Laffittite

Chemical formula: Ag¹⁺Hg²⁺As³⁺S²⁻₃

Laffittite is a rare silver and mercury sulfosalt, forming tiny, dark red to black crystals with a metallic luster.

## Characteristics Laffittite is a rare mineral from the sulfosalt group, chemically a complex sulfide of silver, mercury, and arsenic. It occurs as very small, individual crystals, rarely exceeding 1 mm in size. These crystals are usually well-formed, with a tabular or short prismatic habit. The mineral is opaque and characterized by a metallic luster. Its color ranges from dark red, through reddish-black, to black. In reflected light, it reveals intense, red internal reflections. ## Physical Properties Laffittite is a relatively soft mineral, with a Mohs hardness of 3. It is quite dense, which is due to the presence of heavy elements such as silver and mercury – its density is approximately 5.5 g/cm³. It exhibits a metallic luster. It is brittle and has an uneven fracture. The streak of this mineral is reddish. ## Colors and Varieties This mineral does not form color varieties or commercial varieties. Its color is consistent and ranges from dark red to black. A characteristic feature is the vivid, red internal reflections, especially visible under a microscope. ## History and Name Laffittite was discovered in the Jas Roux quarry in the commune of La Chapelle-en-Valgaudémar (Hautes-Alpes, France). It was described in 1965 by a team of French mineralogists: François Permingeat, Pierre Picot, Roland Pierrot, and Jean-Claude Weil. The mineral is named in honor of Pierre Laffitte (1925-2010), a French geologist and director of the École des Mines de Paris, in recognition of his contribution to metallogeny.

Properties

Mohs hardness: 3.5
Color: Dark red to cherry-red, may be very dark-red with only red glints
Luster: Metallic
Streak: Pale cherry-red to brownish red
Density: 6.11
Cleavage: Good to imperfect on {010}
Fracture: Conchoidal
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying laffittite with the naked eye is practically impossible due to the microscopic size of its crystals. Identification requires advanced laboratory methods, such as chemical analysis (EDS) to confirm the presence of Ag, Hg, As, and S, and X-ray diffraction (XRD) to determine the crystal structure. Under the microscope, characteristic features include small, tabular crystals with a metallic luster and intensely red internal reflections. ## Distinguishing from Similar Minerals Laffittite can be confused with other rare sulfosalts with a red color or streak, such as proustite, pyrargyrite, cinnabar, or realgar. Proustite and pyrargyrite are usually larger and crystallize in the trigonal system. Cinnabar and realgar have lower densities and do not contain silver. Final differentiation is possible only through chemical analysis. ## Crystal Forms Laffittite crystals are monoclinic, usually very small (below 1 mm). They adopt a tabular form, flattened parallel to the {100} plane, or a short prismatic habit. They occur as single, isolated crystals grown on other minerals.

Geological environment

## Genesis Laffittite is a low-temperature hydrothermal mineral. It forms in the final stages of crystallization in ore veins rich in arsenic, antimony, silver, and mercury. In its type locality (Jas Roux), it occurs in dolomites and limestones mineralized during the Alpine orogeny. ## Mineral Associations This mineral co-occurs with other rare sulfosalts and sulfides. In Jas Roux, its associated minerals primarily include realgar, stibnite, pyrite, cinnabar, sphalerite, smithite, wakabayashilite, duranusite, and pierrotite. It also occurs in association with quartz and calcite. ## Localities The most important and classic locality for laffittite is the Jas Roux quarry in the French Alps, from which the best-known specimens originate. Beyond this locality, its presence has been reported in the Balcooma deposit in Queensland (Australia) and in the Getchell mine in Humboldt County, Nevada (USA).

Rarity

Very rare

For collectors

## Quality Criteria The quality of a laffittite specimen is primarily determined by the development and size of its crystals, although these remain microscopic. Specimens with sharp, well-defined crystals exhibiting a distinct red color or intense red reflections are most prized. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to their size, laffittite specimens are almost exclusively the domain of collectors specializing in "micromounts." ## Popular Localities The only source of collectible laffittite specimens is its type locality – Jas Roux in France. Specimens from other localities are extremely rare and are primarily of scientific interest.

Care and storage

## Cleaning Laffittite specimens are extremely small and fragile, so they must be handled with the utmost care. Mechanical cleaning is highly inadvisable. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust particles. Avoid contact with water and any chemicals. ## What to Avoid The mineral is sensitive to chemicals, especially acids. Ultrasonic cleaners, which could damage the delicate crystals, should be avoided. It should not be heated or exposed to prolonged strong light. Due to its mercury and arsenic content, avoid inhaling dust and wash hands after any contact with the mineral. ## Storage Laffittite should be stored under stable conditions, in a closed, padded "micromount" box to protect it from mechanical damage and dust. It should be kept away from moisture and heat sources.

External references

https://www.mindat.org/min-2311.html