Studtite
Studtite is a very rarely occurring mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the monoclinic crystal system with the chemical composition [( UO2) (O2) (H2O ) 2] ( H2O) 2, so it is a hydrous uranyl peroxide. In addition to its anhydrous form Metastudtit it is the only known peroxide mineral.
Studtite developed only small, light yellow to almost colorless crystals acicular crystal habit. He usually can be found in the form of fibrous mineral aggregates or crusty coatings. The transparent to translucent crystals have a glass or wax- like luster. The mineral is generally described as a soft ( Mohs hardness about 1 to 2) and the fine crystalline needles are flexible.
Special Features
The mineral is characterized by its uranium content of up to 63.6 wt - % highly radioactive and has a specific activity of about 113.9 kBq / g (compared to natural potassium 0.0312 kBq / g).
Etymology and history
The mineral was first in 1947 by the Belgian mineralogist Johannes Franciscus Vaes ( 1902-1978 ) in the Shinkolobwe uranium mine in Katanga ( now the Democratic Republic of Congo) found. He held it - probably due to inclusions and impurities - after a chemical analysis first for a Urancarbonat. Vaes named the new mineral after the German geologist Eduard Franz Studt, who in 1908 created a geological map of Katanga. In 1974, Kurt Walenta show on the basis of crystallographic comparisons with known artificially produced crystals that it is the mineral is a uranyl peroxide hydrate. Only in 2003 could finally be enlightened by röntgenografischer single-crystal structure of this mineral by Peter C. Burns and Karrie -Ann Hughes.
Classification
In the outdated, but partly still in use 8th edition of the mineral classification by Strunz the studtite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " uranyl ( [ UO2 ] 2 )- hydroxides and hydrates ", where he with Ianthinit, Metaschoepit, Metastudtit, Paraschoepit and Schoepit the unnamed group IV/H.01 formed.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the studtite also in the department of " Uranyl Hydroxide ." However, this is further subdivided according to the possible presence of other cations and, if available, in addition according to the crystal structure, so that the mineral is found according to its composition in the subdivision " without additional cations " where there is only together with Metastudtit the " Studtitgruppe "with the system number. 4.GA.15 forms.
The mainly common in English-speaking classification of minerals according to Dana assigns the studtite in the class of " oxides and hydroxides " and there in the department of " uranium and thorium oxides " one. He is also, along with Metastudtit in the unnamed group 05:03:01 within the subdivision " uranium and thorium -containing oxides with a cation charge of 8 ( AO4 ), and hydrated " to find.
Education and Locations
Studtite very rare forms as a secondary mineral in uranium deposits. In this case, temperatures of from below 90 ° C and only a small amount of water, perhaps in the form of thin films on the surface of mineral necessary. The only known in the minerals studtite and its crystal anhydrous form Metastudtit peroxide group is formed in the radiolysis of water by the uranium emitted by alpha radiation. This has in an average energy of 5.5 MeV has a range of about 40 microns in water, so that the mineral formation takes place locally on a very limited area. This results in, inter alia, the hydroxyl radical ( • OH), the superoxide radical (O 2 • - ) and the hydroperoxide radical ( HO2 • ) responsive subsequently to hydrogen peroxide (H2O2), so that eventually form peroxide anions. Besides arise also reducing compounds such as hydrogen, which is chemically relatively inert at temperatures below 100 ° C, and can escape from the water without any further chemical reactions. Since the natural radioactivity of the uranium occurs with a relatively low radiolysis, promote long contact times of the uranyl ion with the radiolysis of water decomposed by several hundred thousand years, the formation of studtite. Sample calculations of other authors show that, assuming that the hydrogen peroxide does not degrade for a long period, a sufficiently high concentration for the mineral formation can be formed even after 2100 's. In principle, must be present and focused in the water for the formation of the mineral a sufficient amount of peroxide, which can only be achieved in thin films and long contact times.
The mineral is in Shinkolobwe (Democratic Republic of Congo) associated with uranophane, Rutherfordin and Lepersonnit. From Menzenschwand ( Germany ) parageneses with Billietit, Rutherfordin, barite, quartz, hematite and limonite are known. Finds of studtite in Tengchong (China) show parageneses with Tengchongit, Calcurmolit and Kivuit.
Except in natural uranium deposits could prove studtite in uranium waste of the nuclear facility Hanford Site and in the lavaartigen corium remains of the Chernobyl disaster. Although it is a very rare mineral in nature, it is considered an important aging product of radioactive waste. This is due to the formation conditions of studtite which, although rarely achieved in natural uranium deposits, but can occur more easily on the surfaces of uranium waste. The mineral was detected among others as a major aging product to fuel element shells in cooling ponds. Synthetics could this mineral formation in deionized water with uranium (IV ) oxide ( UO2), which was doped with α -emitters or irradiated from external sources, be verified. The interaction of spent nuclear fuel with groundwater can therefore - in addition to the formation of studtite - Schoepit lead to the formation of secondary uranium minerals such as Uranyloxidhydrat. Since Uranylminerale can reduce the mobility of other radionuclides including through incorporation into the crystal lattice and by the formation of inclusion compounds, they are important factors when considering long-term effects with regard to the solubility of radioactive waste and spent nuclear fuel. Studtite therefore is responsible for the final disposal of nuclear matter.
In addition to the type locality in Shinkolobwe studtite was also found in the Swambo mine and in Lusungu River District, Sud- Kivu. From Germany studtite is known inter alia from the pit Krunkelbach at Menzenschwand, from Wittichen and Oberwolfach. In Austria studtite in Mühlbach was found at the High King and St. Johann im Pongau. Other localities are Linópolis in Brazil, Yingjiang and Tengchong in China, Marianske Lazne and Javorník in the Czech Republic, Lodève, Davignac and several places of the department of Deux -Sèvres in France and Krøderen in Norway.
Crystal structure
Studtite crystallizes in the monoclinic crystal system, space group C2 / c ( Raumgruppen-Nr. 15) with the lattice parameters a = 14.07 Å ( 1 Å = 100 pm ), b = 6.72 Å, c = 8.43 Å and β = 123.36 °, and four formula units per unit cell.
The crystal structure contains only one crystallographically distinguishable uranium atom at the origin of the unit cell ( location coordinates: 0, 0, 0 ), and is reproduced by the present symmetry elements symmetrically equivalent position. Due to its location in the origin of the unit cell, it is the only particle in the crystal structure, which lies on a symmetry element, it is located on an inversion center and has the site symmetry 1 The uranium atom is in the form of a uranyl cation [ UO 2 ] 2 (U -O bond length: 1.77 Å) before, the addition of two peroxide ions O22 - (O- O bond length: 1.46 Å, U - O distance: 2.35 Å and 2.37 Å) and two water molecules H2O (U - O distance: 2.40 Å) is surrounded, resulting in a coordination number of CN = 8 total results. The resulting coordination polyhedron of the uranium atom is a distorted hexagonal bipyramid, where are the oxygen atoms of the uranyl cation at the tips (axial or apical position) and the peroxide ions and water molecules in the hexagonal base of the bipyramid ( equatorial position ).
The [( UO2) (O2 ) 2 ( H2O ) 2] - bipyramids are not isolated in the crystal structure before, but link on the peroxide ions (that is, by common edges ) to form chains running along the crystallographic c- axis () and run with the Niggli formula: can be described. The bipyramids are. Within the chain alternately in the opposite direction inclined ( " zigzag chain " ), the chain motif that is repeated after two polyhedra and 8.43 Å, which corresponds to the lattice parameters of the crystallographic c- axis
The individual chains are linked to each other in the crystal structure by the remaining, not involved in the coordination sphere of the uranium atoms of water molecules ( water of crystallization) via hydrogen bonds, whereby the three-dimensional structure.
Linkage pattern of the chains
Hydrogen bonds between the chains
Metastudtit
In 1983 Deliens and Piret report for the first time from the crystal anhydrous form of Studtits, the Metastudtit (UO2 ) ( O2) (H2O ) 2 They examined several mineral samples from Shinkolobwe, and were able to prove by comparison with synthetically produced material, the first natural occurrence of Metastudtit. These samples are associated with Rutherfordin, Becquerelit, Masuyit, Kasolit, Wölsendorfit, uranophane, Soddyit and uraninite. The mineral is pale yellow, and the fine fibers of up to 3 mm long with a diameter of about 0.001 mm; It shows neither short wavelength or under long wave UV light fluorescence. The lattice parameters are a = 6.51 (1) Å; b = 8.78 (2) Å; c = 4.21 ( 1 ) Å and two formula units per unit cell indicated. The type of mineral is in the Royal Museum for Central Africa in Tervuren, Belgium deposited.
Precautions
Due to the strong radioactivity of the mineral should mineral samples from studtite in a dust - and radiation- tight containers are mainly but never kept in living, sleeping and working areas. Should be definitely prevented and the safety of direct body contact and worn when handling the mineral mask and gloves also because of the high toxicity and radioactivity of uranyl inclusion in the body ( incorporation, ingestion).