Jan Parafiniuk

Methane-Derived Carbonates in a Native Sulfur Deposit: Stable Isotope and Trace Element Discriminations Related to the Transformation of Aragonite to Calcite

Abstract Stable isotope ((13)C, (18)O, (34)S) and trace element (Sr(2+), Mg(2+), Mn(2+), Ba(2+), Na(+)) investigations of elemental sulfur, primary calcites and mixtures of aragonite with secondary, post-aragonitic calcite from sulfur-bearing limestones have provided new insights into the geochemistry of the mineral forming environment of the native sulfur deposit at Machów (SE-Poland). The carbon isotopic composition of carbonates (δ(13)C = -41 to -47‰ vs. PDB) associated with native sulfur (δ(34)S = + 10 to + 15‰ vs. V-CDT) relates their formation to the microbiological anaerobic oxidation of methane and the reduction of sulfate derived from Miocene gypsum. From a comparison with experimentally derived fractionation factors the element ratios of the aqueous fluids responsible for carbonate formation are estimated. In agreement with field and laboratory observations, ratios near seawater composition are obtained for primary aragonite, whereas the fluids were relatively enriched in dissolved calcium during the formation of primary and secondary calcites. Based on the oxygen isotope composition of the carbonates (δ(18)O = -3.9 to -5.9‰ vs. PDB) and a secondary SrSO(4) (δ(18)O = + 20‰ vs. SMOW; δ(34)S = + 59‰ vs. V-CDT), maximum formation temperatures of 35°C (carbonates) and 47°C (celestite) are obtained, in agreement with estimates for West Ukraine sulfur ores. The sulfur isotopic composition of elemental sulfur associated with carbonates points to intense microbial reduction of sulfate derived from Miocene gypsum (δ(34)S ≈ + 23‰) prior to the re-oxidation of dissolved reduced sulfur species.

Slavikite—Revision of chemical composition and crystal structure

Abstract Given its abundant occurrence at Wieściszowice, SW Poland, we have carried out a revision of the chemical composition and crystal structure of the sulfate mineral slavikite. Slavikite crystallizes in the trigonal space group R3̅. The unit-cell parameters, determined using single-crystal X-ray diffraction (R1 = 0.0356) at 100 K, are a = 12.1347(6) Å, c = 34.706(3) Å, and V = 4425.9(5) Å3. The results of chemical analyses reported in the literature and made on material from Wieściszowice unequivocally show that Na is not an essential component of slavikite, at odds with the generally accepted Süsse formula and model of the crystal structure. Our chemical analyses and structure determination lead us to propose a new, more adequate, formula for slavikite: (H3O+)3Mg6Fe15(SO4)21(OH)18·98H2O. The crystal structure consists of infinite layers of Fe-hydroxy-sulfate linked with [Mg(H2O)6]2+ octahedra, forming a honeycomb-like structure. These layers are perpendicular to the Z axis and are built up from two types of SO-24 tetrahedra and two types of Fe octahedra (Fe1 with O and OH, and Fe2 with O, OH, and H2O ligands attached, respectively). Compared to previous studies, the main skeleton of the slavikite structure, i.e., the layers of Fe3+- hydroxy-sulfate, Mg[(H2O)6]2+ octahedra and disordered isolated sulfate ions, remains unchanged. However, on the basis of careful chemical analysis and single-crystal X-ray diffraction studies, we conclude that Na cations are absent from the structure of slavikite and their positions are occupied by disordered protonated water clusters balancing the excess of negative charge in the structure. These protonated water clusters are located at the inversion centers on the 3 axes of symmetry between two [Mg(H2O)6]2+ cations also lying on such axes (but not at the inversion centers). This structure also contains another disordered moiety-an isolated sulfate anion located at the inversion center of the 3 axis. This SO-24 anion is disordered in such a way that each oxygen atom partially occupies 5 positions resulting from 5 different orientations of the anion. This complex anion is linked by hydrogen bonds with the O atoms of ordered water molecules. In consequence, the disordered sulfate anion is surrounded by 12 ordered water molecules, thus forming a spherical water environment around the sulfate.

Applications of Hirshfeld surfaces to mineralogy: An example of alumohydrocalcite, and the classification of the dundasite group minerals

Abstract The crystal structure of alumohydrocalcite was determined using synchrotron X-ray radiation. Alumohydrocalcite crystallizes in the triclinic P1̅ space group with unit-cell parameters: a = 5.71(5), b = 6.54(4), c = 14.6 (2) Å, α = 81.8(3)°, β = 83.9(3)°, γ = 86.5(7)°, and V = 537(7) Å3. This mineral has the formula CaAl2(CO3)2(OH)4·4H2O as opposed to the commonly accepted formula CaAl2(CO3)2(OH)4·3H2O. The fourth water molecule interacts with the strongly bonded polyhedral unit of the structure through hydrogen bonds and connects three adjacent units. This water molecule plays a major role in crystal stability. On heating the sample, this fourth water molecule escapes from the crystal structure as a first one at lower temperature (~105 °C) than the other water molecules in the crystal structure (~128 °C). Analysis and description of the alumohydrocalcite crystal structure and particularly of the intermolecular interactions, together with a comparison to the crystal structures of other minerals with the analog formula M2+M3+2 (CO3)2(OH)4·nH2O, suggests that this mineral is an extension of the dundasite group that should, we propose, be formed for all minerals with the above formula (dundasite, dresserite, strontiodresserite, petterdite, kochsándorite, hydrodresserite, and alumohydrocalcite). They all exhibit very similar patterns on Hirshfeld surfaces. Hirshfeld surfaces appear to be a very useful tool in the analysis of interactions, classification, and validation of mineral crystal structures.

Millimeter scale variations in the isotopic composition of vein sulphide minerals in the Kupferschiefer deposits, Lubin area, SW Poland

A slice of black shale rock cut by various metal sulphide veins of different generations from the Kupferschiefer deposits of Lubin, Poland was subjected to bombardment in a Laser Microprobe Combustion Reactor to produce SO2 for S-isotope analyses. The δ34S values ranged from−22 to−29 ‰ consistent with previous findings using conventional IRMS and attributable to primary generation of H2S by bacterial sulphate reduction. Systematic trends in δ34S values of a few per mil over distances of the order of mm attest to low temperatures of mineralization with accompanying change in the isotope composition of the fluids due to kinetic or equilibrium isotope fractionation. †Revised version of a paper presented at the VIII Isotope Workshop of the European Society for Isotope Research (ESIR), June 25 to 30, 2005, Leipzig-Halle, Germany