Ferri-obertiite

Chemical formula: NaNa<sub>2</sub>(Mg<sub>3</sub>Fe<sup>3+</sup>Ti<sup>4+</sup>)Si<sub>8</sub>O<sub>22</sub>O<sub>2</sub>

Ferri-obertiite is a very rare amphibole group mineral, distinguished by its dark brown to black color and complex chemical composition.

## Characteristics Ferri-obertiite is a complex silicate from the amphibole group, approved as a distinct mineral species by the International Mineralogical Association (IMA) in 2015. It occurs as very small, prismatic crystals, usually not exceeding 1 mm in length, forming aggregates or appearing as single grains within the host rock. Its color is dark brown to black, and visually it is indistinguishable from other dark amphiboles without advanced analysis. ## Physical Properties Crystals exhibit a vitreous to resinous luster. The mineral is opaque. Due to the small size of the crystals, many physical properties, such as hardness or density, have not been precisely measured, but only calculated based on chemical composition and crystallographic data. The calculated density is approximately 3.23 g/cm³. ## Colors and Varieties This mineral has no known color varieties or commercial forms. Its color is uniform, from dark brown to black. ## History and Name The name "ferri-obertiite" refers to its chemical composition – the dominance of ferric iron (ferri) – and honors Roberta Oberti, an Italian crystallographer and mineralogist, for her fundamental contributions to amphibole group research. The mineral was discovered in volcanic tuff at its type locality in Spain and described by a research team led by F.C. Hawthorne. ## Applications Ferri-obertiite has no industrial applications. Its significance is purely scientific and collectible, as an example of a rare member of the large and complex amphibole group.

Properties

Mohs hardness
5-6
Luster
Vitreous to resinous
Streak
Light brown
Density
3.23
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of ferri-obertiite is impossible without advanced analytical techniques. In the field and in collections, it is indistinguishable from other dark amphiboles, such as arfvedsonite, riebeckite, or hornblende. Positive identification requires chemical analysis (e.g., using an electron microprobe - EDS/WDS) to determine the precise composition, especially the content of iron, titanium, magnesium, and sodium, and X-ray diffraction (XRD) to confirm the crystal structure. ## Distinguishing from Similar Minerals It differs from other members of the obertiite group (such as obertiite, ferri-ottoliniite) by the dominance of specific elements at particular positions in the crystal structure. Visually, it is identical to many common amphiboles. ## Crystal Forms It forms elongated, prismatic crystals, often terminated by simple faces. Crystals are typically very small, sub-millimeter in size, and can occur as single, isolated individuals or form small, radial aggregates within the rock.

Geological environment

## Genesis Ferri-obertiite forms under specific volcanic conditions. At its type locality (El Guanaco, Spain), it was found in cavities (voids) within phonolitic ignimbrite tuff. Its crystallization occurred at a late stage, likely from a gaseous phase or as a result of high-temperature metasomatic processes in a sodium-rich and oxidizing environment. ## Mineral Associations This mineral coexists with other rare amphiboles and minerals typical of such an environment. At the type locality, it was found in association with sanidine, nepheline, aenigmatite, astrophyllite, as well as other sodic amphiboles, such as arfvedsonite and mangano-arfvedsonite. ## Localities The only confirmed and detailed described locality of ferri-obertiite in the world is its discovery site: El Guanaco volcano in the Santa Cruz volcanic complex, Santa Cruz de Tenerife province, Canary Islands, Spain.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, ferri-obertiite is primarily valued by collectors specializing in "micromounts" and rare and systematic minerals. Specimens showing well-formed, sharply terminated crystals, clearly distinct from the host rock, are most highly rated. The presence of associated minerals confirming the paragenesis is also important. Crystal size, even if still sub-millimeter, matters – the larger, the more desirable the specimen. ## Popular Localities The only source of ferri-obertiite specimens is its type locality in the Canary Islands, Spain. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Specimens of ferri-obertiite, due to their rarity and microscopic nature, should be cleaned very carefully. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding immersion of the entire host rock. ## What to Avoid Avoid contact with any chemicals, including acids and strong bases, which may react with the mineral. Ultrasonic cleaners should not be used, as they could damage or detach small crystals from the matrix. Protect from high temperatures and sudden temperature changes. ## Storage Specimens should be stored in stable conditions, away from dust and direct sunlight. The best solution is a sealed "micromount" box or display cabinet, which protects delicate crystals from mechanical damage.

External references

Sources

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