Ferro-ferri-obertiite

Chemical formula: NaNa₂(Fe₂+₃Fe₃+Ti₄+)Si₈O₂₂O₂

Ferro-ferri-obertiite is an extremely rare, black amphibole group mineral, characterized by a complex composition with iron in two oxidation states.

## Characteristics Ferro-ferri-obertiite is a complex silicate from the amphibole supergroup, formally approved as a distinct mineral species in 2015. It forms very small, acicular or prismatic black crystals, often gathered in radial or fibrous aggregates. It typically occurs as inclusions in other minerals and rarely reaches sizes visible to the naked eye, making it primarily of interest to micromineral collectors. ## Physical Properties This mineral is characterized by a vitreous luster and is opaque. Its Mohs hardness is about 6, which is typical for amphiboles. The density calculated based on the chemical formula and unit cell parameters is 3.43 g/cm³, resulting from the high content of heavy elements such as iron and titanium. ## Colors and Varieties Ferro-ferri-obertiite is monochromatic – it occurs exclusively in black. No colored or commercial varieties are known. ## History and Name The mineral's name is tripartite and precisely describes its chemical composition and structural affiliation. The particle "ferro-" refers to the presence of divalent iron ions (Fe²⁺), "ferri-" to trivalent iron ions (Fe³⁺), and "obertiite" indicates its connection to the obertite structure, named in honor of Italian crystallographer Roberta Oberti. The mineral was described by Frank C. Hawthorne and collaborators. ## Applications Due to its extreme rarity and occurrence in microscopic quantities, ferro-ferri-obertiite has no industrial applications. It is solely an object of scientific research and advanced collecting.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Grey
Density: 3.43
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Amateur identification of ferro-ferri-obertiite is practically impossible. It requires advanced analytical techniques, such as X-ray microanalysis (EDS/WDS) to determine chemical composition and X-ray diffraction (XRD) to confirm crystal structure. Preliminary indications may include its black color, crystal habit, and specific environment of occurrence. ## Distinguishing from Similar Minerals This mineral is visually indistinguishable from many other black amphiboles (e.g., arfvedsonite, riebeckite, hornblende) and some pyroxenes (e.g., aegirine). Certain differentiation is only possible based on detailed chemical analysis, which will reveal a unique combination of sodium, titanium, and iron in the +2 and +3 oxidation states. ## Crystal Forms Ferro-ferri-obertiite forms elongated, prismatic or acicular crystals. They often occur as radial or tangled aggregates embedded in the host rock.

Geological environment

## Genesis This is a metamorphic mineral. It forms under specific conditions within metamorphosed iron and manganese ore deposits, referred to as skarns or iron formations. ## Mineral Associations At its type locality, ferro-ferri-obertiite co-occurs with minerals such as calcite, diopside, garnet, magnetite, hematite, and other rare amphiboles. ## Localities The only confirmed locality for this mineral in the world is its type locality – the Kärrgruvan mine in the Norberg region, Västmanland, Sweden.

Rarity

Extremely rare

For collectors

## Quality Criteria For such a rare micromineral, the main criterion for value is the quality of the identification itself – the specimen must be well-characterized analytically. From a collector's perspective, samples with clearly formed, undamaged crystals are desirable, even if they are microscopic in size. An additional advantage is their abundance on the host rock and association with other rare minerals. ## Popular Localities The only source of collector specimens is the Kärrgruvan mine in Sweden. Material from this locality is extremely difficult to obtain and is a rarity in specialized collections.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. For heavier soiling, distilled water can be used. Ultrasonic cleaners should be avoided, as they can damage delicate, acicular crystals. ## What to Avoid The mineral is sensitive to strong acids. It should be protected from impacts and shocks, which can lead to the chipping of microscopic crystals from the host rock. It does not show particular sensitivity to light or temperature changes. ## Storage The best way to store specimens is to place them in a closed, padded "micromount" box, which protects against dust and mechanical damage. Each specimen should be precisely labeled with its name and location.

Sources

IMA Mineral List (CNMNC)