Roman Skála

Ca-Fe3+-rich, Si-undersaturated buchite from Želénky, North-Bohemian Brown Coal Basin, Czech Republic

Si-undersaturated Ca-Fe-rich buchite produced by fossil burning of a coal seam occurs in porcelanite deposit at Želenky in the North-Bohemian Brown Coal Basin, Czech Republic. Buchite is highly inhomogeneous, dense (ρ = 2.9–3.1 g/cm 3 ), strongly magnetic rock (10 −2 −10 −1 [SI]) composed of melilite, clinopyroxene (esseneite), spinel-group minerals, hematite, Ca 2 SiO 4 , calcite, aragonite, anorthite, anhydrite, perovskite, barite, two calcium ferrites - orthorhombic Ca 2 Fe 2 3+ O 5 (srebrodolskite) and CaFe 4 3+ O 7 , barium hexaferrite - BaFe 12 3+ O 19 , along with other several poorly identified phases. Clinopyroxene is rich in esseneite component (19–63 mol. %) and in a hypothetical end-member CaFe 3+ Fe 3+ SiO 6 (up to 25 mol. %) due to Fe 3+ substitution in tetrahedral sites (up to 0.26 apfu), and also in a hypothetical component Ca(Fe 2+ Mg) 0.5 Ti 0.5 AlSiO 6 (2–36 mol. %) due to incorporation of Ti (0.012–0.18 apfu), in M1 site. Melilite shows a wide compositional variability (mol. %): gehlenite 21–93, ferrigehlenite 0–34, akermanite 1–49, ferroakermanite 0–25, soda-melilite 0–15. Spinel-group minerals show solid solutions of Mf 12–80, Mt 1–62, Sp 2–68, Hc 0–21 (mol. %). Srebrodolskite, orthorhombic Ca 2 (Fe 3+ Al) 2 O 5 is widely substituted by Si and Ti. There are negative correlations among Fe + Al vs. Si + Ti ( R 2 = 0.998), and Ca (or R 2+ ) vs. total vacancy ( R 2 = 0.94), which are consistent with an exchange vector 2R 4+ □2R 3+ -1 Ca -1 , and suggest a hypothetical end-member Ca □ (SiTi) 2 O 5 . Calcium ferrite associated with srebrodolskite, ideally CaFe 4 3+ O 7 , shows extensive compositional variability: Ca 0.6–0.8 Fe 3+ 2.7–3.6 Al 0.1–0.6 Mg 0.2–0.6 Si 0.1–0.5 O 7 . First natural occurrence of barium hexaferrite, (Ba 0.6–1.1 Ca 0.1–0.2 )(Fe 3+ 10.0–11.7 Al 0.3–0.9 Ti 0.1–0.6 Mg 0.1–0.4 Mn 0.0–0.2 ) O 19 , ideally BaFe 3+ 12 O 19 , is reported. Variations of magnetic susceptibility with temperature were also investigated. Buchite originated by fusion of Tertiary clay and residual ash with overlying Quaternary sediments (loess loam with carbonate horizon) during fossil combustion of Miocene coal seam.

Parascorodite, FeAsO 4 .2H 2 O; a new mineral from Kank near Kutna Hora, Czech Republic

Abstract Parascorodite, a new mineral from Kaňk near Kutná Hora, Central Bohemia, Czech Republic, forms earthy white to white-yellow aggregates associated with scorodite, pitticite, bukovskýite, Kaňkite, zýkaite, gypsum, and jarosite, wet chemical analysis gave (in wt%): As2O5 44.45, P2O5 0.84, SO3 1.53, Fe2O3 34.55, Al2O3 0.17, H2O 17.81, totaling 99.95. The simplified chemical formula is FeAsO4·2H2O. Selected area electron diffraction suggests hexagonal or trigonal symmetry. The extinction symbol is P-c-. Powder X-ray diffraction yielded unit-cell parameters a = 8.9327(5) Å, c = 9.9391(8) Å, V = 686.83 (8) Å3, Z = 6. Densities (measured and calculated, respectively) are Dm = 3.213(3) g/cm3 and Dx = 3.212 g/cm3. SEM and TEM images showed that basal sections of parascorodite are hexagonal in shape; thicker prismatic crystals were also observed. Crystal size varies between 0.1 to 0.5 mm. The strongest lines in the X-ray powder diffraction pattern are [d[I](hkl)]: 4.184(44)(012), 4.076(100)(111), 3.053(67)(202), 2.806(68)(211), 2.661(59)(113), 2.520(54)(212), 2.2891(44)(032). Refractive indexes could not have been measured due to extremely small crystallite size, n̅ (calc) = 1.797. The TG curve shows two weight losses: at 20-150 °C (2.1 wt%, absorbed water) and at 150-620 °C (15.5 wt%, molecular water), respectively. They correspond to the endothermic peaks on the DTA curve at 120 and 260 °C. Strong exothermic reaction observed at 585 °C reflects formation of the phase FeAsO4. Infrared absorption spectra of parascorodite are close to those of scorodite.

Cejkaite, the triclinic polymorph of Na4(UO2)(CO3)3—a new mineral from Jáchymov, Czech Republic

Abstract Cejkaite, a new mineral from Jáchymov, NW Bohemia, Czech Republic, forms a thin earthy efflorescence over a calcite vein associated with disintegrated uraninite. The color is pale yellow to beige, the streak is light yellow, and the luster is vitreous. The broad secondary mineral association includes andersonite and schröckingerite. Chemical analysis (by ICP-MS and TG) gave (in wt%): Na2O = 21.39, MgO = 0.15, FeO = 0.53, UO3 = 53.93, and CO2 = 24.00 (calculated by difference). The simplified chemical formula is Na4UO2(CO3)3. The mineral is triclinic, space group P1 or P1 , a = 9.291(2), b = 9.292(2), c = 12.895(2) Å, a = 90.73(2), b = 90.82(2), g = 120.00(1)°, V = 963.7(4) Å3, Z = 4, Dmeas = 3.67(1) g/cm3, and Dcalc = 3.766(5) g/cm3. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 8.022(92)(11 - 0, 010, 100), 5.080(57)(1 - 02, 01 - 2), 5.024(60)( 1 - 12, 11 - 2), 4.967(68)( 012, 102), 4.639(100)(12 - 0, 21 - 0, 110), 3.221(63)(004), 2.681(60) (33 - 0, 1 - 14, 030, 300). Optical data could not be measured due to the extremely small grain size, but the calculated mean refractive index is 1.5825. Crystal size varies from 0.2 to 0.6 mm and shows an indistinct hexagonal outline. Thermal decomposition of synthetic cejkaite proceeds in three main steps. DTA endotherm at 430 °C corresponds to the decomposition of the uranyl tricarbonate groups. IR spectrum of cejkaite confirms the presence of crystallographically nonequivalent (CO3)2- groups and the absence of water. The average U-O bond length in (UO2)2+, calculated from v3 = 848 cm-1, is RU-O ~ 1.81 Å. A model based on the crystal structure of trigonal Na4(UO2)(CO3)3 was adopted and applied to solve the cejkaite crystal structure by the Rietveld method (7238 unique reflections, Rp = 0.076, Rwp = 0.104). Uranium is eight-coordinated, and forms a [UO2O6] skeleton with almost linear O-U-O that is roughly perpendicular to an irregular cycle formed by six O atoms that, in turn, belong to three more-or-less regular and planar CO3 groups. Atoms Na1, Na1a, and Na2 are octahedrally coordinated, whereas Na3 is pentagonally coordinated. The mineral name honors Jirí Cejka for his notable contributions to the crystal chemistry of U minerals.

Inversion twinning in troilite

Abstract The crystal structure of troilite from chondrites Etter and Georgetown and a troilite analog synthesized by sulfurization of an iron wire was refined using single-crystal X-ray data. Troilite is known to be hexagonal, with space group P62c, which is non-centrosymmetric, allowing two non-identical inversely related spatial arrangements of atoms within the unit cell. All three samples represent the so-called inversion twins. They contain both inversely related atomic orientations instead of a single atomic arrangement. The inversion twinning may have developed as a result of a phase transition from the ideal centrosymmetric NiAs-type structure to troilite-type structure during cooling. In addition, all samples were found to be cation-deficient. The departure from ideal stoichiometry.up to almost 3.5 rel% of metal atoms are missing.is also possibly related to atomic ordering when the crystals cooled.

Description and crystal structure of vajdakite, [(Mo6+O2)2(H2O)2 As3+2O5]· H2O—A new mineral from Jáchymov, Czech Republic

Abstract Vajdakite, a new mineral from Jáchymov, NW Bohemia, Czech Republic, forms minute acicular, gray-green crystals associated with arsenolite, scorodite, parascorodite, kanˇkite, annabergite, köttigite, pyrite, marcasite, nickelskutterudite, and löllingite. Microprobe analysis gave (in wt%): As = 27.72, Mo = 35.39, O = 36.66, total = 99.77. The simplified chemical formula is [(Mo6+O2)2(H2O)2 As3+2O5]·H2O. The mineral is monoclinic, P21/c, a = 7.0515(6), b = 12.0908(9), c = 12.2190(14) Å, b = 101.268(9)°, V = 1021.7(2) Å3, Z = 4, Dmeas = 3.50(2) g/cm3, and Dcalc = 3.509 g/cm3. The strongest lines in the powder X-ray diffraction pattern d(I)(hkl) are: 6.046 (100)(020), 3.324 (59)(023), 6.915 (26)(100), 2.264 (19)(310), 3.457 (16)(200), 2.624 (15)(230), and 3.819 (10)(031). Vajdakite is optically positive, with X || b and Z . c =12; elongation is positive. Its birefrigence is 0.28, with 2Vcalc = 35.1° nα = 1.757(2), nβ= 1.778(2), and nγ = 2.04(1). The pleochroic scheme is X ~ Y = light greenish gray, and Z = yellowish gray. Crystal size varies between 0.1 to 0.5 mm. TG curve and IR spectra show that vajdakite contains two distinct types of water molecules. The crystal structure was solved by direct methods (MoKα radiation) and refined using 1787 unique reflections to R = 0.0455, Rw = 0.1143. There are double chains built up by two individual chains with a sequence -O-As-OMo- interconnected by oxygen atoms from two triangular AsO3 groups and two structurally nonequivalent MoO5(H2O) octahedra. The two vertex-sharing, triangular AsO3 groups form an (As2O5)4- diarsenite group. The first type of water molecule is not included in the coordination, but the second one is in octahedral coordination around Mo. The water molecules are linked by a complicated net of interlayer and intralayer hydrogen bonds

Ab initio X-ray powder structure determination of parascorodite, Fe(H2O)2AsO4

The crystal structure of parascorodite, Fe(H 2 O) 2 AsO 4 , has been solved ab-initio from powder data. The mineral is trigonal, with space group P 3 c 1, Z = 6, a = 8.9232(2) A, c = 9.9296 (3) A. The Rietveld refinement was led up to R p = 4.5 %. The structure is built up by the same coordination polyhedra found in its dimorph scorodite, namely slightly distorted Fe-centred octahedra and As-centred tetrahedra, but their connection lead to quite different crystal structures for the two polymorphs. Parascorodite structure can be described in terms of mixed octahedral-tetrahedral “columns” including Fe2 and As polyhedra, running along c , similar to those found in ferrinatrite and kaatialaite. These columns are interconnected in a threedimensional framework by the sharing of the 01 oxygen between [AsO4] tetrahedra and Fe 1 octahedra, and by the strong hydrogen bond OW…O2.

Trigonal Na4[UO2(CO3)3]

Tetrasodium tricarbonatodioxouranate(VI), Na4[UO2(CO3)3], crystallizes in the trigonal space group P\overline 3c1. Though the symmetry differs from other similar compounds (e.g. the NH4+, K+ and Tl+ salts) which are monoclinic, there is a common structure motif consisting of UO2(CO3)3 groups with a trigonal outline when viewed along the shortest O—U—O bond pair. In Na4[UO2(CO3)3], there are three non-equivalent Na atoms; Na1 (site symmetry \overline 3) and Na2 (site symmetry 3) are in centres of face-sharing octahedra, which form a chain running parallel to the c axis at each unit-cell corner, whereas the Na3 atom is surrounded by a deformed square pyramid of O atoms, forming edge-sharing triplets. The title compound has also a natural dimorph, namely the recently approved triclinic mineral cejkaite.

Leydetite, Fe(UO2)(SO4)2(H2O)11, a new uranyl sulfate mineral from Mas d'Alary, Lodève, France

Abstract Leydetite, monoclinic Fe(UO2)(SO4)2(H2O)11 (IMA 2012−065), is a new supergene uranyl sulfate from Mas d’Alary, Lodève, Hérault, France. It forms yellow to greenish, tabular, transparent to translucent crystals up to 2 mm in size. Crystals have a vitreous lustre. Leydetite has a perfect cleavage on (001). The streak is yellowish white. Mohs hardness is ~2. The mineral does not fluoresce under long- or shortwavelength UV radiation. Leydetite is colourless in transmitted light, non-pleochroic, biaxial, with α = 1.513(2), γ= 1.522(2) (further optical properties could not be measured). The measured chemical composition of leydetite, FeO 9.28, MgO 0.37, Al2O3 0.26, CuO 0.14, UO3 40.19, SO3 21.91, SiO2 0.18, H2O 27.67, total 100 wt.%, leads to the empirical formula (based on 21 O a.p.f.u.), (Fe0.93Mg0.07Al0.04Cu0.01)∑1.05(U1.01O2)(S1.96Si0.02)S1.98O8(H2O)11. Leydetite is monoclinic, space group C2/c, with a = 11.3203(3), b = 7.7293(2), c = 21.8145(8) Å, β = 102.402(3)°, V = 1864.18(10) Å3, Z = 4, and Dcalc = 2.55 g cm−3. The six strongest reflections in the X-ray powder diffraction pattern are [dobs inA ˚ (I) (hkl)]: 10.625 (100) (002), 6.277 (1) (1̅11), 5.321 (66) (004), 3.549 (5) (006), 2.663 (4) (008), 2.131 (2) (0 0 10). The crystal structure has been refined from single-crystal X-ray diffraction data to R1 = 0.0224 for 5211 observed reflections with [I > 3σ(I)]. Leydetite possesses a sheet structure based upon the protasite anion topology. The sheet consists of UO7 bipyramids, which share four of their equatorial vertices with SO4 tetrahedra. Each SO4 tetrahedron, in turn, shares two of its vertices with UO7 bipyramids. The remaining unshared equatorial vertex of the bipyramid is occupied by H2O, which extends hydrogen bonds within the sheet to one of a free vertex of the SO4 tetrahedron. Sheets are stacked perpendicular to the c direction. In the interlayer, Fe2+ ions and H2O groups link to the sheets on either side via a network of hydrogen bonds. Leydetite is isostructural with the synthetic compound Mg(UO2)(SO4)2(H2O)11. The name of the new mineral honours Jean Claude Leydet (born 1961), an amateur mineralogist from Brest (France), who discovered the new mineral.

Telluronevskite, Bi3TeSe2, a new mineral

The new mineral telluronevskite, ideally Bi 3 TeSe 2 , has been found as irregular grains disseminated in quartzite from the volcanic Vihorlat Mts near Kosice in eastern Slovakia, Slovak Republic. The mineral forms massive aggregates up to 2 mm in diameter and/or individual disseminated tabular crystals flattened along (0001). Telluronevskite is megascopically steel grey in colour, with a metallic lustre and black streak. In reflected light, it is white with a yellow tint, very weak pleochroism and moderate anisotropy. Maximum and minimum values of reflectance measured in air for 470, 546, 589 and 650 nm are ( R max / R min in %): 48.5/46.6; 51.1/48.5; 51.9/49.5; 52.8/50.5. Vickers micro-hardness (VHN 10 in kp.mm −2 ) varied in the range 63-137 with mean value of 100. Microprobe analyses yield the empirical formula (Bi 2.92 Pb 0.02 )Te 1.01 (Se 1.73 S 0.32 ). It is trigonal with space group P 3 m 1. The unit-cell dimensions refined from X-ray powder microdiffraction data are a = 4.264(6) A and c = 23.25(3) A with c : a = 5.453 : 1. For Z = 2 the calculated density is D x = 8.08(2) g.cm −3 ; measured density D m = 8.1(2) g.cm −3 . The five strongest lines in the diffraction pattern are ( d (A), I/I 0 , hkl ): 4.66, 19, 005; 3.12, 100, 104; 2.28, 33, 108; 2.13, 36, 110, 109; 1.355, 18, 1.0.16, 214. Constrained structure refinement from combined powder and precession data revealed ordering of Te and Se atoms within the telluronevskite crystal structure. Telluronevskite belongs to the tsumoite subgroup of the tetradymite group.

Tertiary alkaline Roztoky Intrusive Complex, České středohoří Mts., Czech Republic: petrogenetic characteristics

The České středohoří Mts. is the dominant volcanic center of the Ohře (Eger) rift zone. It hosts the Roztoky Intrusive Complex (RIC), which is made up of a caldera vent and intrusions of 33–28-Ma-old hypabyssal bodies of essexite–monzodiorite–sodalite syenite series accompanied by a radially oriented 30–25-Ma-old dike swarm comprising about 1,000 dikes. The hypabyssal rocks are mildly alkaline mostly foid-bearing types of mafic to intermediate compositions. The dike swarm consists of chemically mildly alkaline and rare strongly alkaline rocks (tinguaites). The geochemical signatures of the mildly alkaline hypabyssal and associated dike rocks of the RIC are consistent with HIMU mantle sources and contributions from lithospheric mantle. The compositional variations of essexite and monzodiorite can be best explained by fractional crystallization of parent magma without significant contributions of crustal material. On the other hand, the composition of monzosyenite, leuco-monzodiorite and sodalite syenite reflects fractional crystallization coupled with variable degrees of crustal assimilation. It is suggested that the parent magmas in the Ohře rift were produced by an adiabatic decompression melting of ambient upper mantle in response to lithospheric extension associated with the Alpine Orogeny.