistinguishable from other dark amphiboles, such as common hornblende. It typically forms elongated, prismatic crystals ranging in color from dark green to black. It occurs as embedded crystals in the host rock, less frequently forming granular or fibrous aggregates. ## Physical Properties This mineral is characterized by a Mohs hardness of 5-6, which is typical for amphiboles. It has a vitreous luster, and on cleavage planes, it can be pearly. It is an opaque mineral, only translucent in very thin fragments. Its density ranges from 3.2 to 3.4 g/cm³. ## Colors and Varieties Its color is consistently dark – from dark green, through greenish-black, to black. The mineral's name is closely related to its chemical composition, so no color varieties or commercial varieties are distinguished. Each specimen with this name must correspond to a specific chemical formula. ## History and Name The name "Alumino-ferrotschermakite" reflects its chemical composition: "alumino" indicates a significant aluminum content, and "ferro" indicates iron dominance. The "tschermakite" component honors the Austrian mineralogist Gustav Tschermak von Seysenegg (1836–1927), who studied isomorphic substitutions in amphiboles and pyroxenes. The formal definition of the mineral is a result of the amphibole nomenclature revision by the International Mineralogical Association (IMA) in 2012, which precisely defined the chemical compositions of individual end-members. ## Uses Alumino-ferrotschermakite has no industrial applications. Its significance is purely scientific, mainly in petrology, where its presence and chemical composition provide information about the pressure and temperature conditions during metamorphic processes.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Grayish green
Density: 3.2-3.4
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Accurate identification of Alumino-ferrotschermakite is impossible based on visual characteristics alone. Advanced chemical analyses, such as X-ray microanalysis (EDS/WDS) or X-ray diffraction (XRD), are necessary to precisely determine its composition and structure. In field conditions or in a collection, it can only be identified as a "hornblende group amphibole." ## Distinguishing from Similar Minerals This mineral is visually identical to other dark amphiboles, such as ferro-hornblende, magnesiohornblende, pargasite, or ferrotschermakite. It is distinguished from pyroxenes (e.g., augite) by the characteristic amphibole cleavage angle (approx. 124°/56°), whereas pyroxenes exhibit cleavage at an angle close to 90°. ## Crystal Forms It most often forms columnar, elongated crystals with a hexagonal or rhombic cross-section. It also occurs as granular, fibrous aggregates or as chaotic masses embedded in the rock.

Geological environment

## Genesis Alumino-ferrotschermakite is a typical mineral of metamorphic rocks, forming under amphibolite and granulite facies conditions. It results from medium to high-grade metamorphism of mafic and intermediate igneous rocks (e.g., basalts, andesites) and contaminated carbonate rocks. It can also crystallize as a primary mineral in some igneous rocks, such as diorites and granodiorites. ## Mineral Associations It most commonly co-occurs with plagioclase (especially andesine and labradorite), garnet (mainly almandine), epidote, biotite, quartz, and other amphibole group minerals. ## Localities As a chemically defined mineral, its occurrence is confirmed in many places worldwide where detailed petrological studies are conducted. Documented localities include, among others, the vicinity of Blízko u Žďárce in the Czech Republic, the Val di Fassa valley in Italy, and numerous sites in Austria, Germany, and the USA (e.g., Hampshire County, Massachusetts).

Rarity

Rare

For collectors

## Quality Criteria Since Alumino-ferrotschermakite is not a typical collector's mineral, its evaluation criteria differ from those for ornamental specimens. For the scientific community and specialized collectors, the most valuable specimens are those whose identity has been analytically confirmed. The value of such a specimen increases if it comes from a well-described locality (preferably a type locality) and exhibits clear crystal forms, even if they are microscopic in size (so-called micromounts). ## Popular Localities There are no "popular" localities in a commercial sense. The most prized specimens come from localities that have been thoroughly described in scientific literature, as they provide geological context. Specimens from places like Val di Fassa in Italy or the Klet massif in the Czech Republic are examples of material of research significance.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. Due to its perfect cleavage, ultrasonic cleaners should be avoided, as they can cause cracking and delamination of crystals along natural parting planes. ## What to Avoid The mineral is relatively chemically stable, but contact with strong acids should be avoided. It does not show particular sensitivity to sunlight or temperature changes under household exposure conditions. ## Storage Store in a dry place, protected from dust and mechanical damage. It can be safely stored with other minerals of similar or lower hardness, such as quartz or feldspars, although separate boxes are recommended for better protection.

Sources

IMA Mineral List (CNMNC)