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The low crystallinity of arsenic (As) containing minerals such as arseniosiderite and yukonite has made a detailed understanding of their atomic structures elusive. X-ray pair distribution function (PDF) and transmission electron microscopy (TEM) were used to gain a detailed understanding of the short and medium-range structural features. High-resolution synchrotron powder diffraction data showed long-range order in arseniosiderite occurs along one direction. PDF data confirmed arseniosiderite has the same general structure as robertsite and mitridatite but with the layers occasionally shifted to disrupt long-range order. Use of small box and reverse Monte Carlo modeling of the PDF data gave the first complete set of atomic coordinates for arseniosiderite. TEM confirmed the degree of interlayer order is generally high. PDF and TEM data show that yukonite is a nano-sized analog of arseniosiderite which lacks any long-range order but is locally similar. The structural evolution of arseniosiderite from yukonite was observed for the first time in a natural specimen via TEM. PDF data on several yukonite samples reveal rather sudden drops in the degree of structural correlation as a function of distance. These drops occur from yukonite being composed of small layers made of integer numbers of large nonamers of edge-sharing FeO6 octahedra, as seen in arseniosiderite. However, these layers are 2 or 3 nonamers in width, with little structural correlation between layers. Thus our work provides a more complete nano/microscopic picture of the key structural features in these minerals found in mining ores and wastes around the world.

Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system. Amphiboles are hydrous silicates occurring in many rock types in the continental crust and subduction zones. Here, in situ Raman spectroscopy of grunerite reveals temperature-activated electron-phonon excitations that provide an atomistic insight into the role of amphiboles in the anisotropic lithospheric conductivity.

We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 °C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at ∼335 °C accompanied by an anomalous increase in the monoclinic cell angle β. From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at ∼315 °C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mössbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-Amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.

A crystal-chemistry survey on natural aluminium phosphates, carried out by combining EMPA micronalysis, single-crystal X-ray diffraction and FTIR spectroscopy, was performed starting from our studies on whiteite CaFeMgAl2(PO4)4(OH)2∙8H2O, augelite Al2(PO4)(OH)3, wardite NaAl3(PO4)2(OH)4∙2H2O, lazulite MgAl2(PO4)2(OH)2 and wavellite Al3(PO4)2(OH,F)3·5H2O, and extending the investigation to the most common Al phosphates phases. Main crystallographic features (bonds, angles, interactions) were examined according to the results of structural refinements, and the presence of networks of O-H…O hydrogen bonds with in the structures, based on the Libowitzky (1999) relationship, for the range of D-H...A bond systems in the structure, in order to compare results of OH frequencies from FTIR spectra with those observed by X-ray refinement.

Melt inclusions (MIs) represent pristine melt droplets trapped in early forming phenocrysts, which record the variable physico-chemical conditions of their hosting system. Compared to whole rock studies, the MIs approach shows advantages especially because it provides information on melt composition, volatile content, pressure and temperature at well-defined pre-eruptive stage. A large amount of geochemical data is available for the magmas generated in the Phlegraean Volcanic District (PVD), which is one of the most densely populated and high-risk volcanic areas of the world, but the knowledge provided by those data on their plumbing system, ascent, degassing and extrusion is limited. We investigated the glass/melt inclusions in olivines from the Solchiaro 1 volcanics, erupted on Procida Island, which is located along main tectonic alignment of the PVD. These products represent the least evolved magma of the PVD and could testify for the presence of the “basic” and deepest reservoir from which the Phlegraean magmas upraised and differentiated through various steps. Often invoked in literature, this “primitive” reservoir is basically uncostrained. We performed microprobe and microscopy measures to determine textural and compositional features of the MIs and host-minerals. The measures by Fourier Transform InfraRed spectroscopy provided the content of dissolved H2O and CO2, hence the volatile budget of magmas and entrapment pressures (i.e. depths of crystal growth). Recalculated crystallization depths for the Solchiaro 1 olivines result to be very deep (~350 MPa) and isobars obtained for the MIs describe a degassing trend with high CO2 content, of deep origin. Taking into account the geophysical data and the MIs studies on other Phlegraean products recently published, our dataset depicts a new framework for the evolution of the PVD plumbing system up to the present time that improves both the comprehension of magma evolution and migration through various reservoirs and the volcanic hazards assessment.