Tiemannite

Chemical formula: Hg²⁺Se²⁻

Tiemannite is a rare mercury selenide, forming metallic-lustered, steel-gray to black crystals, valued by systematic mineral collectors.

## Characteristics Tiemannite is a rare mineral from the sphalerite group, chemically classified as mercury selenide. It occurs as well-formed, though usually small, tetrahedral crystals. Its surface is characterized by a strong, metallic luster. The most commonly encountered specimens are steel-gray to almost black, and their surface may be dull or covered with a tarnish due to weathering. ## Physical Properties This mineral is relatively soft, with a hardness of 2.5 on the Mohs scale, meaning it can be scratched by a copper wire. It is also very dense – its specific gravity is approximately 8.19 g/cm³, which is noticeable even in small specimens. It is completely opaque. Its fracture is uneven or conchoidal. ## Colors and Varieties The dominant and only natural color of tiemannite is steel-gray, often transitioning to black. No color or commercial varieties are distinguished. A change in color or dulling of the surface is usually the result of weathering processes. ## History and Name The mineral's name was given in honor of its discoverer, the German chemist Carl Wilhelm Ferdinand Tiemann (1848-1899). Tiemannite was first described in 1855 based on specimens found in the St. Lorenz mine in Clausthal-Zellerfeld, Lower Saxony (Germany), which is its type locality. ## Uses Due to its rarity and toxicity (mercury content), tiemannite has no industrial applications. Its significance is purely scientific and collectible, being a sought-after component for advanced systematic collections.

Properties

Mohs hardness: 2.5
Luster: Metallic
Streak: Black
Density: 8.19
Cleavage: None
Fracture: Irregular/Uneven,Conchoidal
Transparency: Opaque
Crystal system: Isometric

Diagnostic features

## Identification Characteristic features of tiemannite include its high density, metallic luster, steel-gray to black color, and black streak. It is a soft and brittle mineral. Its occurrence in association with other selenides or mercury minerals is a strong diagnostic indicator. ## Distinguishing from Similar Minerals It can be confused with other dark, metallic minerals such as sphalerite, galena, or acanthite. It is distinguished from sphalerite by its significantly higher density and black streak (sphalerite's streak is light). Galena has perfect cleavage in three directions, which tiemannite lacks. Acanthite is usually more malleable and has lower hardness. ## Crystal Forms It crystallizes in the isometric system, most often forming tetrahedral crystals. Crystals can be distorted or form irregular clusters and massive aggregates. It is less commonly found in granular aggregates or coatings.

Geological environment

## Genesis Tiemannite is a hydrothermal mineral, forming under low-temperature conditions. It occurs in ore veins, usually in association with calcite and other selenium minerals. Its presence indicates an environment rich in selenium and mercury, and poor in sulfur. ## Mineral Associations It most commonly co-occurs with other selenides, such as clausthalite, umangite, klockmannite, and berzelianite. It is also accompanied by barite, calcite, and sometimes native mercury. ## Localities The most important tiemannite localities worldwide include the historic type locality in Clausthal-Zellerfeld in the Harz Mountains (Germany). Significant specimens also come from Marysvale, Utah (USA), where well-formed crystals are known. Other known localities include the Tlalpujahua mine in Mexico and some deposits in Bolivia and Argentina.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed crystals with a strong, metallic luster. Rich crystal aggregates and associations with other rare selenides are also highly valued. Crystal size is a key factor – specimens with crystals exceeding a few millimeters are considered exceptional. ## Popular Localities Two localities are considered the best for this mineral: the historic locality in Clausthal-Zellerfeld, Germany, which yielded classic specimens, and the deposits in the Marysvale area, Utah, USA, which have provided some of the sharpest and largest tiemannite crystals.

Care and storage

## Cleaning Tiemannite specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to its mercury content, wet cleaning methods, which could lead to the release of toxic compounds, should be avoided. All operations should be performed with gloves. ## What to Avoid The mineral is sensitive to acids and other chemicals. Ultrasonic cleaners, high temperatures, and prolonged exposure to moisture should be avoided. Due to its toxicity, inhalation of dust and direct skin contact should be avoided. ## Storage Tiemannite specimens should be stored in closed, airtight containers or display boxes, away from other minerals that could undergo a chemical reaction. Store in a dry place, out of reach of children and pets. After each contact with the mineral, wash hands thoroughly.