Clausthalite

Chemical formula: Pb<sup>2+</sup>Se<sup>2-</sup>

Clausthalite is a lead selenide, a mineral with a metallic luster, forming compact, granular aggregates, closely related to galena.

## Characteristics Clausthalite is a relatively rare mineral from the galena group, chemically classified as lead selenide. Its appearance is very similar to the much more common galena (lead sulfide). It most often occurs in the form of granular or massive aggregates, rarely forming small, poorly developed crystals in the shape of cubes or octahedra. Its surface is usually metallic lustrous, ranging in color from lead-gray to tin-white, often with a dull, gray or brownish tarnish resulting from oxidation. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of about 2.5, meaning it can be scratched by a copper wire. It is characterized by a very high density (7.6-8.2 g/cm³), resulting from the presence of lead. It is opaque and has a strong, metallic luster on fresh fracture surfaces. It is also brittle. ## Colors and Varieties Clausthalite has a consistent, characteristic lead-gray color, which on weathered surfaces may be covered with a dull tarnish. It does not exhibit color varieties. It sometimes forms solid solutions with galena (PbS) or altaite (PbTe). ## History and Name The mineral's name comes from its discovery locality – the town of Clausthal-Zellerfeld in the Harz Mountains, Germany. It was first described in 1832 by the German mineralogist François Sulpice Beudant. ## Uses Due to its rarity, clausthalite has no industrial significance as a lead ore. However, it can be a minor source of selenium if it occurs in larger accumulations. For collectors, it is an interesting and valued specimen, especially due to its kinship with galena and its rarity.

Properties

Mohs hardness
2.5
Luster
Metallic
Streak
Black
Density
7.6-8.2
Cleavage
Perfect, cubic along {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Cubic

Diagnostic features

## Identification A key diagnostic feature of clausthalite is its very high density, which can be felt even in small specimens. Other features include its lead-gray color, metallic luster, low hardness (approx. 2.5), and perfect cleavage in three directions, causing the mineral to break into cubic fragments. The streak is black. ## Distinguishing from Similar Minerals Clausthalite is visually almost identical to galena (PbS). Differentiation in the field is very difficult and often requires chemical analysis (EDS/XRF) to confirm the presence of selenium instead of sulfur. Galena is much more common. From other gray metallic minerals, such as stibnite, it is distinguished by its cubic cleavage and higher density. ## Crystal Forms Clausthalite crystals are rare and usually poorly developed, taking the form of cubes or octahedra, often with rounded edges. It is most commonly found as fine-grained, massive, or disseminated aggregates within the host rock.

Geological environment

## Genesis Clausthalite is a hydrothermal mineral. It forms in ore veins developing under low to medium temperature conditions. It typically crystallizes from relatively sulfur-poor, but selenium-rich, hydrothermal solutions. ## Mineral Associations It often co-occurs with other selenides, such as tiemannite (HgSe), klockmannite (CuSe), berzelianite (Cu₂Se), and umangite (Cu₃Se₂). It is also accompanied by other ore minerals, such as galena, chalcopyrite, stibiopalladinite, as well as gangue minerals like calcite, barite, or quartz. ## Localities The most important historical localities, from which classic specimens originate, are in Germany (Clausthal-Zellerfeld and other mines in the Harz Mountains; Tilkerode). Other known localities include Skrikerum in Sweden; Pacajake in Bolivia; Mendoza Province in Argentina; and some deposits in the USA (e.g., in Nevada and Utah).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, even if small, cubic crystals. Rich, granular aggregates with a strong, metallic luster are also highly prized, especially if they contrast with a light host rock (e.g., white calcite). The presence of rare associated minerals, particularly other selenides, is also important. ## Popular Localities Classic, historical specimens come from mines in the Harz region of Germany and are sought after by collectors specializing in minerals from that region. Specimens from Bolivia and Argentina also appear on the collector's market.

Care and storage

## Cleaning Clausthalite specimens should be cleaned very carefully, preferably dry, using a soft brush to remove dust. If necessary, a damp cloth or cotton swab with distilled water can be used, immediately drying the specimen. Due to its lead content, it is recommended to wear gloves during cleaning and wash hands after contact with the mineral. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral's surface. Clausthalite is susceptible to oxidation, so it should be protected from prolonged exposure to moisture and air, which cause it to tarnish. It should not be heated or cleaned ultrasonically. ## Storage It is recommended to store specimens in dry conditions, preferably in sealed boxes or display cases, to limit air and moisture exposure. This will help preserve its natural, metallic luster for longer. It should be stored away from minerals that may release sulfur compounds to avoid chemical reactions.

External references

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