Vallouiseite

Chemical formula: Ag₁+₃Tl₁+₂₁.₅Pb₂+Sb₃+₆₃As₃+₄₁.₅S₂-₁₇₀

Vallouiseite is an extremely rare, lead-gray sulfide with a metallic luster, occurring as microscopic, acicular crystals.

## Characteristics Vallouiseite is a very rare mineral from the sulfosalt group, characterized by a complex chemical composition containing silver, thallium, lead, antimony, arsenic, and sulfur. It occurs as very fine, acicular or hair-like crystals that form tangled aggregates. Its color is lead-gray, and its luster is strongly metallic, which is typical for many sulfosalts. ## Physical Properties This mineral is opaque and has a lead-gray streak. Due to the microscopic size of the crystals, precise determination of hardness and density is difficult. Like other acicular sulfosalts, it is likely brittle. ## History and Name Vallouiseite was first described in 2017 by a team of mineralogists (Y. Moëlo, P. Orlandi, C. Biagioni, I.V. Pekov, W.G. Mumme, N.V. Chukanov, O.V. Kiriukhina, D.I. Belakovskiy, E. Bonaccorsi). Its name comes from the Vallouise valley in the French Alps, where its type locality, the Jas Roux mine, is located. ## Uses Vallouiseite has no industrial applications. Due to its extreme rarity and microscopic size, it is of purely scientific importance and is an object of interest for specialized collectors of rare minerals.

Properties

Luster: Metallic
Streak: Lead-gray
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of vallouiseite is impossible without advanced laboratory techniques such as chemical analysis (EDS/WDS) and X-ray diffraction (XRD). Visually, it appears as tangled, hair-like or acicular aggregates with a metallic, lead-gray luster. ## Distinguishing from similar minerals It can be confused with many other acicular sulfosalts, such as jamesonite, boulangerite, zinkenite, or other rare sulfosalts from Jas Roux (e.g., pierrotite, parapierrotite). Definitive differentiation is only possible based on chemical composition analysis. ## Crystal forms It forms elongated, acicular or hair-like crystals, usually not exceeding a few hundred micrometers in length. These crystals form dense, tangled or felted aggregates in small voids within the host rock.

Geological environment

## Genesis Vallouiseite forms as a result of low-temperature hydrothermal processes. In its type locality (Jas Roux), it crystallizes in the late stage of mineralization, in voids of Triassic rocks (dolomites and limestones) that have been subjected to solutions rich in arsenic, antimony, thallium, lead, and sulfur. ## Mineral associations This mineral co-occurs with other rare thallium and lead sulfosalts, such as pierrotite, parapierrotite, chabournéite, routhierite, as well as with realgar, orpiment, stibnite, sphalerite, pyrite, and calcite. ## Localities The only confirmed occurrence of vallouiseite in the world is its type locality: the former Jas Roux mine, located near La Chapelle-en-Valgaudémar in the Vallouise valley, in the Hautes-Alpes department, France.

Rarity

Extremely rare

For collectors

## Quality criteria The collector's appeal of a specimen depends on the richness and density of vallouiseite aggregates. The most prized samples are those where microcrystals form distinct, dense clusters, contrasting well with the rock matrix. The presence of other rare associated minerals is also important. ## Popular localities The only source of specimens is the historical type locality – the Jas Roux mine in France. Material from this mine is very limited and rarely appears on the market.

Care and storage

## Cleaning Specimens containing vallouiseite should generally not be cleaned. Any attempts at mechanical or chemical cleaning can irreversibly damage the microscopic crystals. If dust removal is necessary, a gentle stream of compressed air can be used from a safe distance. ## What to avoid Contact with water, chemicals, ultrasound, and sudden temperature changes must be strictly avoided. The mineral is sensitive to oxidation, so it should be protected from moisture and direct sunlight, which can accelerate degradation processes. ## Storage Specimens should be stored in stable conditions, preferably in a closed, airtight "perky box" away from sources of light and moisture. Due to its fragility, vibrations and shocks should be avoided.

Sources

IMA Mineral List (CNMNC)