Drysdallite
Chemical formula: Mo<sup>4+</sup>Se<sup>2-</sup><sub>2</sub>
A very rare molybdenum selenide, forming soft, hexagonal flakes with a metallic luster, being the selenium analog of molybdenite.
Description
## Characteristics Drysdallite is an extremely rare mineral from the sulfide group, being the selenium analog of the much more common molybdenite. Chemically, it is molybdenum selenide (MoSe₂). It occurs as small, hexagonal, tabular crystals, as well as lamellar and scaly aggregates. Its appearance is almost identical to molybdenite – it forms soft, lead-gray to black flakes with a strong, metallic luster. ## Physical Properties This mineral is exceptionally soft, reaching a hardness of only 1-1.5 on the Mohs scale, which means it can be scratched with a fingernail. Drysdallite flakes are flexible but inelastic. It possesses perfect cleavage in one plane, allowing for easy separation of thin flakes. It is opaque and relatively heavy for its appearance, with a density of approximately 6.26 g/cm³. ## Colors and Varieties Drysdallite is an idiochromatic mineral, its color directly resulting from its chemical composition. It occurs exclusively in shades from lead-gray to black. It does not form color varieties or commercial varieties. ## History and Name The mineral was first described in 1973 based on samples from the Kapijimpanga copper-uranium deposit in Zambia. Its name commemorates Alan Roy Drysdall, former director of the Geological Survey of Zambia, in recognition of his contribution to geological research in the region. ## Applications Due to its extreme rarity, drysdallite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.
Diagnostic features
## Identification Diagnostic features of drysdallite include its extreme softness (hardness 1-1.5 on the Mohs scale), perfect cleavage in one direction, formation of flexible but inelastic flakes, and a black streak. A strong metallic luster on cleavage surfaces is also characteristic. ## Distinguishing from Similar Minerals Drysdallite is visually almost impossible to distinguish from molybdenite (MoS₂). Both minerals have identical appearance, hardness, and habit. The only certain method of identification is chemical analysis (e.g., EDS) to confirm the presence of selenium instead of sulfur. The geological context – occurrence in a selenium-rich paragenesis – can be a strong indicator. It is distinguished from graphite by its higher density and a slightly different shade of luster. ## Crystal Forms Most often, it forms small, hexagonal, tabular crystals. It usually occurs as scaly, lamellar, or earthy aggregates. Well-formed, isolated crystals are rarely found.
Geological environment
## Genesis Drysdallite is a hydrothermal mineral, crystallizing under low-temperature conditions. Its formation is associated with the presence of selenium-rich solutions, which is a relatively rare environment in the Earth's crust. ## Mineral Associations This mineral co-occurs with other selenides and ore minerals. In its type locality (Zambia), it was found in association with bornite, chalcopyrite, cuprite, uraninite, cuprosklodowskite, cassiterite, wittichenite, and native selenium. ## Localities The most important and historical occurrence is its type locality – the Kapijimpanga deposit in the North-Western Province of Zambia. Its presence has also been confirmed in several other locations worldwide, including the Tolar Grande area in Salta Province (Argentina) and the Bukov uranium mine in the Czech Republic.
Rarity
Very rare
Collector aspects
## Quality Criteria As a mineral of exceptional rarity, every documented drysdallite specimen is valuable to collectors. Specimens with well-formed, macroscopic hexagonal crystals are most highly prized. Samples where drysdallite co-occurs with other rare selenic minerals also hold significant value. Due to its nature, most specimens are micromounts. ## Popular Localities Specimens from the type locality in Kapijimpanga, Zambia, are by far the most sought after by collectors, as they represent the reference material for this mineral species.
Care and storage
## Cleaning Drysdallite specimens are extremely delicate and soft. Any mechanical cleaning, including the use of brushes, should be avoided. Water or any chemicals must not be used. The only safe method for removing dust is to gently blow it off using a photographic air blower or a stream of compressed air from a distance. ## What to Avoid Avoid touching the mineral's surface, as it is so soft that it can be damaged by a fingernail. Protect it from any pressure, scratching, and vibrations. It should not be subjected to ultrasound or sudden temperature changes. ## Storage Drysdallite should be stored exclusively in a stable, padded box (e.g., a "micromount" type), isolated from other minerals that could scratch it. Display should be in a closed cabinet, protecting it from dust and mechanical damage.