Rankachite

Chemical formula: Ca₀.₅(V₄+,V₅+)(W₆+,Fe₃+)₂O₈(OH)·₂H₂O

Rankachite is a very rare vanadium and tungsten oxyhydroxide mineral, forming microscopic, black crystals with a metallic luster.

## Characteristics Rankachite is an extremely rare mineral from the oxide group, with a complex chemical composition containing calcium, vanadium, tungsten, and iron. It occurs as very small, tabular or bladed crystals, which rarely exceed 0.2 mm in length. It typically forms radial or tangled aggregates. Its color is black, and on thin edges, it may be translucent reddish-brown. ## Physical Properties Rankachite crystals exhibit a strong, metallic luster. The mineral is opaque, except for the thinnest fragments. Due to the microscopic size of the crystals, precise determination of hardness and density is difficult; the density calculated based on the formula and cell parameters is approximately 4.45 g/cm³. ## History and Name Rankachite was discovered in the area of the Rankach volcano in the Murcia region of Spain, from which it derives its name. It was approved as a new mineral species by the International Mineralogical Association (IMA) in 2017 (IMA2017-084). The discovery was made by a team of mineralogists, including Christian Rewitzer, Franz-Josef Heker, and Thomas Witzke. ## Applications Rankachite has no industrial applications. Its significance is purely scientific and collector-oriented, as a unique and newly discovered mineral species.

Properties

Luster: Metallic
Streak: Red-brown
Density: 4.45
Cleavage: Perfect on {010}
Transparency: Opaque, Translucent in thin fragments
Crystal system: Monoclinic

Diagnostic features

## Identification Amateur identification of rankachite is practically impossible. It requires advanced analytical techniques, such as Raman spectroscopy or chemical analysis using an electron microprobe (EDS/WDS). A visual, though not unambiguous, characteristic is the black, metallic-lustered, microscopic crystals forming radial aggregates. ## Distinguishing from similar minerals It may be confused with other black, microcrystalline secondary minerals, such as manganese oxides (e.g., pyrolusite, hollandite) or other rare vanadates and tungstates. Definitive differentiation is only possible through laboratory methods. ## Crystal forms Rankachite forms very small, thin, tabular or bladed crystals, often flattened. These crystals combine into characteristic radial or chaotically tangled aggregates.

Geological environment

## Genesis Rankachite is a secondary mineral. It forms in the oxidation zones of polymetallic deposits, in this case within volcanic andesites. It forms as a result of the weathering of primary ore minerals under conditions of low temperature and pressure. ## Mineral associations This mineral co-occurs with other rare secondary minerals. At the type locality, it was found in association with hekerite, rewitzerite, witzkeite, gypsum, jarosite, hematite, and unidentified manganese oxides. ## Localities The only confirmed locality of rankachite in the world is its type locality – Rankach volcano (Cabezo de la Fraila) in the municipality of Vera, in the province of Almería, Andalusia (Spain).

Rarity

Extremely rare

For collectors

## Quality criteria As a "micromount" mineral, the main criterion for evaluation is the quality and richness of the crystal aggregates on the rock matrix. The most desirable specimens are those with clearly formed, radial groupings of black, lustrous crystals, contrasting well with the substrate. Association with other rare minerals from this locality is also important. ## Popular localities The only source of specimens is the type locality in Spain. Material from this location is extremely limited and primarily available within specialized collecting circles and scientific institutions.

Care and storage

## Cleaning Rankachite specimens are extremely delicate and small. Any mechanical cleaning should be avoided. If absolutely necessary, compressed air can be used from a safe distance to remove loose dust particles. ## What to avoid Avoid contact with all chemicals, including water, which may react with the mineral or the host rock. Avoid ultrasonics, high temperatures, and sudden temperature changes. Protect from direct sunlight, which may cause fading or degradation of associated minerals. ## Storage Specimens should be stored exclusively in specialized "micromount" boxes to protect them from mechanical damage and dust. Store in stable room conditions, away from sources of vibration and humidity.

Sources

IMA Mineral List (CNMNC)