Thunderbayite

Chemical formula: TlAg₃Au₃Sb₇S₆

Thunderbayite is an extremely rare thallium, silver, and gold-bearing antimony sulfosalt, discovered in the Thunder Bay gold mine in Ontario, Canada.

## Characteristics Thunderbayite is a very rare sulfosalt mineral, identified only in 2023. It occurs as microscopic, metallic grains, usually not exceeding 100 micrometers in size. Its grains are most often anhedral (lacking their own crystallographic forms) and intergrown with other rare minerals, such as stibnite, native gold, or aurostibite. Due to its size, visual features can only be observed under high magnification with a reflected light microscope. ## Physical Properties As a metallic mineral, thunderbayite exhibits a strong, metallic luster. It is opaque. Its hardness and density have not yet been precisely measured due to its extreme rarity and small grain size; however, based on its chemical composition, it is presumed to have a high density. ## History and Name The mineral's name comes from its discovery locality – the Thunder Bay Amethyst Mine, located in the Thunder Bay District, Ontario, Canada. It was officially described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2023 (IMA2023-059). The discovery was made by a team of mineralogists studying unique ore parageneses at this location.

Properties

Luster: Metallic
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of thunderbayite is impossible without advanced laboratory techniques. It requires analysis by electron microprobe (EDS/WDS) to confirm its unique chemical composition (the presence of Tl, Ag, Au, Sb, and S in appropriate proportions). In reflected light under a reflected light microscope, it has a creamy-white color and exhibits weak pleochroism. ## Distinguishing from Similar Minerals It can be confused with many other microscopic sulfosalts and metal alloys, such as other minerals from the criddleite group, aurostibite, or native gold. Definitive differentiation is only possible based on chemical analysis. ## Crystal Forms To date, it has only been observed in the form of anhedral (irregular) grains ranging from 5 to 100 micrometers, forming intergrowths with other minerals.

Geological environment

## Genesis Thunderbayite forms under low-temperature hydrothermal conditions. Its occurrence is associated with quartz-adularia veins cutting through rocks of an Archean volcanic complex. The mineral crystallizes in the late stages of deposit formation, from solutions rich in antimony, gold, silver, and thallium. ## Mineral Associations This mineral co-occurs with a rich assemblage of other ore minerals. It is most commonly accompanied by stibnite, native gold, aurostibite, getchellite, criddleite, pyrite, chalcopyrite, sphalerite, galena, as well as quartz and calcite as gangue minerals. ## Localities The only confirmed occurrence (type locality) of thunderbayite in the world is the Thunder Bay Amethyst Mine in McTavish Township, Thunder Bay District, Ontario, Canada.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, the collector's value of a thunderbayite specimen is not assessed based on its aesthetics, but on its scientific value and rarity. The most valuable specimens are those in which the presence of thunderbayite has been analytically confirmed. The richness of associated minerals on the same rock fragment enhances its attractiveness. ## Popular Localities The only source of specimens is the type locality in Thunder Bay, Ontario, Canada. Material from this mine is extremely difficult to obtain and primarily goes to scientific institutions and specialized systematic collections.

Care and storage

## Cleaning Specimens containing thunderbayite, due to their microscopic nature and embedding in a rock matrix, are practically not subject to cleaning under collector conditions. Any mechanical or chemical intervention could irrevocably damage the delicate mineral grains. ## What to Avoid All chemicals, ultrasonics, temperature changes, and moisture should be avoided. Minerals containing thallium and antimony can be sensitive to oxidation and acids. ## Storage Specimens should be stored under stable conditions, in sealed, dry containers (e.g., micromount type), away from light and contaminants. The most important aspect is protection against mechanical damage.

Sources

IMA Mineral List (CNMNC)