Ferri-magnesiokatophorite

Chemical formula: NaNaCa(Mg,Fe₃+)₅(Si,Al)₈O₂₂(OH)₂

A rare sodium-calcium amphibole with a complex chemical composition, forming dark, prismatic crystals in metamorphic and alkaline rocks.

## Characteristics Ferri-magnesio-katophorite is a mineral from the amphibole group, belonging to the katophorite series. It is a complex silicate of sodium, calcium, magnesium, and iron. It forms elongated, prismatic crystals, often terminated by steep faces. It also occurs as fibrous aggregates or granular masses. Its crystals are usually small and embedded in the host rock. ## Physical Properties This mineral is characterized by a hardness ranging from 5-6 on the Mohs scale. It has a vitreous luster and is typically opaque, rarely translucent in thin fragments. Its density is approximately 3.23 g/cm³. ## Colors and Varieties Ferri-magnesio-katophorite occurs in shades from dark green to black. Pleochroism is distinct, changing color from yellowish or light brown to dark green or bluish-green depending on the observation direction. ## History and Name The mineral's name refers to its chemical composition – the dominance of iron (ferri) and magnesium (magnesio) and its relation to katophorite. It was formally described as a distinct mineral species relatively recently, as part of the amphibole nomenclature revision by the International Mineralogical Association (IMA).

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Greenish-gray
Density: 3.23
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of ferri-magnesio-katophorite in field conditions is practically impossible and requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) or X-ray diffraction (XRD). In a collection, it can be preliminarily recognized based on its dark color, prismatic habit, co-occurrence with other alkaline minerals, and excellent cleavage typical of amphiboles. ## Distinguishing from Similar Minerals It is visually indistinguishable from other dark amphiboles, such as hornblende, arfvedsonite, riebeckite, or other minerals from the katophorite group. Definitive differentiation is possible only through chemical analysis. ## Crystal Forms It most often forms columnar or prismatic crystals, often with a hexagonal cross-section. It also occurs as fibrous, acicular, or granular aggregates within the rock.

Geological environment

## Genesis Ferri-magnesio-katophorite is a characteristic mineral of alkaline igneous rocks, such as nepheline syenites and phonolites. It also forms under conditions of regional metamorphism, in the glaucophane schist and eclogite facies, especially in metasomatically altered sodium-rich rocks. ## Mineral Associations It often co-occurs with minerals such as aegirine, albite, nepheline, glaucophane, crossite, epidote, titanite, and quartz. ## Localities Confirmed occurrences of this mineral are rare. It has been identified, among others, in the Ilímaussaq igneous complex in Greenland, on the Kola Peninsula in Russia, in Norway (Fen Complex), and also in some localities in Italy and the USA (California).

Rarity

Very rare

For collectors

## Quality Criteria As a rare and difficult-to-identify mineral, specimens with well-formed, analyzable crystals from an analytically confirmed locality have the highest collector value. Precise documentation and a label indicating the conducted research are crucial. The size and aesthetics of the crystals are of secondary importance compared to their scientific value and certainty of identification.

Care and storage

## Cleaning Specimens should only be cleaned with a soft brush to remove dust. The use of water, especially with detergents, is not recommended due to the risk of chemical reactions and damage to delicate aggregates. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral's components. It should not be heated or subjected to ultrasound. Prolonged exposure to moisture can lead to slow chemical changes. ## Storage Store in a dry place, under stable conditions, away from direct sunlight. It is best to keep it in a closed display box to protect it from dust and mechanical damage.