Compressibility of hingganite-(Y): high-pressure single crystal X-ray diffraction study

Behaviour of hingganite-(Y), Y$_2\square$Be$_2$Si$_2$O$_8$(OH)$_2$, on compression to 47 GPa has been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction at room temperature in a diamond anvil cell. In the studied pressure range no obvious phase transitions have been observed. The compression of hingganite-(Y) crystal structure is anisotropic, with b axis showing the maximal compressibility. A fit of the experimental pressure–volume data by the Birch-Murnaghan third-order equation of state yielded the bulk modulus of 131(2) GPa and its pressure first derivative of 3.5(2). The difference between high-pressure behaviour of hingganite-(Y) and structurally related datolite is governed by the different chemical nature of interlayer cations

Crystal Chemistry and Structural Complexity of Natural and Synthetic Uranyl Selenites

Comparison of the natural and synthetic phases allows an overview to be made and even an understanding of the crystal growth processes and mechanisms of the particular crystal structure formation. Thus, in this work, we review the crystal chemistry of the family of uranyl selenite compounds, paying special attention to the pathways of synthesis and topological analysis of the known crystal structures. Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl selenite mineral formation requires heating, which most likely can be attributed to the radioactive decay. Structural complexity studies revealed that the majority of synthetic compounds have the topological symmetry of uranyl selenite building blocks equal to the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved regardless of the interstitial filling of the structures. Whereas the real symmetry of U-Se complexes in the structures of minerals is lower than their topological symmetry, which means that interstitial cations and H2O molecules significantly affect the structural architecture of natural compounds. At the same time, structural complexity parameters for the whole structure are usually higher for the minerals than those for the synthetic compounds of a similar or close organization, which probably indicates the preferred existence of such natural-born architectures. In addition, the reexamination of the crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite is reported. As a result of the single crystal X-ray diffraction analysis of demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6(OH)6](H2O)2, the H atoms positions belonging to the interstitial H2O molecules were assigned. The refinement of the guilleminite crystal structure allowed the determination of an additional site arranged within the void of the interlayer space and occupied by an H2O molecule, which suggests the formula of guilleminite to be written as Ba[(UO2)3(SeO3)2O2](H2O)4 instead of Ba[(UO2)3(SeO3)2O2](H2O)3

Coupled Substitutions in Natural MnO(OH) Polymorphs: Infrared Spectroscopic Investigation

Solid solutions involving natural Mn3+O(OH) polymorphs, groutite, manganite, and feitknechtite are characterized and discussed based on original and literature data on the chemical composition, powder and single-crystal X-ray diffraction, and middle-range IR absorption spectra of these minerals. It is shown that manganite forms two kinds of solid-solution series, in which intermediate members have the general formulae (i) (Mn4+, Mn3+)O(OH,O), with pyrolusite as the Mn4+O2 end-member, and (ii) (Mn3+, M2+)O(OH, H2O), where M = Mn or Zn. In Zn-substituted manganite from Kapova Cave, South Urals, Russia, the Zn2+:Mn3+ ratio reaches 1:1 (the substitution of Mn3+ with Zn2+ is accompanied by the coupled substitution of OH− with H2O). Groutite forms solid-solution series with ramsdellite Mn4+O2. In addition, the incorporation of OH− anions in the 1 × 2 tunnels of ramsdellite is possible. Feitknechtite is considered to be isostructural with (or structurally related to) the compounds (M2+, Mn3+)(OH, O)2 (M = Mn, Zn) with a pyrochroite-related layered structure

Jezekite, Na-8[(UO2)(CO3)(3)](SO4)(2)center dot 3H(2)O, a new uranyl mineral from Jachymov, Czech Republic

International audienceJezekite (IMA2014-079), Na-8[(UO2)(CO3)(3)](SO4)(2)center dot 3H(2)O, is a new uranyl carbonate-sulfate mineral from Jachymov, Western Bohemia, Czech Republic. The new mineral was found on samples from the Geschieber vein in the Svornost mine. It occurs as a crystalline crust composed of thin, bladed prismatic crystals of yellow to sulfuric yellow color on a gangue along with andersonite, cejkaite, schrockingerite and ubiquitous gypsum. It is a supergene, low-temperature mineral formed by hydration-oxidation weathering of uraninite associated with post-mining processes. Jezekite is hexagonal, space group P-62m, with unit-cell parameters a = 9.0664(11), c = 6.9110(6)angstrom and V = 491.97(12)angstrom(3), Z = 1. Crystals are thin blades elongated along [001]. Crystals exhibit the forms {001}, {1-11}, {100} and {010}, commonly forming twins/intergrowths with a twin plane parallel to [001]. Jezekite is light yellow to sulfuric yellow and has a very pale yellow streak. It exhibits a bright greenish white fluorescence under both long-wave and short-wave UV. It is transparent with a vitreous to pearly luster. The mineral has a Mohs hardness similar to 2; it is brittle, with uneven fracture and a perfect cleavage on {001} and along [010]. The calculated density based on the empirical formula is 2.966 g/cm(3). The mineral is optically uniaxial (+), with omega = 1.484(2) and epsilon = 1.547(2) (589 nm). It is non-pleochroic. The chemical composition of jezekite (wt. %, electron-microprobe) is: Na2O 27.92, SO3 18.49, UO3 32.85, CO2 (calc.) 15.08, H2O (calc.) 6.17, total 100.51, which yields the empirical formula Na-7.88 (UO2)(CO3)(3)(S1.01O4)(2)center dot 3H(2)O (based on 22 O apfu). Prominent features in the Raman spectrum include the O-H stretching vibrations, symmetric stretching vibrations of (UO2)(2+) ions, and stretching and bending vibrations of symmetrically non-equivalent CO3 groups and highly disordered SO4 tetrahedra. The eight strongest powder X-ray diffraction lines for jezekite are [d(obs)angstrom(I-rel.) (hkl)]: 7.861(59)(100), 6.925(20)(001), 5.193(100)(101), 4.534(44)(110), 3.415(23)(201), 2.751(17)(112), 2.728(20)(211), 2.618(25)(300). The crystal structure of jezekite (R = 0.043 for 444 reflections with I-obs > 3 sigma[I]) contains finite uranyl tricarbonate clusters linked through the Na-O bonds to form sheets of the composition {Na-2[(UO2)(CO3)(3)]}(2-) parallel to (001). The adjacent sheets of polyhedra are also linked through Na-O bonds to the six Na2 atoms and highly disordered sheets of composition {[(SO4)(2)(H2O)(3)]}(4-) into a sandwich-like structure. The new mineral is named after Professor Bohuslav Jezek (1877-1950), a prominent Czech mineralogist and crystallographe

Jahn-Teller Distortion and Cation Ordering: The Crystal Structure of Paratooite-(La), a Superstructure of Carbocernaite

The crystal structure of paratooite-(La) has been solved using crystals from the type locality, Paratoo copper mine, near Yunta, Olary Province, South Australia, Australia. The mineral is orthorhombic, Pbam, a = 7.2250(3) Å, b = 12.7626(5) Å, c = 10.0559(4) Å, V = 927.25(6) Å3, and R1 = 0.063 for 1299 unique observed reflections. The crystal structure contains eight symmetrically independent cation sites. The La site, which accommodates rare earth elements (REEs), but also contains Sr and Ca, has a tenfold coordination by seven carbonate groups. The Ca, Na1, and Na2 sites are coordinated by eight, eight, and six O atoms, respectively, forming distorted CaO8 and Na1O8 cubes, and Na2O6 octahedra. The Cu site is occupied solely by copper and possess a distorted octahedral coordination with four short (1.941 Å) and two longer (2.676 Å) apical Cu–O bonds. There are three symmetrically independent carbonate groups (CO3)2− with the average <C–O> bond lengths equal to 1.279, 1.280, and 1.279 Å for the C1, C2, and C3 sites, respectively. The crystal structure of paratooite-(La) can be described as a strongly distorted body-centered lattice formed by metal cations with (CO3)2− groups filling its interstices. According to the chemical and crystal-structure data, the crystal-chemical formula of paratooite-(La) can be described as (La0.74Ca0.11Sr0.07)4CuCa(Na0.75Ca0.15)(Na0.63)(CO3)8 or REE2.96Ca1.59Na1.38CuSr0.28(CO3)8. The idealized formula can be written as (La,Sr,Ca)4CuCa(Na,Ca)2(CO3)8. The structure of paratooite is a 1 × 2 × 2 superstructure of carbocernaite, CaSr(CO3)2. The superstructure arises due to the ordering of the chemically different Cu2+ cations, on one hand, and Na+ and Ca2+ cations, on the other hand. The formation of a superstructure due to the cation ordering in paratooite-(La) compared to carbocernaite results in the multiple increase of structural complexity per unit cell. Therefore, paratooite-(La) versus carbocernaite represents a good example of structural complexity increasing due to the increasing chemical complexity controlled by different electronic properties of mineral-forming chemical elements (transitional versus alkali and alkaline earth metals)