Argandite
Chemical formula: Mn<sup>2+</sup><sub>7</sub>(V<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>8</sub>
Argandite is a very rare manganese hydroxo-vanadate, forming characteristic reddish-brown, spherulitic aggregates.
Description
## Characteristics Argandite is a mineral from the vanadate group, chemically classified as a basic manganese vanadate. It occurs as small, spherulitic or rosette-like aggregates, composed of thin, tabular crystals. Its most typical visual feature is its radial clusters of intense, reddish-brown color, which form crusts on the host rock. ## Physical Properties Argandite crystals are translucent and exhibit a vitreous to slightly resinous luster. This mineral is relatively soft, with a Mohs hardness of approximately 3. It is also quite brittle, which, combined with its perfect cleavage, makes specimens susceptible to mechanical damage. Its calculated density is 3.83 g/cm³. ## Colors and Varieties Argandite occurs in a uniform, reddish-brown color. No color varieties or commercial varieties are known. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2011. Its name honors Emile Argand (1879–1940), a distinguished Swiss geologist known for his research on Alpine tectonics. The type locality, from which it originates and where it was first described, is the Valletta mine in Italy. ## Applications Due to its extreme rarity, argandite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for advanced systematic and regional mineral collections.
Diagnostic features
## Identification Characteristic features of argandite include its reddish-brown color, spherulitic or rosette-like aggregates composed of tabular crystals, and low hardness. A key diagnostic indicator is also its specific occurrence environment and mineral paragenesis, and above all, its locality – this mineral is known practically only from one place in the world. ## Distinguishing from Similar Minerals Argandite can be confused with other rare, reddish-brown manganese minerals from the same locality, such as palenzonaite, tiragalloite, or saneroite. Certain differentiation based on visual characteristics is extremely difficult and most often requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Crystal Forms It forms thin, tabular crystals with a hexagonal or rhombic outline, which almost always occur in radial clusters, forming spherulites, rosettes, or fan-shaped aggregates.
Geological environment
## Genesis Argandite forms as a secondary mineral in manganese deposits that have undergone metamorphic processes. It forms in quartz-braunite veins cutting metasediments and ophiolites. It is a product of hydrothermal alterations occurring in manganese- and vanadium-rich rocks. ## Mineral Associations This mineral co-occurs with other manganese minerals. Its most common associations include braunite, ganophyllite, palenzonaite, pyroxmangite, rhodochrosite, rhodonite, saneroite, shattuckite, strontiomelan, and tiragalloite. ## Localities The only confirmed and significant occurrence of argandite in the world is its type locality – the Valletta mine in the Maira Valley, in the municipality of Canosio, in the province of Cuneo (Piedmont, Italy).
Rarity
Very rare
Collector aspects
## Quality Criteria The quality of argandite specimens, which are usually micromounts, is assessed based on several criteria. The most important is the intensity and saturation of the reddish-brown color. Well-defined, complete spherulites or rosettes with a distinct, radial structure are highly valued. Additional value is provided by the aesthetic arrangement of aggregates on a contrasting rock matrix and the presence of well-formed crystals of associated minerals. ## Popular Localities The only source of argandite collector specimens is the Valletta mine in Italy. Material from this location is considered exemplary for this species and is sought after by collectors specializing in rare or systematic minerals.
Care and storage
## Cleaning Argandite specimens are very delicate and should be handled with extreme caution. For dust removal, it is best to use compressed air from a safe distance. If necessary, the specimen can be very carefully rinsed in distilled water using a soft brush, but any pressure should be avoided due to its perfect cleavage. Ultrasonic cleaners are absolutely forbidden. ## What to Avoid The mineral is sensitive to chemicals, especially acids. Contact with any cleaning agents should be avoided. It is also brittle, so it must be protected from impacts, vibrations, and scratching. It should not be heated or exposed to sudden temperature changes. ## Storage It is recommended to store argandite specimens in separate, padded "micromount" boxes to prevent contact with harder minerals. They should be kept in stable conditions, away from dust, moisture, and direct sunlight.