Alcantarillaite

Chemical formula: [Fe<sup>3+</sup><sub>0.5</sub>(H<sub>2</sub>O)<sub>4</sub>][CaAs<sup>3+</sup><sub>2</sub>(Fe<sup>3+</sup><sub>2.5</sub>W<sup>6+</sup><sub>0.5</sub>)(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>O<sub>7</sub>]

Alcantarillaite is an extremely rare, hydrated calcium and iron arsenate, forming microscopic, dark red crystals in the Alcantarilla mine in Spain.

## Characteristics Alcantarillaite is a very complex, hydrated arsenate containing calcium, iron, arsenic, and tungsten. It occurs in the form of very small, prismatic or acicular crystals, whose length does not exceed 0.1 mm. These crystals often form radial or haphazardly arranged aggregates. Due to its microscopic size, it is a typical "micromount" mineral, requiring magnification for observation. Its color is dark reddish-brown to almost black. ## Physical Properties The hardness of the mineral is estimated at about 3 on the Mohs scale. The luster is described as adamantine to submetallic. In thin fragments, it is transparent, but due to its dark color, it appears opaque macroscopically. The calculated density is 4.28 g/cm³. ## Colors and Varieties This mineral is characterized by a uniform, dark color in shades from reddish-brown to black. No color varieties or commercial varieties have been distinguished. ## History and Name Alcantarillaite was recognized as a new mineral species by the International Mineralogical Association (IMA) in 2019. Its name comes from the discovery site and the only known locality – the Alcantarilla mine, located near Belalcázar in the province of Córdoba, in Spanish Andalusia. ## Uses Due to its extreme rarity and microscopic size, alcantarillaite has no industrial or commercial applications. Its significance is purely scientific and collectible, being an object of desire for specialized collectors of rare minerals.

Properties

Mohs hardness
3
Luster
Adamantine to sub-metallic
Streak
Light brown
Density
4.28
Cleavage
Perfect on {010}
Transparency
Transparent in thin fragments
Crystal system
Monoclinic

Diagnostic features

## Identification Amateur identification of alcantarillaite is practically impossible. Key indicators are: locality (Alcantarilla mine only), dark reddish-brown color, habit of small, acicular crystals, and co-occurrence with other rare arsenates. Final confirmation of identity requires advanced laboratory analyses, such as X-ray diffraction (XRD) and chemical microanalysis (EDS). ## Distinguishing from Similar Minerals This mineral can be confused with other dark, secondary arsenic minerals found in the same locality, such as valentinite, schneiderhöhnite, or goethite. It differs from them by its unique chemical composition, which includes calcium and tungsten, and its characteristic crystal structure. Differentiation without specialized equipment is not possible. ## Crystal Forms Alcantarillaite forms elongated, prismatic to acicular crystals, reaching a maximum length of 0.1 mm. These crystals occur individually or form small, divergent, fan-shaped, or haphazard aggregates on the surface of the host rock.

Geological environment

## Genesis Alcantarillaite is a secondary mineral, formed in the oxidation (weathering) zone of polymetallic hydrothermal deposits. It forms as a result of the alteration of primary ore minerals, mainly arsenopyrite, in an environment rich in arsenic, iron, and calcium, with the participation of tungsten derived from the weathering of other minerals. ## Mineral Associations This mineral occurs in association with other secondary arsenates. The most common associated minerals at the type locality include: valentinite, schneiderhöhnite, parasymplesite, pharmacosiderite, scorodite, and iron oxyhydroxides, such as goethite. The primary mineral is arsenopyrite. ## Localities The only confirmed locality of alcantarillaite in the world is its type locality: Alcantarilla mine, Belalcázar municipality, Córdoba province, Andalusia, Spain.

Rarity

Extremely rare

For collectors

## Quality Criteria In the case of a "micromount" mineral like alcantarillaite, the collectible value of a specimen depends on several factors. The most important is the abundance and quality of the crystals – the more numerous, sharper, and better-formed they are, the more valuable the specimen. Specimens with well-defined, isolated crystals or aesthetic aggregates are also highly prized. The presence of other rare minerals on the same rock fragment is also significant. ## Popular Localities All known and commercially available alcantarillaite specimens come from a single location – the Alcantarilla mine in Spain. This is the only source of this mineral.

Care and storage

## Cleaning Alcantarillaite specimens are extremely delicate and small. Mechanical or chemical cleaning is not recommended. Any contaminants, such as dust, should be removed very carefully, preferably with a stream of compressed air from a safe distance. Avoid contact with water and any cleaning fluids. ## What to Avoid As an arsenate, this mineral is toxic – avoid inhaling dust and direct skin contact. It should be protected from high temperatures, humidity, and all chemicals, especially acids, which can damage it. Storage in stable conditions is crucial for preserving the integrity of the specimen. ## Storage The safest storage method is to place the specimen in a sealed, plastic "micromount" box. The box should be clearly labeled with the mineral name and its toxicity. Store in a dry place, away from sunlight and heat sources.

External references

Sources

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