Downsite

Chemical formula: K<sub>2</sub>(Mo<sup>6+</sup>O<sub>3</sub>)<sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)·4H<sub>2</sub>O

Downsite is an extremely rare, secondary potassium molybdate, forming microscopic, colorless crystals in the form of tablets and rosettes.

## Characteristics Downsite is a hydrated potassium molybdate-sulfate. This mineral occurs as extremely small, colorless, tabular crystals, which often combine into rosettes or spherical aggregates. Individual crystals rarely exceed 20 micrometers in length, making them visible only under high magnification. Due to its microscopic size, its physical properties are difficult to precisely determine. ## Physical Properties Downsite crystals exhibit a vitreous luster. Hardness and density have not been measured due to the small size and rarity of the material. It is transparent to translucent. ## History and Name The mineral was named after Dr. Andrew J. Downes (born 1950), a chemist at the Australian Nuclear Science and Technology Organisation (ANSTO), in recognition of his contributions to the chemistry and mineralogy of uranium and molybdenum. It was approved by the IMA in 2013 (IMA2013-010), and its description was published in 2016.

Properties

Luster
Vitreous
Streak
White
Cleavage
Perfect on {010}
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of downsite is impossible without advanced laboratory techniques, such as Raman spectroscopy or X-ray diffraction (XRD), due to its microscopic size. It occurs as fine, colorless, tabular crystals forming rosettes on the host rock. ## Distinguishing from Similar Minerals It can be confused with other secondary molybdates or sulfates, such as ichenbergite, wulfenite, or gypsum. Visual differentiation is practically impossible and requires chemical and structural analysis. Unlike the often colored wulfenite, downsite is colorless. ## Crystal Forms It forms thin, tabular crystals with hexagonal or rhombic outlines, which group into rosettes and spherulitic aggregates up to 0.1 mm in diameter.

Geological environment

## Genesis Downsite is a secondary mineral, forming in the oxidation zones of molybdenum deposits. It forms as a result of the weathering of primary molybdenum sulfides (molybdenite) in the presence of potassium- and sulfate-rich solutions. It forms under low temperature and pressure conditions, directly on the rock surface. ## Mineral Associations It co-occurs with other secondary molybdenum minerals and sulfates. In the type locality (El Teniente mine), it was found in association with ichenbergite, molybdenite, pyrite, gypsum, anhydrite, and tenorite. ## Localities The only confirmed occurrence of downsite in the world is its type locality: the El Teniente copper and molybdenum mine in the O'Higgins Region, Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, the collector's value of downsite primarily depends on the richness and aesthetics of the aggregates on the rock matrix, visible under a microscope. The best specimens are those with numerous, well-formed rosettes, clearly contrasting with the substrate. Precise confirmation of the mineral's identification is also crucial. ## Popular Localities The only source of specimens is the El Teniente mine in Chile. Material from this locality is extremely difficult to obtain and is only available in specialized micromineral collections.

Care and storage

## Cleaning Due to the extreme delicacy and microscopic size of the crystals, any attempt at mechanical cleaning is impossible and will destroy the specimen. It should be protected from dust. ## What to Avoid Avoid contact with water, chemicals, ultrasound, and sudden temperature changes. The mineral is likely soluble in water and acids. It should be protected from all vibrations and shocks. ## Storage Downsite specimens should be stored exclusively in stable, sealed "micromount" containers to protect them from mechanical damage, dust, and humidity changes. It is not suitable for display outside of specialized packaging.

Sources

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