Curzio Cipriani

Iron local structure in tektites and impact glasses by extended X-ray absorption fine structure and high-resolution X-ray absorption near-edge structure spectroscopy

Abstract The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) spectroscopy in order to provide quantitative data on distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean distances and Fe coordination numbers for the three tektite samples are constant within the estimated error ( =2.00 A ± 0.02 A, CN = 4.0 ± 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 × 1.92 A and 2 × 2.08 A). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5. Comparison of the tektites XANES spectra with those of a suite of different impact glasses clearly shows that tektites display a relatively narrow range of Fe oxidation state and coordination numbers, whereas impact glasses data span a much wider range of Fe oxidation states (from divalent to trivalent) and coordination numbers (from tetra-coordinated to esa-coordinated). These data suggest that the tektite production process is very similar for all the known strewn fields, whereas impact glasses can experience a wide variety of different temperature–pressure–oxygen fugacity conditions, leading to different Fe local structure in the resulting glasses. These data could be of aid in discriminating between tektite-like impact glasses and impact glasses sensu strictu.

First evidence of natural superconductivity covellite

The singular conductive properties of natural covellite were investigated through magnetic AC susceptibility measurements. Two natural hexagonal covellite lamellae coming from Calabona (Italy) and Butte (USA), respectively, were investigated from 30 to 1.5 K. At 1.63(5) K, an abrupt increase of the diamagnetism (Meissner effect) was observed, thus evidencing a transition from a poor conducting to a superconducting state. The transition is independent on small applied DC fields (< 50 Oe), whereas larger values prevent the superconducting state to be attained. These features confirm covellite as a Class I superconductor. This peculiar property of covellite, never observed before on natural materials, can be mainly related to its structural features. The layered framework of covellite, where CuS3 planes are isolated by covalently bonded S2 planes, provides an electron excess, which can be easily involved in the bulk conduction.

Genetic evolution of nanocrystalline Fe oxide and oxyhydroxide assemblages from the Libiola mine (eastern Liguria, Italy) structural and microstructural investigations

The Libiola Fe-Cu-sulphide mine, near Sestri Levante (eastern Liguria), represents one of the most extensively exploited sulphide deposits in Italy. In this area, active Acid Rock Drainage (ARD) processes are evident. The major resulting mineral phases are Fe oxides and oxyhydroxides, occurring in varicoloured crusts on the surface of waste rocks and in unconsolidated muds. In this study, the Fe assemblages of the waste rock were investigated by microchemical (SEM), structural (XRD), microstructural (TEM) and spectroscopic (DRS, IR, μ-Raman) techniques, in order to determine the phase composition, the textural relations among the minerals and their genetic evolution. They are characterized by intimate intergrowths of hematite and goethite with minor quartz and lepidocrocite; in some samples, the presence of very minor schwertmannite was detected. TEM and HR-TEM observations revealed that hematite is present within pseudo-elliptical bodies as pseudo-hexagonal to subrounded nanocrystalline lamellae (from 18.9 to 26.5 nm in diameter), whereas goethite occurs either as parallel intergrowths of acicular crystals (from 10 to 16.3 μm in length) or as sheaf-like assemblages. On the basis of the present data, the studied Fe oxide and oxyhydroxide assemblages are found to represent distinct spatial and temporal stages of a nano-scale evolution process.

Silicate-silicate liquid immiscibility and graphite ribbons in Libyan desert glass

Abstract Transmission electron microscopic (TEM) investigation of the dark (brown or bluish) streaks occurring in Libyan Desert Glass reveals the common presence of small glass spherules. The spherules, mostly 100 nm in size, are homogeneously dispersed within the silica-glass matrix. The complete absence of electron diffraction effects confirms their amorphous nature. The spherules are Al-, Fe- and Mg-enriched with respect to the surrounding silica matrix and their (Mg + Al + Fe) : Si ratio is close to 1. The silica-glass matrix and amorphous spherules form an emulsion texture (i.e., globules of one glass in a matrix of another glass), which originates from silicate-silicate liquid immiscibility. This texture has also been observed in other impact-derived glasses. The silica glass also contains carbonaceous inclusions consisting of 5–50 nm thick, polygonalized graphite ribbons that form closed structures up to 200 nm in diameter. These observations are in agreement with an impact origin for Libyan Desert Glass.

Empressite, AgTe, from the Empress-Josephine mine, Colorado, U.S.A.: Composition, physical properties, and determination of the crystal structure

Abstract The chemistry and composition of empressite, AgTe, a rare silver telluride mineral, has been mistaken in the mineralogical literature for the silver telluride stützite (Ag5-xTe3). Empressite from the type locality, the Empress-Josephine deposit (Colorado), occurs as euhedral prismatic grains up to 400 μm in length and contains no inclusions or intergrowths with other minerals. It is pale bronze in color and shows a grey-black to black streak. No cleavage is observed in empressite but it shows an uneven to subconchoidal fracture and Vickers hardness (VHN25) of 142 kg/mm2. Empressite is greyish white in color, with strong bireflectance and pleochroism. Reflectance percentages for Rmin and Rmax are 40.1, 45.8 (471.1 nm), 39.6, 44.1 (548.3 nm), 39.4, 43.2 (586.6 nm), and 38.9, 41.8 (652.3 nm), respectively. Empressite is orthorhombic and belongs to space group Pmnb (Pnma as standard), with the following unit-cell parameters: a = 8.882(1), b = 20.100(5), c = 4.614(1) Å, V = 823.7(3) Å3, and Z = 16. Electron microprobe analyses gave the chemical formula Ag1.01Te0.99. The calculated density (from the ideal formula) is 7.59 g/cm3. The crystal structure has been solved and refined to R = 4.45%. It consists of edge-sharing AgTe4 tetrahedra forming sheets parallel to (010). The connectivity between the sheets is provided by Te-Te contacts (<2.9 Å) to complete the framework. The structural study presented here shows that empressite and stützite have different crystal structures.

Selective depth analyses of the alteration products of bornite, chalcopyrite and pyrite performed by XPS, AES, RBS

The comparative application of three complementary surface analytical techniques allows us to investigate the alteration process and to probe selectively, as a function of depth, the oxidation products of bornite, chalcopyrite and pyrite. Starting from a pristine surface, the samples have been studied at different stages of alteration up to a strongly oxidized state after two years of air exposure. In spite of their different compositions, some similarities exist in their oxidation: iron sulfates seem to be mainly responsible for the increase of alteration whereas copper sulfide layers, due to iron depletion from the subsurface area, act as a barrier to the progress of alteration.

Chemical speciation of Ag in galena by EPR spectroscopy

Abstract Electron paramagnetic resonance (EPR) spectroscopy has been used to study the valence state of silver in “argentiferous” galena samples from the Apuane Alps (Tuscany, Italy) mining district. This method was used to reveal primary metallic silver (Ag0) in galena. Both thermodynamic data and experimental studies suggest that galena and native silver can stably coexist, but have not been reported as a primary (hypogene) assemblage in natural samples. EPR spectroscopy proved to be a suitable tool to solve this problem, because this technique is capable of detecting paramagnetic species down to the ppb level, even in a highly absorbent matrix such as galena. A detailed SEM-EDS investigation could not detect metallic silver (or gold) in galena samples, but did reveal small (few micrometers) inclusions of Ag-bearing phases, in which silver has a formal valence of +1. On the other hand, EPR spectra indicated the presence in galena of pairs and clusters of elemental silver atoms, which may be associated with pairs of metallic gold, or with silver-gold hetero-atomic pairs. Therefore, SEM/EDS and EPR are complementary techniques, revealing the presence of both Ag+ and Ag0. The Ag(Au) metallic species were apparently deposited on the galena surface during its growth from mineralizing fluids. Their scarcity, and the presence of larger amounts of Ag1+ phases, suggest that the assemblage galena-metallic silver was stable only under peculiar physical and chemical conditions. The formation of Ag0 was presumably linked to local and rare chemical fluctuations of the hydrothermal environment, characterized by low activities of S, Sb, Bi (Cu…) and high activity of Ag in the fluids. The occurrence of both homo- and hetero-atomic pairs suggests either different kinetics of pair formation, or possible fluctuations in the composition of the hydrothermal fluids, which alternatively carried Ag or Au species, or both

Ordered distribution of Au and Ag in the crystal structure of muthmannite, AuAgTe2, a rare telluride from Sacarîmb, western Romania

Abstract Muthmannite, AuAgTe2, a rare gold-silver telluride was discovered in a sample from the historical mineralogical collection of the Naturhistorisches Museum of Vienna. The sample is from the gold-telluride deposit of Sacarîmb, Metaliferi Mountains, western Romania. Muthmannite occurs as anhedral grains up to 200 µm associated with large sylvanite crystals and does not show any inclusions or intergrowths of other minerals. The associated minerals are sylvanite, calaverite, and petzite, whereas the gangue mineral is quartz. Muthmannite is pale bronze in color and shows a gray-black streak. No cleavage was observed, the fracture is uneven, and the Vickers hardness (VHN15) is 186 kg/mm2. Muthmannite is grayish white in reflected light, with very low bireflectance and pleochroism. When observed near sylvanite it is darker and shows a gray color with a slightly bluish tint. Reflectance percentages for Rmin and Rmax were found to be 40.1, 40.8 (471.1 nm), 38.3, 38.6 (548.3 nm), 37.9, 38.3 (586.6 nm), and 37.7, 38.1 (652.3 nm), respectively. Muthmannite is monoclinic, space group P2/m, with the following unit-cell parameters: a = 5.124(2), b = 4.419(1), and c = 7.437(2) Å, β = 89.96(1)°, V = 168.4(4) Å3, and Z = 2. Electron microprobe analyses gave the chemical formula Au0.97Ag0.99Te2.03. The calculated density (from the ideal formula) is 11.04 g/cm3. The crystal structure was solved and reÞ ned to R = 5.52%. It is based on the NiAs-type structure, with a distorted hexagonal closest-packed array of Te2- atoms with Au3+ and Ag+ occupying all the octahedral sites. The crystal-chemical relationships with other gold-silver tellurides are outlined.

Cathodoluminescence investigations on the Popigai, Ries, and Lappajärvi impact diamonds

Abstract Twenty impact diamond samples from the Popigai, Ries, and Lappajärvi astroblemes were analysed using cathodoluminescence (CL) at room temperature (RT). Five of the samples were further investigated at liquid nitrogen temperature (LNT). Cathodoluminescence images allowed for the discrimination of diamond from graphite, thus contributing to a better understanding of the reciprocal relationships between these carbon polymorphs and their overall textural features. Cathodoluminescence spectral measurements of the diamonds revealed emission bands and peaks located at 1.8 eV (688 nm), 2.23 eV (556 nm), 2.32 eV (534 nm), 2.39 eV (519 nm), 2.49 eV (498 nm), and 2.8-2.9 eV (443-427 nm). The bands at 2.8-2.9 eV and at 1.8 eV, observed at RT, were related respectively to vibronic levels (involved in electronic transitions), located at dislocation defects and to dislocations. Regarding the other lines, which were only visible at LNT, there may be a relationship between the peaks at 2.32 eV, 2.23 eV, and 2.39 eV, and the content of amorphous carbon phases. Some spectral features may be considered a possible signature of impact diamonds. In particular, the band at 1.8 eV, which is uncommon in terrestrial natural diamonds, and the peaks at 2.23 eV and 2.32 eV, are present in all the samples studied.

Aluminium coordination in tektites: A XANES study

Abstract Al K-edge XANES spectra were recorded for six tektites to obtain information on the local structure around Al. Albite, grossular, and andalusite were used as reference materials for Al in fourfold, sixfold, and five+sixfold coordination, respectively. The spectra of the tektites are very similar, except for the intensity of the main absorption edge. Comparison of the spectral features of the tektite glasses with those of the reference materials indicates that Al is tetrahedrally coordinated.