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Štěpite, U(AsO3OH)2·4H2O, from Jáchymov, Czech Republic: the first natural arsenate of tetravalent uranium
- J. Plášil, K. Fejfarová, J. Hloušek, R. Škoda, M. Novák, J. Sejkora, J. Čejka, M. Dušek, F. Veselovský, P. Ondruš, J. Majzlan, Z. Mrázek
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- Journal:
- Mineralogical Magazine / Volume 77 / Issue 1 / February 2013
- Published online by Cambridge University Press:
- 05 July 2018, pp. 137-152
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Štěpite, tetragonal U(AsO3 OH)2(H2O)4 (IMA 2012-006), is the first natural arsenate of tetravalent uranium. It occurs in the Geschieber vein, Jáchymov ore district, Western Bohemia, Czech Republic, as emerald-green crystalline crusts on altered arsenic. Associated minerals include arsenolite, běhounekite, claudetite, gypsum, kaatialaite, the new mineral vysokýite (IMA 2012-067) and a partially characterized phase with the formula (H3O)+2(UO2)2(AsO4)2˙6H2O. Štěpite typically forms tabular crystals with prominent {001} and {010} faces, up to 0.6 mm in size. The crystals have a vitreous lustre and a grey to greenish grey streak. They are brittle with an uneven fracture and a very good cleavage on (001). Their Mohs hardness is about 2. Štěpite is not fluorescent in either short-wave or long-wave ultraviolet light. It is biaxial (–) with refractive indices (at 590 nm) of α = 1.636(2), β = 1.667(3), γ = 1.672(2) and 2Vobs < ~5°, anomalous greyish to pale yellow interference colours, and no pleochroism. The composition is as follows: 0.12Na2O, 50.19 UO2, 0.04SiO4, 0.09 P2O5, 0.93 As2O5, 1.95 SO3, 16.41 H2O; total 107.90 wt.%, yielding an empirical formula (based on 12 O a. p. f. u.) of (U1.01Na0.02)Σ1.03[(AsO3OH)1.82 (PO3OH)0.04(SO4)0.13(SiO4)0.01]Σ 2.00˙4H2O. Štěpite is tetragonal, crystallizing in space group I41/acd, with a = 10.9894(1), c = 32.9109(6) Å, V = 3974.5(1) Å3, Z = 16 and Dcalc = 3.90 g cm-3. The six strongest peaks in the X-ray powder-diffraction pattern [dobs in Å (I) (hkl)] are as follows: 8.190(100)(004), 7.008(43)(112), 5.475(18)(200), 4.111(16)(008), 3.395(20)(312,217), 2.1543(25)(419). The crystal structure of šteěpite has been solved from singlecrystal X-ray diffraction data by the charge-flipping method and refined to R1 = 0.0353 based on 1434 unique observed reflections, and to wR2 = 0.1488 for all 1523 unique reflections. The crystal structure of štšpite consists of sheets perpendicular to [001], made up of eight-coordinate uranium atoms and hydroxyarsenate 'tetrahedra'. The ligands surrounding the uranium atom consist of six oxygen atoms which belong to the hydroxyarsenate groups and two oxygen atoms which belong to interlayer H2 O molecules. Each UO8 polyhedron is connected to five other U polyhedra via six AsO3OH groups. Adjacent electroneutral sheets, of composition [U4+(AsO3OH)22-]0, are linked by hydrogen bonds involving H2 O molecules in the interlayers and OH groups in the sheets. The new mineral is named in honour of Josef Štěp (1863–1926), a Czech mining engineer and 'father' of the world's first radioactive spa at Jáchymov.
Bêhounekite, U(SO4)2(H2O)4, from Jáchymov (St Joachimsthal), Czech Republic: the first natural U4+ sulphate
- J. Plášil, K. Fejfarová, M. Novák, M. Dušek, R. Škoda, J. Hloušek, J. Čejka, J. Majzlan, J. Sejkora, V. Machovic, D. Talla
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- Mineralogical Magazine / Volume 75 / Issue 6 / December 2011
- Published online by Cambridge University Press:
- 05 July 2018, pp. 2739-2753
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Bêhounekite, orthorhombic U(SO4)2(H2O)4, is the first natural sulphate of U4+. It was found in the Geschieber vein, Jáchymov (St Joachimsthal) ore district, Western Bohemia, Czech Republic, crystallized on the altered surface of arsenic and associated with kaatialaite, arsenolite, claudetite, unnamed phase UM1997-20-AsO:HU and gypsum. Bêhounekite most commonly forms short-prismatic to tabular green crystals, rarely up to 0.5 mm long. The crystals have a strong vitreous lustre and a grey to greenish grey streak. They are brittle with an uneven fracture and have very good cleavage along ﹛100﹜. The Mohs hardness is about 2. The mineral is not fluorescent either in short- or long-wavelength UV radiation. Bêhounekite is moderately pleochroic, α∼β is pale emerald green and γ is emerald green, and is optically biaxial (+) with α = 1.590(2), β = 1.618(4), γ = 1.659(2) (590 nm), 2V (calc.) = 81°, birefringence 0.069. The empirical formula of bêhounekite (based on 12 O atoms, from an average of five point analyses) is (U0.99Y0.03)Σ1.02(SO4)1.97(H2O)4. The simplified formula is U(SO4)2(H2O)4, which requires UO2 53.77. SO3 31.88, H2O 14.35, total 100.00 wt.%. Bêhounekite is orthorhombic, space group Pnma, a = 14.6464(3), b = 11.0786(3), c = 5.6910(14) Å, V = 923.43(4) Å3, Z = 4, Dcalc = 3.62 g cm–3. The seven strongest diffraction peaks in the X-ray powder diffraction pattern are [dobs in Å (I) (hid)]: 7.330 (100) (200), 6.112 (54) (210), 5.538 (21) (020), 4.787 (42) (111), 3.663 (17) (400), 3.478 (20) (410), 3.080 (41) (321). The crystal structure of bêhounekite has been solved by the charge-flipping method from single-crystal X-ray diffraction data and refined to R1 = 2.10 % with a GOF = 1.51, based on 912 unique observed diffractions. The crystal structure consists of layers built up from [8]-coordinate uranium atoms and sulphate tetrahedra. The eight ligands include four oxygen atoms from the sulphate groups and four oxygen atoms from the H2O molecules. Each uranium coordination polyhedron is connected via sulphate tetrahedra with other uranium polyhedra and through hydrogen bonds to the apices of sulphate tetrahedra. The dominant features of the Raman and infrared spectra of bêhounekite are related to stretching vibrations of SO4 tetrahedra (∼1200–950 cm–1), O-H stretching modes (∼3400–3000 cm–1) and H—O—H bending modes (∼1650 cm–1). The mineral is named in honour of František Bêhounek, a well known Czech nuclear physicist.
The crystal structures, solid solutions and infrared spectra of copiapite-group minerals
- J. Majzlan, R. Michallik
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- Mineralogical Magazine / Volume 71 / Issue 5 / October 2007
- Published online by Cambridge University Press:
- 05 July 2018, pp. 553-569
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Copiapite is a mineral of iron- and sulphate-rich acidic environments and has a general formula AFe3+4 (SO4)6(OH)2(H2O)20, where A = Fe2+, 2/3Fe3+, 2/3Al3+, Mg, Zn. The structure is built by infinite tetrahedral-octahedral chains and isolated octahedrally coordinated A sites. Our synthetic and natural copiapite samples can be divided into two large groups based on the orientation of the structural fragments. One group comprises copiapite phases where A = Al3+, Fe2+ or Fe3+ and we designate it as the structural type AL. The other group consists of copiapite with A = Mg2+, Zn2+ or Ni2+ and this is the structural type MG. The solid-solution series between Fe3+ and Al3+ copiapite is continuous. The series between Mg2+-Al3+, Mg2+-Fe3+ and Mg2+-Al3+-Fe3+ copiapite are not continuous; the samples with intermediate compositions contain two copiapite phases, one of the type AL and one of the type MG. The series between Mg2+ and Zn2+ copiapite is continuous only at 25°C. At 75°C, the Zn-rich portion of this systemcrystallizes a copiapite-like phase whose structure may be a superstructure of copiapite. The series between Al-Fe2+ and Mg-Fe2+ copiapite are not continuous and show complex behaviour of the intermediate compositions.
Vysokýite, U4+[AsO2(OH)2]4·4H2O, a new mineral from Jáchymov, Czech Republic
- J. Plášil, J. Hloušek, R. Škoda, M. Novák, J. Sejkora, J. Čejka, F. Veselovský, J. Majzlan
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- Mineralogical Magazine / Volume 77 / Issue 8 / December 2013
- Published online by Cambridge University Press:
- 05 July 2018, pp. 3055-3066
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Vysokýite, U4+[(AsO2(OH)2]4(H2O)4 (IMA 2012–067), was found growing on an altered surface of massive native As in the Geschieber vein, Jáchymov ore district, Western Bohemia, Czech Republic. The new mineral was found in association with běhounekite, štěpite, kaatialaite, arsenolite, claudetite and gypsum. It forms extremely fibrous light-green crystals up to 8 mm long. Crystals have an alabaster lustre and a greenish-white to greyish streak. Vysokýite is brittle with uneven fracture and perfect cleavage along (100) and (001); the Mohs hardness is ∼2. A density of 3.393 g/cm3 was calculated using the empirical formula and unit-cell parameters obtained from a single-crystal diffraction experiment. Vysokýite is non-fluorescent under short or long wavelength UV radiation. It is colourless under the microscope, measured refractive indices are α' = 1.617(3), γ' = 1.654(3); the estimated optical orientation is α' ∼X, γ' ∼Z. The average of five spot wavelength dispersive spectroscopy (WDS) analyses is 29.44 UO2, 1.03 SiO2, 48.95 As2O5, 0.12 SO3, 15.88 H2O (calc.), total 95.42 wt.%. The empirical formula of vysokýite (based on 20 O a.p.f.u.) is U1.00[AsO2(OH)2]3.90(SiO4)0.16 (SO4)0.01·4H2O. The As–O–H and O–H vibrations dominate in the Raman spectrum. Vysokýite is triclinic, space group P, with a = 10.749(2), b = 5.044(3), c = 19.1778(7) Å, α = 89.872(15)°, β = 121.534(15)°, γ = 76.508(15)°, and V = 852.1(6) Å3, Z = 2 and Dcalc = 3.34 g·cm–3. The strongest diffraction peaks in the X-ray powder diffraction pattern are [dobs in Å (Irel.)(hkl)]: 8.872(100)(100), 8.067(50)(002), 6.399(7)(10), 4.773(6)(10), 3.411(10)(30), 3.197(18)(31). The crystal structure of vysokýite was solved from single-crystal X-ray diffraction data by the charge-flipping method and refined to R1 = 0.0595 based on 2718 unique observed reflection, and to wR2 = 0.1160 for all 4173 unique reflections. The structure of vysokýite consists of UO8 square antiprisms sharing all of their vertices with 8 As-tetrahedra to form infinite chains parallel to [010]. These chains are linked by hydrogen bonds involving terminal (OH) groups of the double-protonated As-tetrahedra and molecules of H2O located between the chains. The new mineral is named in honour of Arnošt Vysoký (1823–1872), the former chief of the Jáchymov mines and smelters, chemist and metallurgist.
Crystal structure, thermal behaviour and parageneses of koninckite, FePO4·2.75H2O
- J. Plášil, J. Majzlan, M. Wierzbicka-Wieczorek, B. Kiefer
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- Mineralogical Magazine / Volume 79 / Issue 5 / October 2015
- Published online by Cambridge University Press:
- 02 January 2018, pp. 1159-1173
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The crystal structure of the mineral koninckite was solved from synchrotron powder X-ray diffraction (XRD) data and refined using density-functional theory (DFT) calculations. Koninckite is tetragonal, with the space group P41212, a = 11.9800(5) Å, c = 14.618(1) Å, V = 2097.9(2) Å3, Z = 8. Its structure is a heteropolyhedral framework with zeolite-like tunnels along [001]. Owing to the severe peak overlap in the powder XRD data and the probable intergrowth of enantiomorphic domains in koninckite, the DFT calculations were applied to provide precise atomic positions (including hydrogen). Additionally, the DFT calculations suggest strongly that koninckite is an antiferromagnetic semiconductor, at least at low temperatures. The DFT computations were used to locate H2O molecules in the channels and to complete the structural description. Thermogravimetric analysis and powder XRD data at variable temperatures show that the structure of koninckite dehydrates and eventually collapses between 160–180°C. Negative thermal expansion was observed between 80 and 150°C. A list of the known occurrences of koninckite suggests that this mineral is not as rare as assumed previously; koninckite is often fine-grained, inconspicuous, and thereby easy to overlook. Koninckite is yet another natural example of an Fe-phosphate zeolitic material.