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

Redefinition of thérèsemagnanite, NaCo4(SO4)(OH)6Cl·6H2O: new data and relationship to ‘cobaltogordaite'

Thérèsemagnanite was originally described from the Cap Garonne mine, Var, France. Its ideal formula was reported as (Co,Zn,Ni)6(SO4)(OH,Cl)10·8H2O; without crystal structure data, only the powder X-ray diffraction pattern was given. Revision of the holotype material revealed that thérèsemagnanite is identical to ‘cobaltogordaite’ (IMA2014-043), recently described from the Blue Lizard mine, Utah, USA. Thérèsemagnanite is thus redefined in accordance with the new data obtained for the neotype specimen from Blue Lizard (formerly the holotype specimen of ‘cobaltogordaite’) and ‘cobaltogordaite’ has been discredited by the International Mineralogical Association Commission on New Mineral Nomenclature and Classification (IMA CNMNC). Thérèsemagnanite has the ideal, end-member formula NaCo4(SO4)(OH)6Cl·6H2O. The empirical formulae of the holotype (Cap Garonne) and the neotype (Blue Lizard), both based on microprobe analyses and calculated on the basis of 17 O + Cl atoms per formula unit (with fixed 6 OH groups and 6 H2O molecules; H content is calculated by stoichiometry) are (Na0.64K0.09)Σ0.73(Co2.35Zn1.22Ni0.50)Σ4.07S1.02O3.98(OH)6Cl1.02·6H2O and Na1.01(Co1.90Zn1.37Ni0.48Cu0.15Mn0.05)Σ3.95S1.03O4.09(OH)6Cl0.91·6H2O, respectively. Thérèsemagnanite is trigonal, Pforumla$\overline 3$⁠, a = 8.349(3), c = 13.031(2) Å, V = 786.6(4) Å3 and Z = 2 (neotype). The strongest powder X-ray diffraction lines are [dobs in Å (hkl) (Irel)]: 13.10 (001)(100), 6.53 (002)(8), 4.173 (110)(4), 3.517 (112)(5), 2.975 (104, 10forumla$\overline 4$⁠)(4), 2.676 (211)(5) and 2.520 (12forumla$\bar 2$⁠)(5) (neotype). Thérèsemagnanite is a cobalt analogue of gordaite, NaZn4(SO4)(OH)6Cl·6H2O. These minerals represent the gordaite group, accepted by the IMA CNMNC

Crystal Structure Evolution of Slawsonite SrAl2_{2}Si2_{2}O8_{8} and Paracelsian BaAl2_{2}Si2 _{2}O8_{8} upon Compression and Decompression

The high-pressure behavior of slawsonite, SrAl$_{2}$Si$_{2}$O$_{8}$ , has been studied using in situ single-crystal X-ray diffraction (SCXRD) and Raman spectroscopy up to 31 GPa. Slawsonite undergoes displacive phase transition between 6 and 8 GPa with the formation of slawsonite-II, featuring fivefold coordinated silicon and aluminum. The results have been confirmed by the changes in vibrational modes using Raman spectroscopy. High-pressure evolution of the Raman spectra of isotypic paracelsian, BaAl$_{2}$Si$_{2}$O$_{8}$ , was studied upon compression and decompression up to 37.5 GPa. Raman data for paracelsian upon compression are in good agreement with previously obtained SCXRD data, which demonstrated three phase transitions at ∼6, 28, and 32 GPa with the formation of AlO$_5$, SiO$_5$, AlO$_6$, and SiO$_6$ polyhedra. Raman data upon decompression show the possibility to quench the high-pressure modification, containing AlO5 polyhedra. The comparison of the high-pressure behavior of slawsonite with paracelsian reveals that the increasing size of extra framework cation from Sr$^{2+}$ to Ba$^{2+}$ reduces the phase transition pressure but does not change the transformation pathway

Vorontsovite, (Hg5cu)\u3a36TLAs4S12, and ferrovorontsovite, (Fe5Cu)\u3a36TlAs4S12: The Tl- and Tl-Fe-analogues of galkhaite from the vorontsovskoe gold deposit, northern Urals, Russia

Two new mineral species, vorontsovite, ideally (Hg5Cu)TlAs4S12, and ferrovorontsovite, ideally (Fe5Cu)TlAs4S12, the Tl- and Tl–Fe-analogues of galkhaite, respectively, have been discovered at the Vorontsovskoe gold deposit, Northern Urals, Russia. They occur as anhedral grains up to 0.5 mm (vorontsovite) and 0.2 mm (ferrovorontsovite) embedded in a calcite-dolomite matrix. The chemical composition of vorontsovite (wt %) is: Hg 35.70, Fe 5.36, Zn 1.26, Cu 3.42, Ag 0.64, Tl 11.53, Cs 0.35, Pb 0.04, As 15.98, Sb 2.35, Te 0.41, S 22.70, Se 0.02, total 99.76. The empirical formula, calculated on the basis of 23 atoms pfu, is: [(Hg3.02Fe1.63Zn0.33)Σ4.98(Cu0.91Ag0.10)Σ1.01](Tl0.96Cs0.04)Σ1.00(As3.62Sb0.33Te0.05)Σ4.00S12.01. The composition of ferrovorontsovite (wt %) is: Hg 25.13, Fe 9.89, Zn 1.16, Cu 3.95, Ag 0.45, Tl 12.93, Cs 0.44, Pb 0.04, As 17.83, Sb 2.15, Te 0.40, S 24.91, total 99.28. The empirical formula, calculated on the basis of 23 atoms pfu, is: [(Fe2.74Hg1.94Zn0.27)Σ4.95(Cu0.96Ag0.06)Σ1.02](Tl0.98Cs0.05)Σ1.03(As3.68Sb0.27Te0.05)Σ4.00S12.00. Both minerals are cubic, space group I-43m, with a = 10.2956(6) Å, V = 1091.3(1) Å3, Z = 2 (vorontsovite); and a = 10.2390(7) Å, V = 1073.43(22) Å3, Z = 2 (ferrovorontsovite). The crystal structures of both minerals were refined to R = 0.0376 (vorontsovite) and R = 0.0576 (ferrovorontsovite). Vorontsovite and ferrovorontsovite have been approved by the IMA-CNMNC under the numbers 2016-076 and 2017-007, respectively. The first one is named after the type locality, but also honors the mining engineer Vladimir Vasilyevich Vorontsov. The second is named for its chemical composition, as the Fe-analogue of the first. Both species are isostructural with galkhaite, being its Tl- and Tl–Fe analogues, respectively, and forming altogether the galkhaite group