Arsenmedaite

Chemical formula: Mn₂+₆As₅+Si₅O₁₈(OH)

Arsenmedaite is a rare, pinkish manganese and arsenic silicate, forming small, tabular crystals in metamorphic manganese deposits.

## Characteristics Arsenmedaite is a very rare mineral from the silicate group, chemically classified as a manganese and arsenic silicate. It occurs as very small, tabular or bladed crystals, rarely exceeding 0.3 mm in length. These crystals form disseminated clusters or aggregates within the host rock. Due to the small size of the crystals, its visual features are difficult to observe without magnification. ## Physical Properties Arsenmedaite crystals exhibit a vitreous luster. The Mohs hardness is approximately 5. The mineral is brittle and characterized by an uneven fracture. Its calculated density is 3.69 g/cm³. ## Colors and Varieties Arsenmedaite is typically light pink to pinkish in color. No color varieties or commercial forms are known. ## History and Name The mineral's name refers to its chemical composition – the presence of arsenic (Arsen) – and the fact that it was discovered in manganese deposits in Val Graveglia, Italy, similar to medaite (Medaite), with which it is structurally related. It was officially approved as a new mineral by the International Mineralogical Association (IMA) in 1999. Its discovery and description are attributed to a group of researchers, including G. C. Piccoli, who found it in the Valgraveglia mine in Liguria. ## Applications Arsenmedaite has no industrial applications. Its significance is purely scientific and as a collector's item, being a very rare and specific mineral.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 3.69
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying arsenmedaite in the field or in a collection is practically impossible without specialized equipment. It requires chemical analysis (EDS) and X-ray diffraction (XRD) to confirm its composition and structure. Visually, it is simply a pinkish mineral forming microscopic crystals. ## Distinguishing from Similar Minerals It can be confused with other pink manganese minerals found in the same environment, such as rhodochrosite, rhodonite, or medaite. Differentiation requires advanced analytical methods. Medaite, to which it is related, has a similar appearance but differs in chemical composition (containing vanadium instead of arsenic). ## Crystal Forms It forms very small, tabular or bladed crystals, often gathered in small, radial or disordered aggregates. The crystals are usually too small for their form to be visible to the naked eye.

Geological environment

## Genesis Arsenmedaite forms as a result of metamorphic processes acting on sedimentary manganese and silica deposits. It is a product of low-grade metamorphism under greenschist facies conditions. It occurs in quartz veins cutting through manganese-rich rocks (braunite). ## Mineral Associations This mineral coexists with quartz, braunite, ganophyllite, tiragalloite, saneroite, wallkilldellite-(Mn), and medaite. ## Localities The only confirmed occurrence of arsenmedaite in the world is its type locality – the Valgraveglia mine, located in the Val Graveglia valley in Liguria, Italy. It is an endemic mineral to this single site.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of an arsenmedaite specimen is determined by the abundance and visibility of microscopic crystals on the rock matrix. As the mineral is extremely rare, any specimen with confirmed arsenmedaite presence is valuable for specialized collectors of rare minerals (so-called "micromounts"). The best specimens are those where the pink crystals form distinct clusters contrasting with the quartz matrix. ## Popular Localities The only source of specimens is the Valgraveglia mine in Liguria, Italy. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Due to the extreme rarity and small size of crystals embedded in the host rock, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid Avoid all chemicals, acids, and ultrasonics, which can damage delicate crystals or the host rock. The mineral is brittle, so it should be protected from impacts and scratches. ## Storage Specimens should be stored in stable conditions, in a closed box or display case, to protect them from dust and mechanical damage. It is best to store the specimen in its original form, without attempting to isolate microscopic crystals.

Sources

IMA Mineral List (CNMNC)