Threadgoldite
Chemical formula: Al(U<sup>6+</sup>O<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>(OH)·8H<sub>2</sub>O
Threadgoldite is a rare, secondary uranium mineral of the phosphate group, forming characteristic yellow, platy crystals and coatings.
Description
## Characteristics Threadgoldite is a hydrated uranyl aluminum phosphate. It occurs as thin, platy crystals with a square or rectangular outline, often with truncated edges. These crystals typically form rosette-like aggregates, fan-shaped clusters, or thin crusts and coatings on the host rock. Its most characteristic visual feature is an intense, lemon-yellow to greenish-yellow color. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is translucent. On perfect cleavage surfaces, it exhibits a pearly luster, and on other surfaces or fractures, a waxy to dull luster. Its calculated density is 3.91 g/cm³. It shows strong, yellow-green fluorescence under ultraviolet light. ## Colors and Varieties Threadgoldite occurs in uniform shades from pale yellow and lemon-yellow to greenish-yellow. No color or commercial varieties are distinguished. ## History and Name The mineral was described in 1979 by Michel Deliens and Paul Piret. The name honors Ian Malcolm Threadgold (born 1929), an Australian mineralogist from the University of Sydney, in recognition of his contributions to uranium mineral research. The type locality is the Kobokobo pegmatite in South Kivu Province, Democratic Republic of Congo. ## Uses Due to its rarity, threadgoldite has no industrial applications. It is of scientific interest only and is a valued collector's mineral. Like all uranium minerals, it is radioactive.
Diagnostic features
## Identification Key diagnostic features include its lemon-yellow color, habit of thin, platy crystals, often in rosette form, and pearly luster on cleavage planes. The most important confirming tests are its strong radioactivity and intense yellow-green fluorescence under UV light. ## Distinguishing from Similar Minerals Threadgoldite can be confused with other yellow secondary uranium minerals, such as autunite, meta-autunite, or phosphuranylite. Autunite crystallizes in the tetragonal system and usually has thicker platy crystals with more intense green fluorescence. Distinguishing it from the very similar-looking phosphuranylite or mundite often requires advanced analysis, e.g., X-ray diffraction (XRD). ## Crystal Forms It forms very thin, square or rectangular platy crystals. They most commonly occur as radial or fan-shaped aggregates, rosettes, and as thin coatings covering rock surfaces.
Geological environment
## Genesis It is a secondary mineral, formed in the oxidation (weathering) zones of uranium deposits. It forms as a result of the alteration of primary uraninite in the presence of phosphorus and aluminum, most often within granitic pegmatites. ## Mineral Associations Threadgoldite often co-occurs with other secondary uranium phosphates, such as meta-autunite, torbernite, phosphuranylite, bergenite, mundite, and phuralumite. The primary mineral in its paragenesis is uraninite. ## Localities The most important and globally renowned locality for threadgoldite, from which the best collector specimens originate, is the Kobokobo pegmatite in the Democratic Republic of Congo. This mineral has also been found in the Lake Boga granites in Victoria, Australia.
Rarity
Very rare
Collector aspects
## Quality Criteria The most highly valued specimens are those with rich, dense aggregates of well-formed, thin plates forming striking rosettes. The intensity and purity of the lemon-yellow color are crucial. Contrast with the dark host rock and the presence of other well-defined associated minerals also enhance its appeal. ## Popular Localities Undoubtedly, the best and most sought-after threadgoldite specimens come from the type locality – the Kobokobo pegmatite in the Democratic Republic of Congo. Specimens from other localities are extremely rare on the collector's market.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush or a stream of compressed air to remove dust. Due to its significant softness, any friction should be avoided. Contact with water should be minimized, as it is a hydrated mineral. ## What to Avoid Threadgoldite is a radioactive mineral. Inhaling dust and direct skin contact should be avoided, and hands should be washed after any contact with the specimen. The mineral is sensitive to high temperatures, which can cause dehydration and destruction. It should be protected from acids and other chemicals. It is brittle and soft, so it is easily susceptible to mechanical damage. ## Storage Threadgoldite specimens should be stored in separate, tightly sealed containers to limit potential dusting and protect other minerals from radiation. Containers should be clearly labeled as radioactive material. The collection should be kept away from areas of permanent human presence, especially bedrooms and living rooms.