Tiragalloite

Chemical formula: Mn₂+₄As₅+Si₃O₁₂(OH)

Tiragalloite is a very rare, reddish-orange manganese and arsenic silicate, forming small, platy crystals.

## Characteristics Tiragalloite is a hydrated manganese and arsenic silicate, occurring as very small, platy or lamellar crystals, rarely exceeding 1 mm. It typically forms radial or randomly oriented aggregates. Its most characteristic feature is an intense, reddish-orange color. ## Physical Properties Crystals exhibit a vitreous luster. The mineral is transparent to translucent. It has a hardness of approximately 5 on the Mohs scale and a density close to 3.96 g/cm³. ## Colors and Varieties Tiragalloite occurs in a characteristic reddish-orange color. No color varieties or commercial varieties are known. ## History and Name The mineral was discovered in the Molinello manganese mine in Italy. Its name honors Ettore Tiragallo (1889-1980), an Italian chemist and mineral collector from Genoa, who first noticed this mineral and submitted it for research. It was described and approved as a new mineral species in 1980. ## Uses Due to its extreme rarity and small crystal size, tiragalloite has no industrial applications. It is solely an object of interest for specialized collectors and scientific institutions.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: Light orange
Density: 3.96
Cleavage: Good on {100}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of tiragalloite include its intense reddish-orange color, the habit of small, platy crystals forming aggregates, and its specific occurrence environment in manganese deposits. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Distinguishing from Similar Minerals Tiragalloite can be confused with other rare, orange or red minerals from manganese deposits, such as piemontite or some garnets (spessartine). It is distinguished from them by its crystal form (thin plates), lower hardness, and unique chemical composition containing arsenic and silicon. ## Crystal Forms Crystals are very small, thin, and platy, often with a hexagonal or rectangular outline. They form radial or disordered clusters and aggregates on the surface of the host rock.

Geological environment

## Genesis Tiragalloite forms as a result of metamorphic processes within manganese and silica deposits. It occurs in fractures and small rock cavities in rocks rich in braunite and quartz. ## Mineral Associations This mineral coexists with other manganese minerals. It is most commonly found in association with braunite, quartz, ganophyllite, rhodonite, parsettensite, and wallkillite. ## Localities It is an extremely rare mineral, known practically only from one locality in the world – the Molinello mine, located in the Graveglia Valley in Liguria, Italy. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromineral, tiragalloite is primarily evaluated based on the quality and size of its crystals, even though they rarely exceed 1 mm. The most desirable specimens are those with well-formed, sharp, transparent crystals of intense, reddish-orange color. Rich crystal aggregates covering a significant surface of the rock matrix and aesthetic associations with other rare minerals are also highly valued. ## Popular Localities The only source of collector specimens is the historical type locality – the Molinello mine in Italy. Material from this location is very limited and extremely difficult to acquire on the market.

Care and storage

## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to the small size of the crystals and their fragility, wet or mechanical cleaning is not recommended. ## What to Avoid Avoid contact with chemicals, acids, and detergents. Do not subject specimens to ultrasound. Protect from high temperatures and sudden temperature changes. ## Storage Tiragalloite specimens, as microminerals, should be stored in specialized "micromount" boxes that protect them from mechanical damage and dust. Avoid exposure to direct sunlight, which theoretically could affect the mineral's color.

Sources

IMA Mineral List (CNMNC)