Matteo Zoppi

Single-crystal diffraction and transmission electron microscopy studies of “silicified” pyrochlore from Narssârssuk, Julianehaab district, Greenland

Abstract Pyrochlore-group minerals that exhibit high Si contents are fairly common in geochemically evolved parageneses. However, the role of Si in the structure of these minerals is unclear. Different explanations have been invoked to clarify the way in which Si is incorporated in natural pyrochlores. These include the presence of impurities, the presence of Si in an amorphous or dispersed state, and its presence as an essential part of the structure. This paper reports an integrated XREF, SEM, EMPA, and TEM study on pyrochlore samples with high SiO2 content (up to 11.51 wt%) from Narssârssuk, Julianehaab district (Greenland). TEM observations reveal that Si-poor areas have strong and sharp diffraction peaks, whereas the Si-rich areas showed weaker spots with the diffuse diffraction halo typical of a metamict material. No evidence of crystalline phases other than pyrochlore was observed. Two single crystals having the unit-cell parameter a = 10.4200(7) and 10.3738(7) Å, respectively, were analyzed by X-ray diffraction and the structure was refined to Robs = 2.62 and 4.35%. On the basis of both the refined site scattering and the octahedral bond distance and the results of the TEM investigation, only a fraction (~30.50%) of the Si detected by EMPA is incorporated in the structure. A comparison with structural data of Si-free pyrochlores reported in the literature supports this assumption and allows a linear multiple regression to model the effect of the substitution of (Nb,Ta) by Ti and Si. The remaining 50.70% of the total silicon detected is incorporated in the radiation-damaged portions of pyrochlore.

Raman Characterization of Ambient Airborne Soot and Associated Mineral Phases

Airborne particulate matter samples were collected in an urban and a rural–suburban monitoring stations of the city of Rome, Italy, and the particles were analyzed through the Raman microspectroscopy. A careful examination of the spectral bands, performed with a five-(Voigt) curve deconvolution model previously described by the literature and here adapted to the purpose, lead to the characterization of the graphitic and carbonaceous material plus the identification of the mineral particles associated with it. Statistical analysis of the full-width at half-maximum (FWHM) values of the bands, as well as of their intensity ratio, revealed the presence of two classes of soot particles that can be ascribed to a different degree of crystallinity. The population of soot collected at the urban site, where the vehicular emission component prevails, exhibits mostly crystalline characteristics (with a D1 FWHM of 150–155 cm−1), whereas the population collected at the rural–suburban site, particularly the coarse fraction, shows a prevailing amorphous nature (with a D1 FWHM of ∼175 cm−1). A similar aspect emerges for the pure black carbon particles, mainly crystalline, and the black carbon particles associated with minerals, generally disordered. These results add useful information and characterization of the soot, a relevant component of the ambient air, and its different features with respect to the urban or rural–suburban areas. Copyright 2014 American Association for Aerosol Research

Metamict aeschynite-(Y) from the Evje-Iveland district (Norway) heat-induced recrystallization and dehydrogenation

To study the possible structural-geometrical variations affecting the structure of aeschynite when heated in air, the heat-induced recrystallization process of a metamict sample from Evje-Iveland was monitored by recording X-ray powder diffraction data. The progressive restoring of crystallinity starts at 350°C and appears to be nearly completed at 450°C. During this process the water contained in the untreated amorphous material is released. Structural data were obtained by Rietveld refinements after each successive heat treatment (450–1050°C temperature range, in step of 100°C). With the increase of heating temperature, other phases than aeschynite progressively crystallized ( i.e. pyrochlore, rutile, brannerite).The transition to the high-temperature phase (euxenite-type) occurs at approximately 1100°C and appears to be more temperature- than time-dependent. Formula balance requirements and structural evidence lead to the assumption that an appreciable amount of OH - replaces O 2- at the O4 site. The presence of OH - groups is confirmed by Fourier Transform Infrared Spectroscopy. Crystal-chemical features and chemical data yielded the following chemical formula: Y 0.51 REE 0.25 Th 0.15 U 4+ 0.06 Ca 0.04 ) Σ=1.01 (Ti 1.56 Nb 0.33 Ta 0.05 W 0.02 Fe 2+ 0.03 ) Σ=1.99 O 5.52 (OH) 0.48 . Upon heating the sample in air at temperatures higher than 550°C, an oxidation-dehydrogenation reaction takes place, resulting mainly in a lengthening of the donor-acceptor distance and a related shortening of the A-O4 and B-O4 bond distances. The observed variations of the cell parameters appear to be related to the oxidation-dehydrogenation process rather than to changes of the cation site-population.

The dual behavior of the β-As4S4 altered by light

Abstract Among the polymorphs of the compound As4S4, realgar and β-As4S4 exhibit an interesting phenomenon of light-induced alteration that eventually leads to the transformation to pararealgar and arsenolite through the structural modification of the As4S4 molecule. The mechanism generally invoked to explain the transformation assumes reaction with oxygen, subsequent modification of the molecule through an insertion of a sulfur atom and the eventual production of arsenolite according to the reaction 5As4S4 + 3O2 → 4As4S5 + 2As2O3. Early studies showed that the light-induced transition from realgar to pararealgar is reversible through heat and that implies a transition through the χ-phase, even though the presence of arsenolite was not observed. To further assess the action of the oxygen during the process, we carried out experiments of light-induced alteration of β-As4S4 under ambient air and under isopropyl alcohol. The material was investigated by means of X-ray powder diffraction (XRPD) using quantitative phase analysis (QPA) and the Rietveld method. The further study of the heat-induced transformation of the products showed that β-As4S4 exhibits a dual behavior: if the light-induced alteration occurs under air, arsenolite plus an amorphous phase is produced and the transformation is not reversible, if the alteration occurs without any contact to air none of such phases is produced and the transformation is reversible. These new experimental evidences suggest that the production of arsenolite is not strictly required for the transformation of the β-As4S4 into pararealgar and that the current model invoked to explain the mechanism of alteration should be modified to take into account the dual behavior of the β-As4S4 altered by light.

From ancient pigments to modern optoelectronic applications of arsenic sulfides: bonazziite, the natural analogue of β-As4S4 from Khaidarkan deposit, Kyrgyzstan

Abstract Bonazziite is a new mineral from Khaidarkan deposit, Kyrgyzstan and represents the natural analogue of the β-form of the well known As4S4 compound. It occurs as rare crystals up to 100 mm across associated with realgar, sulfur, wakabayashilite, alacra’nite, non-stoichiometric As4S4+x sulfides and stibnite in a calcite matrix. In thick section, bonazziite is opaque with a resinous lustre and a dark-orange streak. It is brittle; the Vickers hardness (VHN15) is 70 kg/mm2 (range: 60-76) (Mohs hardness of ~2½). In planepolarized incident light, bonazziite is strongly bireflectant and pleochroic from orange to light red. The mineral shows orange to red internal reflections. Between crossed polars, the mineral is strongly anisotropic with greyish to light-blue rotation tints. Reflectance percentages in air for Rmin and Rmax are 19.9, 22.2 (471.1 nm), 19.1, 21.3 (548.3 nm), 18.8, 19.7 (586.6 nm) and 17.8, 18.9 (652.3 nm), respectively. Bonazziite is monoclinic, space group C2/c, with a = 9.956(1), b = 9.308(1), c = 8.869(1) Å, b = 102.55(2)° and V = 802.3(2) Å3, Z = 4. The crystal structure [R1 = 0.0263 for 735 reflections with Fo > 4σ(Fo)] is based on the As4S4 cage-like molecule, in which each As atom links one As and two S atoms. The As4S4 molecule is identical to that found in the structure of realgar. The six strongest powder diffraction lines [d in Å (I/I0) (hkl)] are: 5.74 (100) (1̄11); 4.10 (60) (021); 3.92 (50) (1̄12); 3.12 (60) (022, 310); 2.95 (50) (221, 202); 2.86 (80) (2̄22, 1,31). A mean of six electron microprobe analyses gave the formula As3.95S4.05, on the basis of eight atoms. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA No. 2013-141) and named for Paola Bonazzi, in recognition of her seminal contributions to the study of arsenic sulfides and their alteration induced by exposure to light.

An insight into the inverse transformation of realgar altered by light

Abstract The light-induced alteration of realgar and β-As4S4 is a well-known phenomenon but still displays interesting aspects not completely explained. In the transformation from realgar to pararealgar the molecule As4S4 undergoes a structural modification, and ever since the initial studies two important issues have been highlighted: the influence of the oxygen and the reversibility of the process. The previous study on the reversibility of the altered β-As4S4 points out that this polymorph exhibits a dual behavior. When the light-induced alteration occurs with the presence of the air, pararealgar and arsenolite, along with amorphous material, are the products, while if the air is not present β-As4S4 turns completely into pararealgar. Moreover, when annealing the altered material in the realgar stability fields (220 °C), in the first case pararealgar and amorphous material turn into stoichiometric alacranite, while in the second case the alteration is completely reversible. Similarly, the present study focuses the attention on the question if realgar, when altered by means of the light and when annealed, might behave as β-As4S4 does. These results display that the phenomenon is more complex. The alteration of realgar with the presence of the air yields pararealgar along with arsenolite, a small quantity of uzonite and amorphous material, and when the air is not present pararealgar is the only product. In the first case, when annealing the products of alteration at 220 °C, alacranite, β-As4S4, realgar, and little amorphous material occur along with arsenolite. In the second case at first β-As4S4 crystallizes, then it turns into realgar, but this process yields orpiment and amorphous material as it moves forward, showing that from the products of alteration of realgar it is not possible to obtain the starting material. Examination of the stoichiometry of the products let to infer that the amorphous material occurring in the two cases has very different content of arsenic and sulfur.

Chemical variability in wakabayashilite: a real feature or an analytical artifact?

Abstract Wakabayashilite is a rare mineral with ideal formula [(As,Sb)6S9][As4S5]. Its structure consists of an [M6S9] bundle-like unit (M = As, Sb) running along the [001] axis and [As4S5] cage-like molecules. In this study, samples of wakabayashilite from different occurrences (Khaidarkan, Kyrgyzstan; Jas Roux, France; White Caps mine, USA; Nishinomaki mine, Japan) were selected to verify the possible presence of different molecular groups replacing the As4S5 molecule. Given the chemical (electron probe microanalysis-wavelength dispersive spectroscopy), spectroscopic (micro-Raman) and structural (single-crystal X-ray diffraction) results obtained, it appears evident that only the As4S5 molecular group is present in the wakabayashilite structure and that the apparent non-stoichiometry reported in literature is actually due to unreliable chemical analyses. The structural role of the minor elements (Cu, Zn and Tl) in wakabayashilite is also discussed.

Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data.

A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO4·2H2O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octahedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO4·2H2O), with which it forms a solid solution. The framework structure is based on AsO4 tetrahedra sharing their vertices with AlO4(H2O)2 octahedra. Three of the four H atoms belonging to the two water molecules in cis positions take part in O—H⋯O hydrogen bonding.

The proof is in the... growing? A peculiar case of kidney stones

A 45-year-old woman affected by MEN-1 syndrome with a history of bilateral nephrolithiasis due to parathyroid adenoma (primary hyperparathyroidism) and celiac disease presented to the nephrologist for recurrent episodes of renal colic and urinary passage of small stones with an unusual morphology despite previous surgical removal of the parathyroid adenoma. A complete diagnostic workup ought to be able to establish the type of stones, but contemporary medicine, with its reliance on protocols, procedures and scientific evidence, may have lost sight of the importance of good communication with the patient.