Uranium minerals oxide hydrates

The two most important sources of uranium are the minerals carnotite, where uranium occurs in the hexavalent oxide or hydrated oxide, and pitchblende, where uranium occurs mostly in the tetravalent state as a compound salt with other metals. It also occurs as a mixed oxide with titanium, thorium, and niobium in the tetravalent form. The tetravalent uranium minerals appear to have been geologically formed in the presence of reducing agents such as hydrocarbon minerals, graphite, native metals, and sulfide minerals, while such association is rarely observed with the hexavalent uranium minerals. 

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). 

Most uranium minerals occur in all of the several types of ore deposits. A given deposit usually has no more than two reduced minerals. The oxidized minerals that occur in the deposit depend on the Eh-pH conditions and the availability of reactive anions. In the absence of reactive anions, hydrated oxides and uranates form. The uranyl ion is, however, fairly soluble and groundwater can effectively disperse it a considerable distance from the reduced source. The uranyl minerals that are then deposited are complex compounds that employ available oxyanions. The rate of formation of these secondary minerals can be very rapid, as is evidenced by mineral formation on the walls of mine drifts in a matter of months after the drifts have been opened. In all deposits there is usually a zonation of mineralogy in which a reduced mineral... 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

Evidently, in the oxidation of uraninite these higher oxides have a strong affinity for water and form hydrated compounds instead of simple oxides. Natural specimens usually show a massive uraninite core surrounded by a yellow to orange microcrystalline zone that is usually termed gummite —a complex mixture of uranium oxide hydrates. It may contain such minerals as ianthinite that are not fully oxidized, but this zone usually consists of U minerals. If oxidation were to occur in the absence of water, one of the UsOg forms would probably form—a-UsOs being the most likely. Further oxidation usually produces either a-UOs or 7-UO3. The a-form is structurally related to the UsOg forms. 

Although the list of 231 X-ray patterns has been essentially restricted to accepted uranium mineral species, several poorly described unnamed minerals have been included along with the best characterized uranyl oxide hydrates. These latter synthetic phases have been listed because of the possible existence in the gummite alteration rinds often associated with uraninite. 

Uranium is found in Earth s crust at an average concentration of about 2 ppm, and is more abundant than silver or mercnry. The most common uranium-containing mineral is uraninite, a complex nraninm oxide. Other uranium-containing minerals are autunite, a hydrated calcinm nranium phosphate, and carnotite, a hydrated potassium uranium vanadate. 

Uranium in its highest valence state forms a large number of colourful minerals that may deposit in the oxidized zone associated with the primary deposit or the uranium may go into solution and be transported a considerable distance from its source area before reprecipitation. Minerals that form at the source may mimic the original phases by direct replacement, but more often they form a nondescript mass that destroys any original structure. These minerals are usually hydrated uranyl oxides, silicates or phosphates. Further from the source the minerals usually form as one or more of the many hydrated uranyl oxysalts. 

---