Carnotite

Vanadium is found in about 65 different minerals among which are carnotite, roscoelite, vanadinite, and patronite, important sources of the metal. Vanadium is also found in phosphate rock and certain iron ores, and is present in some crude oils in the form of organic complexes. It is also found in small percentages in meteorites. 

Originally, radium was obtained from the rich pitchblende ore found in Joachimsthal, Bohemia. The carnotite sands of Colorado furnish some radium, but richer ores are found in the Republic of Zaire and the Great Lake region of Canada. Radium is present in all uranium minerals, and could be extracted, if desired, from the extensive wastes of uranium processing. Large uranium deposits are located in Ontario, New Mexico, Utah, Australia, and elsewhere. 

Uranium, not as rare as once thought, is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. 

Secondary (predominantly hexavalent uranium) Carnotite K20 2 U03 V203 n H20... 

Another area for research is how groundwater fluxes of vanadium (critical for carnotite precipitation) and phosphate (enhances uranium mobility) are controlled by geology and geomorphology. This may assist in targeting economic from sub-economic targets. 

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. 

Occurrence. Important minerals are carnotite K(U02)V04 3/2 H20, more important as a uranium ore, vanadinite Pb5(V04)3Cl and some complex sulphides. It occurs also in certain crude oils and may be recovered from dusts after combustion. 

Vanadium is not found in its pure state. Small amounts of vanadium can be found in phosphate rocks and some iron ores. Most of it is recovered from two minerals vanadinite, which is a compound of lead and chlorine plus some vanadium oxide, and carnotite, a mineral containing uranium, potassium, and an oxide of vanadium. Because of its four oxidation states and its ability to act as both a metal and a nonmetal, vanadium is known to chemically combine with over 55 different elements. 

Vanadiums principal ores are roscoelite, patronite, vanadinite, and carnotite, which are found in the states of Idaho, Montana, Arkansas, and Arizona as well as in Mexico and Peru. It is also a by-product from the production of phosphate ores. 

U/ U ratios in carnotite consistent with such a source. Metasomatic-related... 

The discovery of the calcrete hosted surfical uranium deposits of Namibia demonstrated the presence of widespread uranium in calcrete filled palaeochannels (Hambleton-Jones 1984) and similar mineral deposits have been observed elsewhere in Southern Africa, USA and Australia (Carlisle 1978 Hambleton-Jones 1978 Mann Deutscher 1978). The host rocks are typically lenticular bodies of alluvium, soil or detritus material cemented by calcite, gypsum, palygorskite, and other mineral phases. Uranium mineralogy is dominated by the mineral Carnotite [K2(U02)2(V04)2.3(H20)] as the main mineral in these channels. However other phases such as andersonite (Na2K3U03(C03)3(H20)6), liebigite (Ca2U02(C03)3(H20)io ... 

Fig. 1. Distribution of calcrete-hosted carnotite deposits in central Namibia (from Hambleton-Jones 1984).

Fig. 2. Matrix plot for key groundwater parameters, calcrete-hosted carnotite deposits, Namibia.

Geochemical Controls The speciation of uranium and vanadium has been determined for groundwaters from the calcrete-hosted carnotite deposits. These were completed in Geochemists Workbench v.7 using in-... 

The uranyl dicarbonate species is extremely stable in aqueous solutions but by evaporation can become sufficiently concentrated by evaporation to generate the low solubility mineral carnotite in the pH range 6 to 8, the natural range of pH values in the Namibian groundwaters. 

At the prevailing pH in the Namibian groundwaters, the predicted solubility of carnotite is low and close to saturation. From one hole in the Tubas deposit, carnotite saturation is close to 0 and predicted to be over saturated around the water-table zone and in the near-surface upper 2m of the gypcrete. Where Eh is positive carnotite is predicted to be nearsaturation. This indicates that carnotite accumulation at or above the regional water-table can occur by upward diffusion of uranyl carbonate species with possible precipitation due to nucleation on clay minerals or gypsum, as evidenced in the Tubas River. 

Understanding the formation of carnotite in calcrete is important with respect to not just understanding the formation of the deposits but also has implications for exploration of such deposits (Nash et al. 1981 Hambleton-Jones et al. 1984). A... 

Field measurements in sedimentary fluvial-type calcrete deposits also suggest that present-day groundwater in these areas may also display potential to both dissolve and precipitate uranium in the near surface. Chemical dilatancy and evaporation-driven diffusion that promote de-complexing, diffusion, and reprecipitation mechanisms are seen to play integral parts in the continued chemical reworking and modification of these calcrete-hosted carnotite deposits. 

Where uranium and vanadium concentrations are in pg/L and potassium and bi-carbonate are in mg/L. Where the CSI is equal to zero then groundwater chemistry and carnotite saturation are in equilibrium and the mineral has the potential to be present. The assessment of groundwater chemistry in the vicinity of calcrete-hosted carnotite deposits indicates that a wide geochemical halo exists and that this halo can be identified during exploration. In Australia direct analysis of groundwater and the CSI have been demonstrated as suitable methods for exploration. In Namibia, in reality although the approach may be useful for... 

Arakel, A.V. 1988. Carnotite mineralization in inland drainage areas of Australia. Ore Geology Reviews, 3, 289-311. 

Characterization of carnotite uranium deposit in calcrete channels, Trekkopje, Namibia. In Proceedings of Africa Uncovered Mineral Resources for the future. SEG-GSSA 2008 Conference, 7 -1 o" July 2008, 114-121. Boyle, D.R. 1984. The genesis of surficial uranium deposits. In Toen, P.D. (ed) Surficial Uranium Deposits. lAEA-Tecdoc-322. Vienna, 45-52. 

Mann, A.W. Deutscher, R.L. 1978. Genesis principles for the precipitation of Carnotite in calcrete drainages in Western Australia. Economic Geology, 73, 1724-1737. 

Fig. 2. Eh-pH diagram for the system K-U-V-C-S-H2O at 25 °C showing the stability of secondary carnotite.

Uranium occurs in nature in many rocks, minerals and sediments. The principal uranium minerals are pitchblende, carnotite, uranitite, tobernite, uranophane, autunite, and davidite. Uranium also is found in very small quantities in monazite sand, phosphate rock, and lignite. Although uranium is present in very smaU quantites, these sources also are used for commercial recovery of the metal. 

Vanadium is recovered from several sources vanadium minerals, vanadium-bearing phosphates, boder residues, and spent vanadium catalysts. One major vanadium mineral is patronite, a greenish-black, amorphous sulfide ore used extensively for many years to produce vanadium. This mineral, found in Peru, has depleted gradually. The metal also is recovered commercially from carnotite and roscoelite. 

Plutonium is the only transuranium element which has been found in nature. Until its properties were known it would have been impossible to detect it in the minute amounts in which it occurs, but when its behavior was understood, Seaborg and his co-workers were able to find it in pitchblende, monazite ores, and carnotite in concentrations of about one part in 1014 (63, 73, 76). Peppard and his group found it in somewhat greater amounts in pitchblende from the Belgian Congo (77). Seaborg believes that most of this plutonium arises by fission of the uranium in the ore, though other processes may also be involved (77, 78). 

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