Jonassonite

Chemical formula: Au(Bi,Pb)<sub>₅</sub>S<sub>₄</sub>

Jonassonit is an extremely rare gold bismuth sulfide, occurring as microscopic, tabular crystals with a metallic luster.

## Characteristics Jonassonit is a gold bismuth sulfide with the formula AuBi₅S₄, where bismuth can be partially replaced by lead. It forms microscopic, elongated or tabular crystals, reaching sizes up to 100 micrometers. They usually occur as intergrowths with other ore minerals. The mineral has a steel-gray to tin-white color and is opaque. ## Physical Properties It is characterized by a strong, metallic luster. Its hardness and density have not been measured due to the small size of the crystals. The calculated density is 8.51 g/cm³, and the hardness is estimated to be 2.5-3 on the Mohs scale. Cleavage and fracture have not been determined. ## Colors and Varieties The mineral is uniform in color, ranging from steel-gray to tin-white. No varieties have been distinguished. ## History and Name Jonassonit was recognized as a new mineral by the International Mineralogical Association (IMA) in 2004. Its name honors Peter Jonasson, a Swedish mineral collector who provided the material for research. It was discovered in historical samples from the Săcărâmb deposit in Romania. ## Applications Due to its extreme rarity and microscopic size, jonassonit has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Luster: Metallic
Streak: Black
Transparency: Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Identification of jonassonit is impossible without advanced analytical techniques. It requires chemical composition analysis using an electron microprobe (EDS/WDS) and crystal structure studies (XRD). A clue is its occurrence in association with other rare gold and bismuth sulfides and tellurides in material from Săcărâmb. ## Differentiation from similar minerals Jonassonit is visually indistinguishable from many other metallic, gray ore minerals, such as bismuthinite, nagyágite, buckhornite, or other bismuth sulfosalts. Certain differentiation is only possible based on chemical analysis. ## Crystal forms It forms elongated, bladed, or tabular crystals. They often occur as irregular aggregates and intergrowths with other minerals, forming complex aggregates visible only under high magnification.

Geological environment

## Genesis Jonassonit forms under hydrothermal conditions, within epithermal high-sulfidation gold deposits. It is a product of crystallization from solutions rich in gold, bismuth, lead, and sulfur. ## Mineral associations This mineral co-occurs with native bismuth, gold, nagyágite, buckhornite, and other, often unnamed, mineral phases containing copper, gold, bismuth, sulfur, and selenium. ## Localities The only confirmed locality of jonassonit in the world is its type locality - the Săcărâmb mine (formerly Nagyág) in Hunedoara County, Romania.

Rarity

Extremely rare

For collectors

## Quality criteria As a microscopic mineral, jonassonit is not evaluated for aesthetics in the traditional sense. The value of a specimen is determined by its richness (quantity and size of microcrystals on the rock matrix), analytically confirmed identity, and the presence of rare associated minerals. The most valuable samples are those with well-formed, though still microscopic, crystals. ## Popular localities The only source of specimens is the historical Săcărâmb deposit in Romania. Collector material comes almost exclusively from old, archival museum and collection samples.

Care and storage

## Cleaning Jonassonit specimens are typically micromounts that do not require cleaning. If necessary, dust can only be removed with a photographic blower bulb. Any contact with water, chemicals, and brushes should be avoided. ## What to avoid The mineral is potentially sensitive to moisture and atmospheric oxygen, which can lead to its oxidation and tarnishing. Avoid sudden temperature changes, shocks, and exposure to chemicals and ultrasound. ## Storage Specimens should be stored in stable conditions, preferably in tightly sealed "micromount" boxes, which protect against dust, moisture, and mechanical damage.

Sources

IMA Mineral List (CNMNC)