Uranium silicates

A large variety of secondary uranium minerals are known, many are brilliantly colored and fluorescent. The commonest are gummite (a general term like limonite for mixtures of various secondary hydrated uranium oxides with impurities) hydrated uranium phosphates of the phosphuranylite type, including autunite (with calcium), saleeite (magnesium), and torbernite (with copper) and hydrated uranium silicates such as cof-finite, uranophane (with calcium), and sklodowskite (magnesium). 

Outerbridge W. F. et al. Weeksite, a new uranium silicate from the Thomas Range, Juab county, Utah. Am. Miner., 45, 1960, 39-52. 

Removal of uranium from solution can take place by various mechanisms. Calcareous and siliceous organisms can efficiently remove uranium from... 

With the exception of actinium, which is found naturally only in traces in uranium ores, these elements are by no means rare though they were once thought to be so Sc 25, Y 31, La 35 ppm of the earth s crustal rocks, (cf. Co 29ppm). This was, no doubt, at least partly because of the considerable difficulty experienced in separating them from other constituent rare earths. As might be expected for class-a metals, in most of their minerals they are associated with oxoanions such as phosphate, silicate and to a lesser extent carbonate. 

Thorium is widely but rather sparsely distributed and its only commercial sources are monazite sands (see p. 1229) and the mineral conglomerates of Ontario. The former are found in India, South Africa, Brazil, Australia and Malaysia, and in exceptional cases may contain up to 20% Th02 but more usually contain less than 10%. In the Canadian ores the thorium is present as uranothorite, a mixed Th,U silicate, which is accompanied by pitchblende. Even though present as only 0.4% Th02, the recovery of Th, as a co-product of the recovery of uranium, is viable. 

SIMS techniques have occupied somewhat of a narrower niche in uranium-series analysis, but have significantly improved Th isotope analysis relative to TIMS for chemically separated samples. The major improvement relative to TIMS is an improvement by about an order of magnitude in efficiency or sample size requirements for silicates. For uranium and/or thorium rich minerals such as carbonates and zircons, both SIMS and laser-ablation MC-ICPMS have been used for the direct in situ analysis of U and Th isotopes (Reid et al. 1997 Stirling et al. 2000) on very small (pg to ng levels of total U and Th) samples, at 10-100 pm scale resolution. 

Van den Bogaard P, Schimick C (1995) " °Ar/ Ar laser probe age of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States. Geology 23 759-762 Van Orman J.A., Grove T.L., Shimizu N (1998) Uranium and thorium diffusion in diopside. Earth Planet Sci Lett 160 505-519... 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

Thorium is widely but rather sparsely distributed its only commercial sources are monazite (together with the rare earths) and uranothorite (a mixed Th, U silicate). Uranium is surprisingly common and more abundant than mercury, silver or cadmium in the earth s crust. It is widely distributed and it is found scattered in the faults of old igneous rocks. Concentration by leaching followed by re-precipitation has produced a number of oxide minerals of which the most important are uranite (also called pitchblende) U308 and carnotite, K UC HVO -SF O. 

Both humic acids and fulvic acids have a strong affinity for particulate and crystalline substances possessing oxygen atoms at their surfaces and they have been reported to bring about the dissolution of iron phosphate, calcium phosphate (61), uranium dioxide (65), hydrated magnesium alumino-silicates (66) and limonite, a complex mixture of hydrated ferric oxides (67). 

Alteration at Michelin is dominated by intense soda metasomatism and potash depletion, coupled with locally intense hematization much of the mineralization consists of fine-grained uraninite within sodium-rich silicates such as aegirine and arfvedsonite. The recently discovered Jacques Lake deposit, which is an important resource in its own right (about 17 million pounds UsOs), has many similarities to Michelin, although its metavolcanic host rocks are compositionally distinct. The age of the uranium mineralization in these deposits is... 

Domine, F. Velde, B. 1986. Preliminary investigation of the process governing the solubility of uranium in silicate melts. Bulletin de Mineralogie, 108, 755-766. 

Douglas, M., Clark, S. B., Buck, E. C. Hanson, B. D. 2003. Neptunium association with uranium (VI) silicate solid phases. Migration OJ, Gyeongju (Kyongju), Korea, 21-26 September 2003. Abstract volume. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Nonferrous Metal Production. Nonferrous metal production, which includes the leaching of copper and uranium ores with sulfuric acid, accounts for about 6% of U.S. sulfur consumption and probably about the same in other developed countries. In the case of copper, sulfuric acid is used for the extraction of the metal from deposits, mine dumps, and wastes, in which the copper contents are too low to justify concentration by conventional flotation techniques or the recovery of copper from ores containing copper carbonate and silicate minerals that cannot be readily treated by flotation (qv) processes. The sulfuric acid required for copper leaching is usually the by-product acid produced by copper smelters (see Metallurgy, EXTRACTIVE Minerals RECOVERY AND PROCESSING). 

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