Clogauite

Chemical formula: Pb₂+Bi₃+₄Te₂-₄S₂-₃

Clogauite is a rare lead bismuth tellurosulfide, forming tiny, metallic inclusions in quartz gold veins.

## Characteristics Clogauite is a mineral from the sulfide group, specifically a lead bismuth tellurosulfide. It occurs as very small, anhedral (lacking its own crystalline form) grains, usually not exceeding 200 micrometers in size. It most commonly forms inclusions in other minerals, mainly in tetradymite and native gold. Its grains are opaque and have a metallic luster. ## Physical Properties Due to the microscopic size of its crystals, most physical properties, such as hardness, density, cleavage, or fracture, have not been precisely determined. The mineral is opaque and exhibits a strong, metallic luster. In reflected light, its color is creamy white, often with a greenish tint. ## Colors and Varieties The mineral is homogeneous and does not exhibit color variation. Its typical color in reflected light is creamy white with a greenish tint. No varieties have been distinguished. ## History and Name The mineral's name comes from its discovery locality – the Clogau St. David's gold mine in Gwynedd, Wales (United Kingdom). It was approved as a new mineral species by the International Mineralogical Association (IMA) in 1992, and its first description was published in 1994. The authors of the description were R.J. Callow and A.J. Criddle. ## Applications Clogauite has no industrial applications. Its significance is purely scientific and collectible, although due to its microscopic size and extreme rarity, it is of interest only to specialized collectors of rare minerals (so-called micromounts).

Properties

Luster: Metallic
Transparency: Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Identification of clogauite is possible only with advanced laboratory techniques. It requires analysis under a reflected light microscope (ore microscope), where its characteristic creamy white color with a greenish tint, strong anisotropy (color change upon stage rotation), and co-occurrence with tetradymite and native gold are observed. Final confirmation of identity requires chemical analysis using an electron microprobe (EDS/WDS). ## Distinguishing from Similar Minerals Clogauite is visually almost impossible to distinguish from other bismuth tellurides and sulfosalts, such as tetradymite, tellurobismuthite, or joseite, with which it often co-occurs. Differences can only be determined based on precise chemical analysis, which will show the presence of lead, bismuth, tellurium, and sulfur in proportions characteristic of clogauite. ## Crystal Forms This mineral forms exclusively anhedral grains and aggregates, meaning they lack their own crystal faces. They usually occur as irregular inclusions within other minerals, primarily tetradymite.

Geological environment

## Genesis Clogauite is a hydrothermal mineral. It forms in quartz veins containing gold, under low to medium temperature conditions. Its crystallization is associated with ore mineralization processes, where solutions rich in bismuth, tellurium, lead, and sulfur migrate through fractures in rocks. ## Mineral Associations This mineral most often occurs in close association with tetradymite, in which it forms inclusions. Other associated minerals include native gold, pyrite, chalcopyrite, sphalerite, galena, altaite, and other rarer bismuth tellurides. ## Localities The only confirmed and described occurrence of clogauite in the world is its type locality – the Clogau St. David's gold mine, located near Bontddu, Gwynedd, North Wales (United Kingdom).

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a clogauite specimen is assessed entirely differently than for macroscopic minerals. The key is the mere confirmation of the mineral's presence through analysis. Collectible value depends on the size and "purity" of the analyzed grain, as well as the richness of the mineral association on the sample (e.g., visible native gold, well-formed tetradymite). Specimens are typically rock fragments or polished sections, and their value lies in their rarity and scientific significance, not in aesthetics. ## Popular Localities The only source of specimens is the historical type locality in Wales. The mineral is not commercially mined, and specimens in collections come almost exclusively from archival material or historical mine production.

Care and storage

## Cleaning Specimens containing clogauite, which are typically polished sections or rock fragments, do not require cleaning. Any attempts at mechanical or chemical cleaning can irreversibly damage the microscopic mineral grains. If necessary, dust can be removed with a gentle stream of compressed air. ## What to Avoid Avoid contact with any chemicals, acids, and bases. Specimens should not be subjected to ultrasound or high temperatures. The surface of the polished section or rock should be protected from scratching, which could damage the clogauite inclusions. ## Storage Specimens should be stored under stable conditions, away from dust, moisture, and extreme temperatures. The safest method is to keep them in sealed "micromount" boxes or special containers for mineralogical polished sections, which protects them from mechanical damage.

Sources

IMA Mineral List (CNMNC)