Fettelite

Chemical formula: Ag₁+₁₆Hg₂+As₃+₄S₂-₁₅

Fettelite is a rare sulfosalt of arsenic, silver, and mercury, forming thin, hexagonal plates with a metallic luster and a cherry-red color.

## Characteristics Fettelite is a mineral belonging to the sulfosalt group, composed of silver, mercury, arsenic, and sulfur. It occurs as very small, thin, hexagonal plates or lamellae, rarely exceeding 1 mm in diameter. Crystals often form rosette-like or fan-shaped aggregates. A characteristic feature of fettelite is its intense, cherry-red to dark red color and metallic luster, which can be dulled by surface alteration. ## Physical Properties This mineral is very soft, with a Mohs hardness of 3.5. It is opaque, and its thin fragments may appear red in transmitted light. It has a relatively high density, approximately 6.28 g/cm³, which is typical for minerals containing heavy metals like silver and mercury. It exhibits perfect cleavage in one plane, parallel to the base of the hexagonal crystals. ## Colors and Varieties Fettelite occurs in shades from cherry-red to dark red and reddish-brown. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1996 by W.H. Paar, Y. Moëlo, P.K. R.G. Sobott, and T.T. Chen. Its name honors Markus Fettel (1936–1989), a German geologist and mineralogist, who was the first to collect samples of this mineral in the Odenwald mine in Germany and recognized it as a potentially new species. ## Uses Due to its extreme rarity and small crystal size, fettelite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness: 3.5
Luster: Metallic
Streak: Cherry-red
Density: 6.28
Cleavage: Perfect on {0001}
Fracture: Sectile
Transparency: Opaque, Translucent in thin splinters
Crystal system: Trigonal

Diagnostic features

## Identification Diagnostic features of fettelite include its unique combination of cherry-red color, metallic luster, hexagonal shape of thin plates, and co-occurrence with other silver sulfosalts. A red streak is also characteristic. Due to the small size of the crystals, identification often requires magnification (microscope) and advanced analytical methods (e.g., EDS). ## Distinguishing from similar minerals - **Proustite:** Has a similar red color and contains silver, but crystallizes in the trigonal system, most often forming columnar or prismatic crystals, not thin plates. It is also transparent to translucent. - **Cinnabar:** Has a bright red color and high density, but forms different crystal habits and has distinct mineral associations. - **Realgar:** Has an orange-red color, but is much lighter, very unstable in light, and forms prismatic crystals. ## Crystal Forms Fettelite forms thin, tabular (platy) crystals with a hexagonal outline. These crystals often group into rosette-like, fan-shaped, or almost spherical aggregates. They rarely occur as single, isolated plates.

Geological environment

## Genesis Fettelite is a low-temperature hydrothermal mineral. It forms in the final stages of crystallization in ore veins rich in silver, arsenic, and mercury. ## Mineral Associations This mineral often occurs in association with other rare silver and mercury sulfosalts. The most common associated minerals include: proustite, samsonite, stephanite, pyrargyrite, quartz, calcite, as well as other rare minerals like laffittite or rathite. ## Localities The most important and classic occurrences of fettelite are: - **Germany:** Odenwald mine in the Bergstrasse region, Hesse (type locality). - **Switzerland:** Lengenbach in the Binn Valley, Valais canton – known for world-class microcrystals of rare sulfosalts. - **Chile:** Chañarcillo mine, near Copiapó, Atacama region.

Rarity

Very rare

For collectors

## Quality Criteria The most valuable fettelite specimens are those with well-formed, sharp crystals of intense, cherry-red color and strong luster. Aggregates in the form of aesthetic rosettes or fans, contrastingly set on a light matrix (e.g., on quartz or calcite), are highly prized. Crystal size is crucial – specimens with plates exceeding 1 mm are exceptionally rare and desirable. The presence of rare associated minerals on the same specimen is also important. ## Popular Localities The most famous and highly valued specimens by collectors come from the Lengenbach quarry in Switzerland, which is known for supplying the world's best fettelite microcrystals. Specimens from the type locality in Germany and from Chañarcillo in Chile are also sought after, but generally yield in quality to those from Switzerland.

Care and storage

## Cleaning Fettelite is extremely delicate and sensitive. Mechanical cleaning should be avoided. If absolutely necessary, a very soft brush can be used to remove loose dust. Water or any liquids should not be used, as they may react with or damage the mineral. ## What to avoid - **Light:** Prolonged exposure to strong light, especially sunlight, can cause darkening and degradation of the mineral's surface. - **Chemicals:** Contact with all acids, bases, and solvents should be avoided. The mineral is chemically reactive. - **Water:** Contact with water can lead to oxidation and the formation of secondary coatings. - **Shocks and impacts:** Crystals are very soft and brittle, and their cleavage makes them prone to delamination. ## Storage Fettelite specimens should be stored in stable conditions, in closed, padded boxes (of the "micromount" type), to protect them from mechanical damage, dust, and light. It is best to keep them in a dry place, away from heat sources and sudden temperature changes.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FFettelite