Tyrrellite

Chemical formula: Cu²⁺(Co³⁺,Ni³⁺)₂Se²⁻₄

Tyrrellite is a very rare copper, cobalt, and nickel selenide, forming small, metallic grains of black color with a brownish-red streak.

## Characteristics Tyrrellite is a very rare mineral from the thiospinel group, belonging to the sulfide class (although chemically it is a selenide). It is composed of copper, cobalt, and nickel. It forms very small, most often rounded or irregular grains, rarely exceeding 0.5 mm in diameter. It usually occurs as inclusions in other minerals. It is opaque and black in color. ## Physical Properties This mineral is characterized by a metallic luster. Its Mohs hardness is approximately 3.5, and its density is high, around 6.4 g/cm³. It exhibits no cleavage, and its fracture is uneven. ## Colors and Varieties Tyrrellite is a mineral of consistent, black color. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from Joseph Burr Tyrrell (1858-1957), a Canadian geologist and historian, who was a consultant at the mine where the discovery was made. The mineral was first described in 1932. ## Uses Due to its extreme rarity and occurrence only as microscopic grains, tyrrellite has no industrial application. It is of purely scientific significance and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 3.5
Color: Bronze
Luster: Metallic
Streak: Black
Density: 6.6
Cleavage: {001}
Fracture: Sub-Conchoidal
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Identifying tyrrellite is extremely difficult without specialized equipment. Enriched samples can be recognized by their black color, metallic luster, and characteristic brownish-red streak. It reacts with nitric acid. However, final confirmation requires advanced methods such as EDS analysis in a scanning electron microscope (SEM) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Tyrrellite can be confused with other black, metallic selenides or sulfides with which it co-occurs, such as umangite, berzelianite, clausthalite, or bornhardtite. A key distinguishing feature in field conditions (though difficult to observe on microscopic grains) is its brownish-red streak, in contrast to the black or gray streak of many similar minerals. ## Crystal Forms Tyrrellite crystallizes in the isometric system, but almost never forms well-developed crystals. It is observed almost exclusively as massive, fine, rounded or irregular grains and aggregates, often as inclusions in other minerals.

Geological environment

## Genesis Tyrrellite is a hydrothermal mineral. It forms in low-temperature calcite-quartz veins that cut uranium deposits in sandstones. Its crystallization is associated with the presence of solutions rich in selenium, copper, cobalt, and nickel. ## Mineral Associations This mineral co-occurs with other selenides and sulfides. It is most often accompanied by: clausthalite, umangite, berzelianite, bornhardtite, chalcopyrite, pyrite, as well as calcite, hematite, and uranium minerals. ## Localities Tyrrellite is an extremely rare mineral, known from only a few localities worldwide. The most important occurrences are: * **Canada** - Eagle Mine (formerly Goldfields), Athabasca Lake region in Saskatchewan province (type locality). * **Czech Republic** - Předbořice uranium deposit in central Bohemia and Habří near Tišnov in Moravia. * **Argentina** - Sierra de Cacho District, La Rioja Province.

Rarity

Very rare

For collectors

## Quality Criteria For a mineral as rare as tyrrellite, the main criterion for value is the mere confirmation of its presence on a specimen. Since it occurs as microscopic grains, specimens are evaluated based on the abundance and size of these grains within the rock matrix. The most highly prized samples are those where tyrrellite forms rich clusters visible under slight magnification, clearly distinct from the surrounding minerals. The presence of rare associated minerals, especially other selenides, also increases the scientific and collector's value of the specimen. ## Popular Localities The most classic and sought-after specimens by collectors come from the type locality in Canada (Athabasca Lake) and from Czech localities (Předbořice, Habří), which provided material for research and enriched institutional collections.

Care and storage

## Cleaning Specimens containing tyrrellite, due to its microscopic nature and occurrence as inclusions, practically do not require cleaning. If dust needs to be removed from the rock matrix, a soft brush or compressed air from a safe distance can be used. ## What to Avoid Avoid contact with chemicals, especially acids, which can react with sulfides and selenides. Do not subject specimens to ultrasound or sudden temperature changes. Although not sensitive to light, prolonged exposure to moisture can lead to slow oxidation. ## Storage Specimens containing tyrrellite are best stored under stable conditions, in closed collector boxes or display cases, to protect them from dust and mechanical damage. Due to the microscopic size of the grains, magnification is often required for viewing, so it is important to properly mark the mineral's location on the specimen.