Oldsite-(K)

Chemical formula: K<sub>2</sub>Fe<sup>2+</sup>[(U<sup>6+</sup>O<sub>2</sub>)(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>]<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>

Oldsite-(K) is a very rare, secondary uranyl mineral, forming microscopic, yellow tabular crystals.

## Characteristics Oldsite-(K) is a hydrated uranyl, potassium, and iron sulfate. It occurs as extremely small, tabular or bladed crystals, rarely exceeding 0.2 mm in length. These crystals form scattered aggregates, rosettes, or thin coatings on the surface of the host rock. Due to its microscopic size, it is a mineral primarily accessible to specialized micromount collectors. ## Physical Properties The crystals exhibit yellow-green fluorescence under ultraviolet light (both longwave and shortwave). The mineral is strongly radioactive, which is typical for uranyl minerals. Hardness and density have not been precisely measured due to the small size of the crystals. ## Colors and Varieties Oldsite-(K) has a characteristic, bright yellow color. No color varieties or commercial varieties are known. ## History and Name The mineral was named after Travis Olds, a professor of materials science and mineralogist at Michigan Technological University, in recognition of his contributions to uranium mineralogy. The "-(K)" suffix in the name indicates the dominance of potassium in its chemical structure. It was recognized as a new mineral species by the International Mineralogical Association in 2022 (IMA2022-044). ## Uses Oldsite-(K) has no industrial applications. Its significance is purely scientific and for collectors, especially those specializing in uranium minerals and micromounts.

Properties

Luster
Vitreous
Streak
Light yellow
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of Oldsite-(K) is possible almost exclusively through advanced analytical methods, such as Raman spectroscopy or chemical analysis using an electron microprobe (EDS/WDS). Under collecting conditions, identification relies on appearance (microscopic, yellow, tabular crystals), locality (North Mesa mine), and associated minerals. ## Distinguishing from Similar Minerals It can be confused with many other secondary yellow uranyl sulfates, such as natrozippeite, zippeite, or johannite. Certain differentiation without specialized equipment is impossible. Oldsite-(K) is distinguished by its specific chemical composition with a dominance of potassium and iron. ## Crystal Forms It forms very small, thin, tabular crystals with a rectangular or bladed outline. These crystals often group into small, radial aggregates resembling rosettes or occur as single flakes on the rock.

Geological environment

## Genesis Oldsite-(K) is a secondary mineral that forms in the oxidation zone of uranium deposits. It crystallizes under post-mining conditions, on the walls of mine workings, as a result of the interaction of sulfate-rich waters with primary uranium minerals (uraninite) in the presence of potassium and iron derived from pyrite decomposition. ## Mineral Associations It most often co-occurs with gypsum, halotrichite, metavoltine, as well as other uranyl sulfates such as natrozippeite and johannite. These minerals collectively form multicolored, fragile coatings and efflorescences. ## Localities The only confirmed occurrence (type locality) of this mineral is the North Mesa mine in San Juan County, Utah, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria The collecting appeal of an Oldsite-(K) specimen depends on the richness and aesthetics of the microcrystal aggregates. Specimens with well-formed, isolated crystals or dense, rosette-like aggregates of intense yellow color are most valued. Contrast with the rock matrix is also important. Due to their microscopic nature, specimens are evaluated under magnification. ## Popular Localities The only source of specimens is the North Mesa mine in Utah, USA. Material from this locality is highly sought after by specialized collectors of uranium minerals and micromounts.

Care and storage

## Cleaning Oldsite-(K) specimens should generally not be cleaned. The crystals are extremely small, fragile, and water-soluble. Any attempt at mechanical cleaning or using liquids will almost certainly destroy the specimen. ## What to Avoid Contact with water and other chemicals must be strictly avoided. The mineral is sensitive to changes in humidity. As a secondary mineral, it is unstable under conditions other than those in which it formed. It should be protected from vibrations and shocks. ## Storage Specimens should be stored in sealed, closed containers (micromount boxes) to protect them from dust, moisture, and mechanical damage. Due to radioactivity, it should be stored away from other radiation-sensitive minerals, in an area isolated from permanent human presence and marked with appropriate warning symbols.

Sources

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