Search

In this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mössbauer spectroscopy and powder X-ray diffraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400–450 °C), then it undergoes a rapid (within ~50 °C) dehydrogenation, forming oxo-riebeckite, which is stable up to ~900 °C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (Ea) for the riebeckite to oxo-riebeckite transition is 166 ± 6 kJ/mol. Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750–800 °C); however, in the 550 < T < 700 °C range, it undergoes a significant re-arrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700–800 °C range has the same chemical formula as riebeckite but has a disordered and nonstandard cation distribution at the octahedra, i.e., M(1)(Fe3+Fe2+)M(2)(Fe3+Fe2+)M(3)Fe2+; we call this phase “CR3+ disordered riebeckite”. For T ≥ 800 °C, it decomposes to aegirine + fayalite + cristobalite + H2O. External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures.

Geothermal systems in terrains affected by polyphase deformation have reservoirs with a 3D geometry that is always difficult to predict. In this paper we describe a fossil exhumed geothermal system exposed in eastern Elba Island that developed in a polyphase folded and faulted setting, which can help us to understand how geothermal fluids circulate in geological bodies with inherited structures. Geothermal circulation at Elba allowed the deposition of Fe-ore deposits (haematite/magnetite and pyrite) and altered rock volumes, which represent tracers of the palaeo-fluid flow. Normal and oblique-slip faults dissected a polyphase folded metasiliciclastic succession and produced a secondary permeability in the range of 5 × 10−13 to 5 × 10−16 m2. From the permeable fault zones acting as feeder conduits, geothermal fluids permeated the hydraulically connected metasiliciclastic rock bodies previously deformed by two generations of folds. Geothermal fluids followed the already defined geometry, thus giving rise to apparent folded mineralized levels. Fluid migration into the metasiliciclastic rocks was possible due to their chemical aggression, which favoured the dissolution and reprecipitation of quartz, and Fe-oxide and sulphide deposition. Renewed fluids maintained their chemical properties (pH value and temperature, mostly). This conclusion provides inputs for reconstructing geothermal conceptual models and evaluating the geothermal potentiality of exploitable areas developing in similar geological settings.

In the Lesina Marina village area, the cropping out gypsum rocks, that rose up from the deep and thick Upper Trias Burano Fm anhydrite layer, have been analyzed to investigate the trigger mechanism for their ascent. In this regard, we focus on the anhydrite-to-gypsum transformation starting from the deep source layer. Indeed, petrographic observations of the widespread corroded anhydrite crystals embedded in these gypsum rocks revealed unequivocal evidences of the anhydrite-to-gypsum transformation. In addition, and referring to gypsum, microstructural features indicate that ductile deformation mechanisms initially operated under higher temperature conditions. This temperature should be close to 107 °C, namely the value of the upper temperature limit of stability of gypsum from Lesina Marina, above which it starts to transform and dehydrate, as revealed by microthermometry heating/dehydration experiments combined with micro-Raman analyses. All these evidences are in favor of the dramatic density decrease and volume increase due to anhydrite-to-gypsum transformation by hydration in the deep source layer; these variations of physical conditions, triggered by hydration, promoted diapirism of the gypsum mass, in other words “hydration diapirism”. As revealed by seismic lines interpretation, the diapirism, which gave rise to the Lesina diapir, occurred during Plio-Pleistocene and was genetically related to the Lesina graben-type structure. Hydration at depth was favored by the downward circulation of water-rich fluids channeled in faults, and the gypsum mass used the weakened zone of the southern fault to pierce the overlying Mesozoic and Tertiary sediments.

The thermal behavior of melanterite from the Fornovolasco mine (Tuscany, Italy) has been investigated via differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature X-ray powder diffraction (XRPD) and Fourier-transform infrared spectroscopy (FTIR). The DTA curve showed endothermic peaks at 70, 100, 260, 500–560 and 660 °C whereas the TG curve evidenced a total mass decrease of ~68%, in keeping with the loss of all H2O and SO4 groups. Rietveld refinements were performed for all the collected patterns in the 25–775 °C range and converged at 1.57 ≤ R (%) ≤ 2.75 and 1.98 ≤ Rwp (%) ≤ 3.74. The decomposition steps FeSO4·7H2O → FeSO4·4H2O (25 ≤ T ≤ 50 °C) → FeSO4·H2O (50 &lt, T ≤ 100 °C) → FeOHSO4 (75 &lt, T ≤ 200 °C) → Fe2(SO4)3 (400 &lt, T ≤ 500 °C) → Fe2O3 (500 &lt, T ≤ 775 °C) were obtained. The high-temperature infrared analysis confirmed that melanterite undergoes a three-step dehydration in the 25–300 °C temperature range. The FeOHSO4 phase is stable over a wide range of temperature and transforms partially to Fe2(SO4)3 without the formation of Fe2O(SO4)2. The findings highlight a different behavior of the studied sample with respect to the synthetic salt.

A multi-methodical characterization of a sauconite (Zn-bearing trioctahedral smectite) specimen from the Skorpion ore deposit (Namibia) was performed by combining X‑ray powder diffraction (XRPD), cation exchange capacity (CEC) analysis, differential thermal analysis (DTA), thermo‑ gravimetry (TG), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM‑HRTEM‑AEM). The X‑ray diffraction pattern exhibits the typical features of turbostratic stacking disorder with symmetrical basal 00l reflections and long‑tailed hk bands, as confirmed by TEM observations. Besides sauconite, the sample contains minor amounts of kaolinite, dioctahedral smectite, and quartz. CEC analysis provides a total of Ca (~69%), Mg (~26%), Na (~4%), and K (0.7%) exchangeable cations. Therefore, Zn is located exclusively within the octahedral site of sauconite. TG analysis of the sample yields a total mass loss of about 17%. Three endothermic peaks can be observed in the DTA curve, associated with dehydration and dehydroxylation of the material. An exothermic peak at 820 °C is also present as a consequence of decomposition and recrystallization. The infrared spectrum shows the typical ZnOH stretching signature at 3648 cm, whereas, in the OH/HO stretching region two bands at 3585 and 3440 cm can be attributed to stretching vibrations of the inner hydration sphere of the interlayer cations and to absorbed HO stretching vibration, respectively. Diagnostic bands of kaolinite impurity at ~3698 and 3620 cm are also found, whereas 2:1 dioctahedral layer silicates may contribute to the 3585 and 3620 cm bands. Finally, using the one-layer supercell approach implemented in the BGMN software, a satisfactory XRPD profile fitting model for the Skorpion sauconite was obtained. These findings have implications not only for economic geology/recovery of critical metals but also, more generally, in the field of environmental sciences., The XRPD laboratory at the Dipartimento di Scienze della Terra and Geoambientali, University of Bari “Aldo Moro”, was funded by Potenziamento Strutturale PONa3_00369 “Laboratorio per lo Sviluppo Integrato delle Scienze e delle TEcnologie dei Materiali Avanzati e per dispositivi innovativi (SISTEMA)”. The XRPD facility at the DiSTAR, University “Federico II” Napoli, is acknowledged. The authors acknowledge Annett Steudel and Katja Emmerich for CEC measurement at the Competence Center for Material Moisture, University of Karlshrue, Germany. GB and NM thank Maria Boni, invaluable scientific guide who promoted the study of the economic geology of worldwide nonsulfide ore deposits, and G. Arfè who firstly characterized the Skorpion samples and for continuous help and fruitful discussions. This work was partly supported by DiSTAR fund 2017 (University of Naples Federico II, Italy) granted to G. Balassone, as well as by the research projects PGC2018-094573-B-100 from the Spanish Government and the Research Group RNM-179 of the Junta de Andalucía granted to F. Nieto. Two anonymous referees are thanked for insightful comments that helped to improve the quality of the manuscript.

We report in this paper a new crystal-chemical study of synthetic basic ferric sulfate FeOHSO4. The structure solution performed by the Endeavour program, from new X-ray powder diffraction (XRPD) data, indicated that the correct space group of the monoclinic polytype of FeOHSO4 is C2/c. Selected Area Electron Diffraction (SAED) patterns are also consistent with this structure solution. The arrangement of Fe and S atoms, based on linear chains of Fe3+ octahedra cross-linked by SO4 tetrahedra, corresponds to that of the order/disorder (OD) family. The positions of the hydrogen atoms were located based on DFT calculations. IR and Raman spectra are presented and discussed according to this new structure model. The decomposition of FeOHSO4 during heating was further investigated by means of variable temperature XRPD, thermogravimetry, and differential thermal analysis as well as IR and Raman spectroscopies.

This study is focused on a barian titanian phlogopite found in an alkaline ultramafic dyke transecting Mesozoic limestones of the Gargano Promontory (Apulia, Italy). The rock containing the barian titanian phlogopite, an olivine-clinopyroxene-rich lamprophyre with nepheline and free of feldspars, has been classified as monchiquite. The present study combines chemical analyses, single crystal X-ray diffraction and Raman spectroscopy. Chemical variations suggest that the entry of Ba into the phlogopite structure can be explained by the exchange Ba + Al = K + Si. The crystal structure refinement indicates that the Ti uptake is consistent with the Ti–oxy exchange mechanism. The structural parameters associated with the oxy substitution mechanism are extremely enhanced and rarely reported in natural phlogopite: (a) displacement of M2 cation toward the O4 site (~0.7), (b) M2 octahedron bond-length distortion (~2.5), (c) very short c cell parameter (~10.14 Å). Raman analysis showed most prominent features in the 800–200 cm−1 region with the strongest peaks occurring at 773 and 735 cm−1. Only a weak, broad band was observed to occur in the OH-stretching region. As concerns the origin of the barian titanian phlogopite, the rock textural features clearly indicate that it crystallized from pockets of the interstitial melt. Here, Ba and Ti enrichment took place after major crystallization of olivine under fast-cooling conditions, close to the dyke margin.

The investigation of the deep geothermal systems is a challenging task in active geothermal systems. In order to decrease the mining risk, the study of the analogue exhumed systems sheds light on the relationships between fluid circulation and geological structures through the analyses of faults and ore deposits distributions. In the Las Minas area (Central Mexico), ore deposits are quite diffuse at the boundary between crystalline and sedimentary rocks and in fault zones. This is a consequence of the interaction between cooling of Miocene felsic magmas, hydrothermal fluids and coeval fault activity. We investigated the role of the faults in channeling the hydrothermal fluids by fieldwork and analysis of fractures at outcrops. The field mapping was carried out at 1:10000 scale (60 km2). When possible, kinematic data on recent fault planes influencing the permeability and geothermal fluid paths were collected. This includes information on the main structural trends and the orientation of the intermediate kinematic axis.The evolution and origin of the hydrothermal fluids circulating in the exhumed geothermal system of Las Minas area (Central Mexico) were investigated by i) structural and minero-petrographic studies and, ii) fluid inclusion and isotope analyses carried out on skarn and hydrothermal alteration minerals.Two families of faults have been recognized, NNW-SSE and SW-NE oriented, respectively. The SW-NE trending faults often controlled the emplacement of dykes, indicating that the magmatic fluid was channeled and driven by the faults induced permeability. Their activity is at least encompassed between Miocene and Quaternary. The kinematic relation between these two fault systems could be explained in a extensional framework, assuming that the NNW-SSE fault system acted as transfer faults. Fluid inclusions recorded the circulation of: 1) high-temperature (up to 650°C), high-salinity (up to 60 wt.% NaCl equiv.) fluid of magmatic origin; 2) high-temperature (470-650°C) aqueous-carbonic fluid produced during fluid-rock interaction with carbonate basement rocks and 3) relatively low-salinity (up to 2 wt.% NaCl equiv.) fluid of meteoric origin. A general evolution from high- to low-temperature fluid circulation characterized the geothermal system.

The crystal structure of ferrinatrite, Na3[Fe(SO4)3]·3H2O, was refined based on a new single-crystal X-ray diffraction experiment on a sample from the type locality Sierra Gorda, Chile. The data allowed H to be successfully located and the H-bonding system to be defined. Infrared and Raman spectra are presented and discussed for this compound on the basis of the crystal structure. The Oacceptor···H–Odonor bond distances determined from the structure refinement agree well with the geometric correlation obtained from spectroscopic data. The thermal stability and dehydration process of ferrinatrite was investigated by in situ high temperature (HT) synchrotron X-ray powder diffraction, Raman and Fourier transform infrared spectroscopies.

The garnierite ore at Campello Monti occurs as dark green colloform concretions covering surfaces, fractures, and filling veins in harzburgite rocks. The representative composition (Ni2.45Mg0.14Cu0.12Co0.05)Σ2.76Si2.10O5(OH)4 is consistent with a 7 Å phase, namely pecoraite or népouite. Relevant chemical features are an exceptionally high Ni/Mg ratio, a significant level of Cu substituting for Ni, and a low content of S, possibly in tetrahedral sites. Olivine and orthopyroxene in the harzburgite host rock are only partially serpentinized, do not contain detectable Ni, and are almost iron free. The green coating probably originated from groundwater solutions that leached nearby weathered peridotites and sulfide ores, and deposited less-mobile elements along fractures and voids of the host peridotite, just outside their provenance area. Bulk techniques such as X-ray powder diffraction and infrared spectroscopy do not confidently distinguish between népouite and pecoraite, although the comparison with synthetic, implicitly pure polymorphs indicates népouite as the best matching phase. On the other hand, HRTEM clearly shows that garnierite is mostly constituted by plumose aggregates made of curved crystals with frayed tips, a few nanometers thick along the stacks and a few tens of nanometers long (nanoflakes). All known lizardite stacking sequences, namely 1T, 2H1, and 2H2, have been locally observed, even though most crystals show stacking disorder. The recorded nanostructure suggests possible explanations for the recurrent anomalies (low oxide totals, high IVT/VIM cation ratios, etc.) found in EMP analyses of garnierites. The small grain size, the high density of defects, and the structural arrangement actually intermediate between lizardite and chrysotile probably explain the ambiguities that occurred during the characterization with bulk techniques. The results obtained in this study may have important implications for ore processing methods

The present study focuses on the assessment of the effects of different activation methods on carbonate-rich clays, to understand the mineralogical differences originated and to exploit such information to industry for traditional and innovative applications, especially as a precursor for alkali activated binders. Illite carbonate-rich clay samples were subjected to thermal activation in ox/red atmosphere between 400 and 900 °C, mechanical activation (grinding for 5, 10 and 15 min) and to a combination of such treatments. Mineralogical and textural changes in the activated samples were evaluated through X-ray powder diffraction, Fourier transform infrared spectroscopy and thermal techniques. The activated samples with the highest content of amorphous phase underwent leaching tests in a 3 M NaOH solution by means of inductively coupled plasma-mass spectrometry. The application of the three processing routines, yielded three types of activated clays with different leaching modes of Si, Al, K and Ca: (1) high energy grinding preferentially delaminates clay minerals and reduces the grain size of calcite. K leaching reaches the highest values; (2) thermal heating at 800 °C increases relatively the Si/Al solubility ratio, but the absolute concentrations of these elements are equal or lower than those obtained from ground clays. The relatively higher leaching of Ca is influenced by the formation of non-stoichiometric and poorly crystalline Ca-silicates and -aluminosilicates; (3) high energy grinding combined with heating treatment yields an extended amorphisation, mainly at the expense of clay minerals, with the highest leaching of Si and Al, and the lowest of Ca. New formed K-feldspars inhibit the concentration of K in alkaline solution.

Fluorine-, boron- and magnesium-rich metamorphosed xenoliths occur in the Campanian Ignimbrite deposits at Fiano (southern Italy), at ∼50 km northeast of the sourced volcanic area. These rocks originated from Mesozoic limestones of the Campanian Apennines, embedded in a fluid flow. The Fiano xenoliths studied consist of ten fluorophlogopite-bearing calc-silicate rocks and five carbonate xenoliths, characterized by combining mineralogical analyses with whole-rock and stable isotope data. The micaceous xenoliths are composed of abundant idiomorphic fluorophlogopite, widespread fluorite, F-rich chondrodite, fluoborite, diopside, Fe(Mg)-oxides, calcite, humite, K-bearing fluoro-richterite and grossular. Of the five mica-free xenoliths, two are calcite marbles, containing subordinate fluorite and hematite, and three are weakly metamorphosed carbonates, composed only of calcite. The crystal structure and composition of fluorophlogopite approach that of the end-member. The Fiano xenoliths are enriched in trace elements with respect to the primary limestones. Comparisons between the rare-earth element (REE) patterns of the Fiano xenoliths and those of both Campanian Ignimbrite and Somma-Vesuvius marble and carbonate xenoliths showthat the Fiano pattern overlaps that of Somma-Vesuvius marble and carbonate xenoliths, and reproduces the trend of Campanian Ignimbrite rocks. Values of δ13C and δ18O depict the same trend of depletion in the heavy isotopes observed in the Somma-Vesuvius nodules, and is related to thermometamorphism. Trace-element distribution, paragenesis, stable isotope geochemistry and data modelling point to infiltration of steam enriched in F, B,Mg and As into carbonate rocks at a temperature of ∼300–450°C during the emplacement of the Campanian Ignimbrite.

Magnetite nanoparticles (NPs) were synthetized by partial oxidation of hot Fe(II) sulfate aqueous solutions upon addition of variable amounts of alkali/nitrate ions in a N2 atmosphere. The resulting NPs were extensively characterized by Scanning Electron Microscopy (SEM), X-Ray Powder Diffraction (XRPD), Dynamic Light Scattering (DLS) and Brunauer-Emmett-Teller (BET) analysis. Depending on the final pH after synthesis, partially oxidized magnetite NPs were obtained with average crystallite size ranging from 29 to 116 nm. The potential of NPs as arsenic adsorbents was investigated through batch adsorption experiments. The analysis of the adsorption isotherms performed on the basis of both Freundlich and Langmuir models, revealed that arsenic adsorption capacity was more efficient for magnetite NPs synthetized at final pH ≤ 6.56, characterized by polydisperse submicrometric aggregates with coarse macro- and mesopore distributions.

A multi-proxy record based on calcareous plankton, dinocysts, pollen, mineralogy and grain size has been acquired from the “Ideale” section, the portion of Montalbano Jonico succession (Southern Italy) that could host the GSSP for the Middle Pleistocene. The direct correlation between marine and terrestrial proxies provides a valuable indication of atmospheric and oceanic connections. The record shows a distinct climate variability well expressed by the vegetational changes, sediment input into the basin and sea surface–water modifications. Biotic proxies indicate that, during the study interval, periods of prevailing subpolar/transitional surface–water conditions were concurrent with steppe and halophytic vegetation. Erosional processes on the hillslopes and terrigenous supply, during these periods, were likely favored due to reduced arboreal vegetation, as attested to by sediment composition and pollen assemblages. Temperate/subtropical surface water conditions were concomitant with prevalent expansions of thermophilous arboreal plants and moister conditions on land, which likely promoted chemical weathering leading to an enhanced supply of kaolinite and smectite in the basin. A relationship between mineral content and the nature of the outcropping units in the source area, as well as sea-level fluctuations, has also been considered. A well-defined period of polar–subpolar water incursion, ranging from 793.2 ka to 788.1 ka, matches the major expansion of steppic vegetation on land and the increase of terrigenous supply into the basin. It likely represents the local response to North Atlantic ice-rafted debris deposition. Short-lived warming/cooling episodes punctuate the latest part of MIS 20 and are tentatively correlated with analogue climate phases predating MIS 1 inception. Full interglacial conditions of MIS 19c, lasting 10.6 kyr, started at 784.3 ka and were characterized by warm, oligotrophic and stratified surface–waters coupled with forest expansion, suggesting prevalent warm and humid conditions on land. A potential sapropel-like interval is identified within MIS 19c and correlated to i-cycle 74. The MIS 19c/19b transition is marked, at 773.2 ka, by the reestablishment of millennial-scale variability, similarly to North Atlantic marine and ice-core records. The results improve the paleoclimate framework of a Middle Pleistocene reference section and add further insight regarding climate development across MIS 19 and on potential similarities with its closest Holocene analogue.

The crystal structure of fibroferrite, Fe(OH)SO 4 ·5H 2 O, was studied by means of single-crystal X-ray diffraction and vibrational (FTIR and Raman) spectroscopies. The new diffraction data allowed to successfully locate eleven H positions and to completely define the H bond system that ensures the cohesion of the Fe-O-S chains in the fibroferrite structure. Infrared and Raman spectra are presented for the first time for this compound and commented on the basis of the crystal structure and literature data for sulfate minerals. Both FTIR and Raman spectra show, in the fundamental water stretching region, a very broad absorption extending from 3600 to 2600 cm −1 ; peaks at 3522, 3411 and 3140 cm −1 can be resolved in the Raman pattern. The bands present in the low-wavenumber ( −1 ) region are assigned on the basis of the literature data for similar substances, and the observed multiplicity is in agreement with a symmetry reduction of the sulfate ion in the structure of fibroferrite.

The crystal structure of hohmannite, Fe3+2[O(SO4)2]·8H2O, was studied by means of single-crystal X-ray diffraction (XRD) and vibrational spectroscopy. The previous structural model was confirmed, though new diffraction data allowed the hydrogen-bond system to be described in greater and more accurate detail. Ab initio calculations were performed in order to determine accurate H positions and to support the experimental model obtained from XRD data. Infrared and Raman spectra are presented for the first time for this compound and comments are made on the basis of the crystal structure and the known literature for sulfate minerals.

The crystal structure of calcium tartrate esahydrate, CaC4H4O6·6H2O, has been solved by the charge-flipping method from single-crystal X-ray diffraction data and refined toR= 0.021, based on 1700 unique observed diffractions. Salient crystallographic data are:a= 7.7390 (1),b= 12.8030 (2),c= 5.8290 (1) Å,Z= 2, and space groupP21212. During the refinement step it was possible to locate all H atoms by difference Fourier synthesis. The tartrate molecule has a (−)-gaucheconformation and is coordinated to two calcium ions to form infinite chains along theaaxis which alternate Ca polyhedra with tartrate molecules. The chains are interlinked by a three-dimensional network of hydrogen bonds from four water molecules surrounding the Ca ion, reinforced by hydrogen bonds from one interstitial water molecule. Micro-Raman and FT–IR spectroscopic data are provided.

The thermal dehydration of hohmannite, Fe2[O(SO4)2]·8H2O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature X-ray powder diffraction and in situ high-temperature Fourier transform infrared spectroscopy. Combination of the data from both techniques allowed determining the stability fields and reaction paths for this mineral and its high temperature products. Five main dehydration/transformation steps for hohmannite have been identified in the heating range of 25–800 °C. Temperature behavior of the different phases was analyzed, and the heating-induced structural changes are discussed.

The thermal dehydration process of fibroferrite, FeOH(SO4)·5H2O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO4)·4H2O, appearing at about 80 °C due to release of one interstitial H2O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.

The thermal dehydration process of fibroferrite, FeOH(SO4)·5H2O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO4)·4H2O, appearing at about 80 °C due to release of one interstitial H2O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.

A crystal-chemical investigation of vauxite, ideally FeAl 2 (PO 4 ) 2 (OH) 2 ·6H 2 O, from Llallagua (Bolivia) has been performed using a multi-methodological approach based on WDS-electron microprobe, single-crystal X-ray diffraction, and vibrational spectroscopies (Raman and FTIR). The structure was refined in the triclinic P space group, with the following unit-cell constants: a 9.1276(2), b 11.5836(3), c 6.15960(10) A, α 98.3152(10)°, β 92.0139(10)°, γ 108.1695(9)°, and V 610.05(2) A 3 . The vauxite structure is based on a building unit oriented parallel to the c axis and composed of a chain of Fe2 and Al2 edge-sharing octahedra and two chains of corner-sharing P2 tetrahedra and Al1 octahedra, interconnected via corners and P1 tetrahedra. Neighboring building units are interconnected by Al3 octahedra and via Fe1 octahedra. The framework is completed with two non-coordinated water molecules. The latter, together with the two hydroxyl groups and the other four coordinated water molecules, form a complex hydrogen bonding network whose interactions further compact the whole framework. Both FTIR and Raman spectra show, in the H 2 O stretching region, a broad absorption consisting of several overlapping components due to the six water molecules plus the OH groups. The band multiplicity observed in the low-wavenumber region ( −1 ) is compatible with the presence of two distorted PO 4 tetrahedra.

A suite of Ti-bearing garnets from magmatic, carbonatitic, and metamorphic rocks was studied by electron probe microanalysis (EPMA), X-ray powder diffraction (XRPD), single-crystal X-ray diffraction (SCXRD), Mossbauer spectroscopy, and secondary ion mass spectrometry (SIMS) to better characterize their crystal chemistry. The studied garnets show TiO 2 varying in the range of 4.9(1) to 17.1(2) wt% and variable Fe 3+ /∑Fe content. SIMS analyses allowed quantification of light elements yielding H 2 O in the range 0.091(7)–0.46(4), F in the range 0.004(1)–0.040(4), and Li 2 O in the range 0.0038(2)–0.014(2) wt%. Mossbauer analysis provided spectra with different complexity, which could be fitted to several components variable from one ( Y Fe 3+ ) to four ( Y Fe 2+ , Z Fe 2+ , Y Fe 3+ , Z Fe 3+ ). A good correlation was found between the Fe 3+ /∑Fe resulting from the Mossbauer analysis and that derived from the Flank method. X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit-cell parameters as recently found by other authors. Single-crystal X-ray refinement using anisotropic displacement parameters were performed in the Ia 3 d space group and converged to 1.65 ≤ R 1 ≤ 2.09% and 2.35 ≤ wR 2 ≤ 3.02%. Unit-cell parameters vary in the range 12.0641(1) ≤ a ≤ 12.1447(1) A, reflecting different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are: Y R 4+ + Z R 3+ ↔ Z Si + Y R 3+ (schorlomite substitution); Y R 2+ + Z R 4+ ↔ 2 Y R 3+ (morimotoite substitution); Y R 3+ ↔ Y Fe 3+ (andradite substitution); in the above substitutions Y R 2+ = Fe 2+ , Mg 2+ , Mn 2+ ; Z R 4+ = Ti; Y R 3+ = Fe 3+ , Al 3+ , Cr 3+ ; Z R 3+ = Fe 3+ , Al 3+ . Minor substitutions, such as 2 Y Ti 4+ + Z Fe 2+ ↔ 2 Y Fe 3+ + Z Si, (SiO 4 ) 4− ↔ (O 4 H 4 ) 4− , F − ↔ OH − , and Y R 4+ + X R + ↔ Y R 3+ + X Ca 2+ , with Y R 4+ = Ti, Zr; Y R 3+ = Fe 3+ , Al, Cr 3+ ; X R + = Na, Li also occur.

We deal with a single-crystal X-ray diffraction study of boulangerite from a plumose sample from the Bottino mine, Apuan Alps, Italy. Chemical composition of the analyzed sample is Pb 4.89 (Sb 4.08 As 0.04 Bi 0.01 ) ∑4.13 (S 10.98 Se 0.02 ) ∑11.00 . The crystal structure was investigated and evaluated in terms of the order–disorder (OD) theory. Boulangerite belongs to the subcategory Ia of OD structures composed of equivalent layers with symmetry P ( n )2 1 m . Two polytypes with maximum degree of order (MDO) are possible: MDO1 with space group P 12 1 / a 1 and unit-cell parameters a 1 ≈ 21.61, b 1 ≈ 23.54, c 1 ≈ 8.05 A, β 1 ≈ 100.7°; MDO2 with space-group P 2 1 / n 11 and unit-cell parameters a 2 ≈ 21.24, b 2 ≈ 23.54, c 2 ≈ 8.05 A, α 2 ≈ 90°. Single-crystal X-ray diffraction patterns (Mo K α, CCD detector) show strong reflections pointing to an orthorhombic substructure (the “family structure” in the OD terminology) and additional weaker reflections that correspond to the polytype MDO 1 . Frequent twinning with (100) as the twin plane was observed. The MDO1 structure was refined on a twinned crystal to R = 0.062 for 8495 reflections with F o > 4σ( F o ). Unit-cell parameters are a 21.554(4), b 23.454(4), c 8.079(2) A, β 100.76(1)°. The structure of boulangerite is composed of rods of SnS archetype six atomic layers thick and three pyramids wide. The central portion of these thick rods is characterized by ribbons of coordination pyramids of Pb and Sb atoms alternating along [001]; the marginal portions contain ribbons of coordination pyramids primarily occupied by Sb. In the refined structure, there are 18 independent cation sites: 10 are pure lead sites, six are pure antimony sites, two mixed positions split into two close sites occupied by Pb and Sb, respectively. On the basis of an OD interpretation, we assess the relationships between monoclinic and orthorhombic structures reported in the literature for boulangerite.

The crystal chemistry of Ti-rich phlogopite from deposits of the oldest activity (∼740 ka) of Mt. Vulture, Potenza, Italy, was investigated to identify the substitution mechanisms in Ti-rich phlogopite and to determine its potential as a geohygrometer. Substitution mechanisms were determined by electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), Mossbauer spectroscopy, and micro-Fourier transform infrared (FTIR) spectroscopy. Magnetite and sanidine from the mica-rich host rocks were also analyzed. Use of the biotite-magnetite-sanidine geohygrometer in these volcanics (essentially trachytic-phonolitic ignimbrites) is exploited.

The iron sulfate Na 3 Fe(SO 4 ) 3 studied here has been obtained as a high-temperature (HT) product (~400 °C) from the thermal decomposition of sideronatrite from Sierra Gorda (Chile) having composition Na 2 Fe(SO 4 ) 2 (OH)·3H 2 O. The structure determination was carried out using synchrotron X‐ray powder diffraction. Structural data refined by the Rietveld method, up to R p = 11.95%, are: space group R 3, lattice parameters a = b = 13.6231(1) A and c = 9.0698(1) A, V = 1457.76(2) A 3 , and Z = 6. The structure of Na 3 Fe(SO 4 ) 3 can be described in terms of FeO 6 octahedra connected to sulfate tetrahedra by corner-sharing to form infinite chains [Fe(SO 4 ) 3 ] ∞ , running along c . These chains are joined together by Na atoms to build up a three-dimensional network of strong (Fe-O-S) and weak (Na-O) bonds. The topological relationships of Na 3 Fe(SO 4 ) 3 to the structure of some analog minerals are also discussed.

ab S tract The kinetics of the Fe-oxidation/deprotonation process in a natural Fe-rich phlogopite from Mt. Vulture (Potenza, Italy) was studied under isothermal conditions by in situ high-temperature singlecrystal X-ray diffraction. Isothermal annealing experiments were performed at five temperatures in the range 640–750 °C on five crystals with similar chemical composition and lattice parameters. The Fe-oxidation/deprotonation process at high temperature occurs with a reduction of unit-cell parameters and cell volume. The changes in unit-cell parameters measured at high temperature and during cooling show that the same degree of Fe-oxidation/deprotonation process was achieved at all temperatures. Changes in unit-cell parameters with temperature and time show that the kinetics of Fe-oxidation/ deprotonation in phlogopite follows an exponential law, and the temperature dependence follows the Arrhenius relation. A kinetic analysis was performed and good agreement was obtained with the onedimensional diffusion model. An apparent activation energy of 195(4) kJ/mol was determined.

Metasideronatrite was obtained as the first dehydration product of sideronatrite, Na 2 Fe(SO 4 ) 2 (OH)·3H 2 O, from Sierra Gorda, Chile. The crystal structure of metasideronatrite was solved by direct methods and refined by full-matrix least-squares to R = 0.039, using 574 independent reflections with I > 3.0σ(I). It is orthorhombic, space group Pbnm, with a = 7.3959(8), b = 16.0979(15), c = 7.1607(8) A, V= 852.5(2) A 3 , Z= 4. The crystal-chemical formula derived from this structural study is Na 2 Fe(SO 4 ) 2 (OH)·H 2 O. The backbone of the structure is the same as that in sideronatrite: infinite [Fe 3+ (SO 4 ) 2 (OH)] 2- chains of interconnected octahedra and tetrahedra parallel to the c axis. These chains are linked primarily by Na atoms to build a 3-dimensional network of strong (Fe-O-S) and weak (Na-O) bonds. Another prominent feature of the structure is the arrangement of distorted (NaO 5 H 2 O) octahedra, which alternately share one edge and one face to form columns parallel to the [Fe 3+ (SO 4 ) 2 (OH)] chains. Subsidiary intra-chain bonds are provided by H atoms belonging to OH - groups shared by adjacent Fe octahedra, and to the unique water molecule shared between two adjacent (NaO 5 H 2 O) octahedra. At normal conditions of relative humidity (RH) and temperature (i.e., RH > 60% and T < 40 °C), metasideronatrite rehydrates rapidly to sideronatrite. The structure solution has allowed us to: (1) investigate the strong relation between sideronatrite and metasideronitrite; (2) elucidate the mechanism involved in the transformation of metasideronatrite into the order/disorder (OD) structure of sideronatrite; and (3) get insight into the stability of this mineral from the valence-matching principle applied to the main structural unit [Fe 3+ (SO 4 ) 2 (OH)] 2 and Na + interstitial species. The weak hydrogen bonds and the particular arrangement of the face-sharing adjacent [NaO 5 (H 2 O)] octahedra are the main factors affecting the stability of metasideronatrite.

The structural evolution of a trioctahedral mica from Cava St. Antonio, Mt. Vulture, Potenza, Italy, has been studied in the temperature range 100–1023 K using in situ single-crystal X-ray diffraction techniques. The sample used shows a Ti-rich composition close to the phlogopite-annite join with the following crystal-chemical formula: (K0.85Na0.11Ba0.03)(Al0.17Mg1.98Mn0.01Fe0.292+Fe0.373+Ti0.18)(Si2.75Al1.25) O10.66(F0.13OH1.20Cl0.01). In the present study, the chemical and structural changes and the deprotonation process involved during heating have been characterized. Analysis of the data showed that a , b , and c cell parameters expand almost linearly up to 823 K, while the β angle remains constant. A sharp decrease in the unit-cell dimensions was observed at 873 and 1023 K. Linear trends obtained during reversal experiments from 1023 K to room temperature demonstrated the irreversibility of these events. Structure refinements of single-crystal XRD data collected at 100, 200, 298, 473, 673, 873, and 1023 K converged to 2.14 ≤ R (%)≤ 8.47, 2.47 ≤ R w (%) ≤ 10.83. In the temperature range 100 673 K, the thermal expansion along the c direction is mainly due to interlayer thickness dilation. The tetrahedral ring approaches the ideal hexagonal shape with increasing temperature to match the expanding octahedral sheet. In the range 873–1023 K, a strong shrinking of the interlayer is associated with the shortening of the M1-O4 and M2-O4 distances and to the consequent reduction of octahedral thickness. Such structural features indicate the occurrence of Fe oxidation process, involving loss of structural H, which is responsible for a phase transition. Mossbauer spectroscopy supported this hypothesis.

Fluorophlogopite, a new F-dominant mineral of the mica group, was found at Monte Calvario, Biancavilla, lower southwestern flanks of Mt. Etna volcano (Catania, Sicily, Italy). The mineral occurs in autoclasts of gray-red altered benmoreitic lavas, primarily associated with fluoro-edenite, alkali-feldspars, clino- and ortho-pyroxenes, fluorapatite, hematite, and pseudobrookite. It was formed by metasomatism of the original lava rocks from very hot fluid enriched in F, Cl, and other incompatible elements. Fluorophlogopite occurs as very thin laminae with a diameter of 200 to 400 pm. Main physical properties are pale yellow in color; yellowish-white in thin section; vitreous to resinous luster; transparent; non-fluorescent; Mohs' hardness 2-3; brittle and malleable; perfect cleavage on {001}; biaxial (-), alpha(calc) = 1.5430(8), beta = 1.5682(5), gamma = 1.5688(5) (lambda = 589 nm); 2V(meas) 17(2)degrees; (alpha = acute bisectrix perpendicular to (001); nonpleochroic; D-calc = 2.830 g/cm(3) (using empirical formula and single-crystal unit-cell parameters), D-calc - 2.842 g/cm(3) (using empirical formula and powder cell constants). Infrared spectrum did not show a significant absorption band in the OH-stretching region (3800-3600 cm(-1)) confirming that the F content of the fluorophlogopite from Biancavilla is close to the stoichiometric value. Unit-cell parameters from X-ray powder-diffraction data (114.6 rum diameter Gandolfi camera, CuK alpha) are a = 5.305(2), b = 9.189(3), c - 10.137(4) angstrom, beta = 100.02(3)degrees. These data agree with those obtained by single-crystal X-ray studies on a very thin (similar to 15 pm) fluorophlogopite crystal, i.e., Monoclinic (1M polytype); Space Group C2/m; a = 5.3094(4), b = 9.1933(7), c = 10. 1437(8) angstrom, beta = 100.062(5)degrees, V= 487.51(6) angstrom(3), Z= 2. Structure refinements using anisotropic displacement parameters converged at R = 3.50, R-w = 4.37, R-sym = 3.72%. Electron microprobe analysis performed on the same crystal used for X-ray investigation gave: SiO2 = 45.75(39), TiO2 - 1.05(5), Al2O3 = 9.60(19), MgO = 27.92(30), MnO = 0.16(3), FeOtot = 1.25(6), BaO = 0.09(5), K2O - 8.22(11), Na2O = 0.61(30), Cl = 0.02(1) wt%. Secondary Ion Mass Spectrometry (SIMS) was used to estimate light elements [Li2O = 0.30(1) and H2O = 0. 16(2) wt%] and fluorine content [F - 8.69(24) wt%]. The new mineral fluorophlogopite and its name were approved by IMA-CNMMN (2006/011).

The crystal structure of cyanotrichite, having general formula Cu4Al2(SO4)(OH)12·2H2O, from the Dachang deposit (China) was studied by means of conventional transmission electron microscopy, automated electron diffraction tomography (ADT) and synchrotron X-ray powder diffraction (XRPD). ADT revealed the presence of two different cyanotrichite-like phases. The same phases were also recognized in the XRPD pattern, allowing the perfect indexing of all peaks leading, after refinement to the following cell parameters: (1) a = 12.417(2) Å, b = 2.907(1) Å, c = 10.157(1) Å and β = 98.12(1); (2) a = 12.660(2) Å, b = 2.897(1) Å, c = 10.162(1) Å and β = 92.42(1)°. Only for the former phase, labeled cyanotrichite-98, a partial structure, corresponding to the [Cu4Al2(OH)12 2+] cluster, was obtained ab initio by direct methods in space group C2/m on the basis of electron diffraction data. Geometric and charge-balance considerations allowed to reach the whole structure model for the cyanotrichite-98 phase. The sulfate group and water molecule result to be statistically disordered over two possible positions, but keeping the average structure consistent with the C-centering symmetry, in agreement with ADT results.

Trioctahedral mica samples, collected at Cava St. Antonio (Mt. Vulture, Italy) were studied by combining electron-microprobe and C-H-N elemental analyses, single-crystal X-ray diffraction refinement, Mossbauer and Fourier transform infrared spectroscopies. Electron-microprobe analyses show the crystals to be quite homogeneous with TiO2 similar to 3 wt % and F ranging from 0.42 to 0.59 wt% Quantitative analyses of H combined with ferric/ferrous ratios from Mossbauer data allowed reliable crystal-chemical formulae to be derived. The results suggest that the entry of both Ti4+ and Fe3+ in the structure occurs through R-oxy substitution mechanisms involving deprotonation at O(4). This inference is supported by X-ray structure-refinement results (notably the c cell-parameter. the off-centering of the M2 cation towards O(4), the bond-length distortions of the cis-M2 octahedron) obtained using anisotropic thermal parameters in space group C2/m. The amount of oxy-substitutions from both electron-microprobe and X-ray data is in agreement with carbon-hydrogen-nitrogen analyses which give an average anion composition (OH1.25O0.65F0.10). Polarized-light infrared spectroscopy shows a complex OH-stretching spectrum which is composed of several overlapping (at least five) components. These can be assigned to the main octahedral local configurations that are compatible with the chemical composition. Pleochroic Fourier transform infrared spectroscopy measurements done along the principal optical directions show that the O-H bond axis is tilted from [001] and provide the average orientation of the O-H dipole in the structure: O-H (boolean AND) alpha similar to 23 degrees and O-H (boolean AND) gamma similar to 56 degrees.

The iron sulfate FeOHSO 4 studied was obtained as a dehydration product of metahohmannite Fe 2 (H 2 O) 4 [O(SO 4 ) 2 ] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO 4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma , lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 A ( a J , b J , and c J are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final R p value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2 n + 1, while sharp reflections for h = 2 n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO 4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO 4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm . Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2 1/2 and 2 −1/2 , and yields an orthorhombic structure with space group Pnma and lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 A. MDO2 results from the 2 1/2 |2 1/2 |2 1/2 ... sequence of symmetry operators and yields a monoclinic structure with space group P 2 1 / c , a M = 7.33, b M = 7.14, c M = 7.39 A, and β = 119.7°. The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit ( R p = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

The crystal chemistry of trioctahedral micas-1 M from Mt. Vulture phonolitic-trachitic ignimbrites has been investigated by single crystal X-ray diffraction, microprobe analysis and plasma emission spectroscopy. Chemical analyses have shown that Mt. Vulture trioctahedral micas belong to the phlogopite-annite join, with minor component of kinoshitalite-ferrokinoshitalite solid solution. In addition, samples with remarkably different Mg/Fe ratio can coexist in the same volcano-stratigraphic level, Fe-rich samples having also higher Ti content. The crystal structure of all of the analysed samples is affected, to varying degrees, by structural disorder, due to ± b /3 slips of octahedral sheet along the [0 1 0], [3 1 0] and [3 −1 0] directions. The latter has been reported in the literature for a handful of cases. In the present work, for the first time, the disorder has been successfully interpreted and managed in the structure refinement. It is demonstrated that it leads to desymmetrization of the TOT layer and therefore of the whole structure, with reduction of symmetry from C 2/ m to C 2. For the sample mostly affected by disorder (17 %) the R -factor drops from 9.97% ( C 2/ m ) to 2.74 % ( C 2). The chemical and geometrical features show that Mt. Vulture micas are homo-octahedral, even if some structural details suggest a preferential partitioning of Ti at M2 sites. However the mechanism of Ti incorporation in the structure seems to be different in Mg-rich and Fe-rich samples. For the former the Ti-vacancy substitution mechanism seems to hold: \(2^{VI}Mg^{2}{+}\ {\longleftrightarrow}\ ^{VI}Ti^{4{+}}\ {+}\ ^{VI}[]\) For the latter, the occurrence of Li, as detected by emission spectroscopy plasma measurements, structural evidence and charge balance considerations suggest the following novel substitution mechanism, \(^{VI}Ti^{4{+}}\ {+}\ ^{VI}Li^{{+}}\ {+}\ O^{2{-}}\ {\longleftrightarrow}\ 2\ ^{VI}M^{2{+}}\ {+}\ OH^{{-}}\) , to be active in the structure.

This paper relates an infrared (IR) (6000–2500; 1400–400 cm–1) spectroscopic study of sideronatrite, Na2Fe3+(SO4)2(OH)·3H2O. Spectra in the 600–1250 cm–1 range are dominated by fundamental S–O absorption bands. Factor-group analysis based on the space group P212121(D42) is in accordance with the observed band multiplicity. A broad and convolute absorption centred around 3400 cm–1 and a sharp band at 3605 cm–1 are observed in the 2900–3900 cm–1 frequency region. The resolution of the broad band is significantly improved at liquid nitrogen temperature; seven components can be fitted to the pattern and these can be assigned to H2O/OH molecules in the structure. All components in the broad band and the sharp absorption at 3605 cm–1 are strongly polarized for E//a, in accordance with the structure analysis results. Infrared transmission spectroscopy is a useful tool to identify these sulfate minerals occurring in specific geological environments.

"The thermal stability of sideronatrite, ideally Na2Fe3+(SO4)(2)(OH)center dot 3(H2O), and its decomposition products were investigated by combining thermogravimetric and differential thermal analysis, in situ high-temperature X-ray powder diffraction (HT-XRPD) and Fourier transform infrared spectroscopy (HT-FTIR). The data show that for increasing temperature there are four main dehydration\/transformation steps in sideronatrite: (a) between 30 and 40 A degrees C sideronatrite transforms into metasideronatrite after the loss of two water molecules; both XRD and FTIR suggest that this transformation occurs via minor adjustments in the building block. (b) between 120 and 300 A degrees C metasideronatrite transforms into metasideronatrite II, a still poorly characterized phase with possible orthorhombic symmetry, consequently to the loss of an additional water molecule; X-ray diffraction data suggest that metasideronatrite disappears from the assemblage above 175 A degrees C. (c) between 315 and 415 A degrees C metasideronatrite II transforms into the anhydrous Na3Fe(SO4)(3) compound. This step occurs via the loss of hydroxyl groups that involves the breakdown of the [Fe3+(SO4)(2)(OH)] (a) (2-) chains and the formation of an intermediate transient amorphous phase precursor of Na3Fe(SO4)(3). (d) for T > 500 A degrees C, the Na3Fe(SO4)(3) compound is replaced by the Na-sulfate thenardite, Na2SO4, plus Fe-oxides, according to the Na3Fe3+(SO4)(3) -> 3\/2 Na-2(SO4) + 1\/2 Fe2O3 + SOx reaction products. The Na-Fe sulfate disappears around 540 A degrees C. For higher temperatures, the Na-sulfates decomposes and only hematite survives in the final product. The understanding of the thermal behavior of minerals such as sideronatrite and related sulfates is important both from an environmental point of view, due to the presence of these phases in evaporitic deposits, soils and sediments including extraterrestrial occurrences, and from the technological point of view, due to the use of these materials in many industrial applications"

Fluorine is an important proxy for magmatic differentiation processes in the shallow parts of volcanic plumbing systems. Fluorphlogopite is one of the more important fluorine carriers in magmatic rocks. In the present study, a full crystal chemical investigation of fluorophlogopite 1 M from Piano delle Concazze, Mt. Etna volcano, Italy, is carried out. The fluorophlogopite occurs in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive center; ~15 ka BP). It is primarily associated with fluorapatite covered with amorphous SiO 2 and crystallized during syn/post-eruption pneumatolytic stages. The mica sample studied here is among the most Fe- and Ti-rich fluorophlogopite found in nature. EPMA data yielded the following mean chemical formula for this mineral ( K 0.83 Na 0.13 ) ( Fe 0.44 2 + Fe 0.09 3 + Mg 2.18 Al 0.05 Ti 0.23 Mn 0.01 ) ( Al 0.92 Si 3.08 ) O 10.64 ( Cl 0.01 F 1.35 ) . Structure refinements on four fluorophlogopite crystals, performed in space group C 2/ m , converged at R = 0.03–0.04, with cell parameters in the ranges a = 5.323–5.324, b = 9.219–9.222, c = 10.116–10.119 A, β = 100.1–100.3°. Major substitutions are OH − ↔ F − ,M 3+ -oxy ( VI M 2+ +OH − ↔ VI M 3+ +O 2− ) and Ti-oxy substitution: VI M 2+ +2(OH) − ↔ VI Ti 4+ +2O 2− . The fluorophlogopite from Piano delle Concazze exhibits the shortest c -parameter with respect to other fluorophlogopites found in nature. The short c parameter is essentially due to the absence of the hydroxyl group in favor of F − and especially of O 2− and to the thus increased attractive interaction between the interlayer cation and the anion content (F − , O 2− ) located at the O4 site. A comparison with other natural fluorophlogopites (namely from Biancavilla, Etna and Presidente Olegario, Brazil) show intermediate crystal-chemical features for the Piano delle Concazze fluorophlogopite. Particularly at Etna, differences in the chemical composition of the crystallized fluorophlogopites could be related to the various extent of enrichments by transfer of a gas phase achieved in specific parts of the volcanic plumbing system.

The aim of this work is to investigate the efficiency of the phyllomanganate birnessite in degrading catechol after mechanochemical treatments. A synthesized birnessite and the organic molecule were grounded together in a high energy mill and the xenobiotic-mineral surface reactions induced by the grinding treatment have been investigated by means of X-ray powder diffraction, X-ray fluorescence, thermal analysis and spectroscopic techniques as well as high-performance liquid chromatography and voltammetric techniques. If compared to the simple contact between the birnessite and the organic molecule, mechanochemical treatments have revealed to be highly efficient in degrading catechol molecules, in terms both of time and extent. Due to the two phenolic groups of catechol and the small steric hindrance of the molecule, the extent of the mechanochemically induced degradation of catechol onto birnessite surfaces is quite high. The degradation mechanism mainly occurs via a redox reaction. It implies the formation of a surface bidentate inner-sphere complex between the phenolic group of the organic molecules and the Mn(IV) from the birnessite structure. Structural changes occur on the MnO6 layers of birnessite as due to the mechanically induced surface reactions: reduction of Mn(IV), consequent formation of Mn(III) and new vacancies, and free Mn2+ ions production.

A natural Fe- and Ti-rich phlogopite-1 M of volcanic origin with an average composition (K 0.86 Na 0.08 Ba 0.04 ) (Mg 1.97 Al 0.17 Mn 0.01 Fe 0.29 2+ Fe 0.37 3+ Ti 0.19 )(Si 2.72 Al 1.28 )O 10.66 (OH 1.22 F 0.12 ) was studied using in situ low/high temperature neutron powder diffraction and the full-profile Rietveld refinement technique. Data sets were collected at 10, 298, 673 and 873 K on the D2B high resolution diffractometer at the ILL (Institute Laue-Langevin) Facility. The cell parameters expand linearly from room temperature up to 873 K [α a = 1.43(5) · 10 −5 K −1 , α b = 1.47(4) · 10 −5 K −1 , α c = 1.99(19) · 10 −5 K −1 , α V = 4.90(12) · 10 −5 K −1 ]. Site-occupancy refinements show that in the octahedral sheet Ti and Al tend to order into the M2 and M1 sites, respectively. As a consequence, Fe 2+ and Fe 3+ are preferentially ordered at the M1 and M2 sites, respectively. The H position was accurately refined at 10 K. The large components of the atomic displacement tensor of H suggest that hydrogen disorders over two symmetry mirror-related sites, around the average position ( x ,1/2, z ). Hydrogen can also act as an “atomic probe” of the Al—Si distribution over the tetrahedral sites. A comparison with the high-temperature results previously obtained using single-crystal X-ray diffraction on the same sample shows that, in the present study, dehydrogenation/oxidation processes involving hydroxyl group coordinated to Fe 2+ have occurred at a very modest rate, as confirmed by FTIR spectroscopy measurements.

The crystal chemistry of red phlogopites from Mt. Vulture (Italy) ignimbrites has been studied by electron microprobe, secondary ion mass spectrometry (SIMS), single crystal structural investigation and Fourier transform infrared (FTIR) spectroscopy. The analysed phlogopite has Fe/(Fe + Mg) ∼ 0.35, TiO2 (wt%): 2.8–5.0 and H2O (wt%): 1.24–3.37. Infrared spectra revealed the presence of bands due to the NH 4 + and H2O stretching and bending vibrations. The samples belong to the 1M polytype. The bimodal behaviour of several structural parameters allows red micas to be clustered into two distinct groups: K+ ↔ NH 4 + , H2O and M3+-vacancy substitutions dominate in the first group; M3+,4+-oxy, in the second group. It has to be pointed out that quantitative analysis of hydrogen (via SIMS) together with the characterization of the local environment of the anionic site (via FTIR) are fundamental in assessing the correct structural formula and the substitution mechanisms in micas.

In this study we investigate a new family of arsenate-bearing phases belonging to one set of structures (Basic Ferric Sulfate-BFS: monoclinic–orthorhombic FeOHSO4) that are significant as an industrial arsenic control material in the environment or a cathodic material in rechargeable Li-ion battery cells. We determine for the first time (after two decades of its known existence in the processing industry) the average crystallographic structure of Basic Ferric Arsenate Sulfate-BFAS: Fe(AsO4)1−x(SO4)x(OH)x·(1−x)H2O) that is a member of this family of phases and how it relates to its parent BFS structure. Moreover, we demonstrate how the substitution of AsO4 ↔ SO4 affects the crystallographic structure of these phases, the phase(s) that are formed and their material properties as environmentally stable arsenic controls or Li-ion battery cathodes.