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Being the high-pressure polymorph of serpentine, antigorite is a candidate mineral to transport “water” into subduction zones. Its expected dehydration under elevated pressure-temperature conditions markedly influences physico-chemical properties and processes in these regions and is considered to trigger subduction zone seismicity. In order to map hydration in subduction zone environments, laboratory data on the (anisotropic) sound wave velocities of serpentine minerals are needed. Here, we present high-pressure acoustic wave velocities, measured within the a-c-plane of a natural antigorite singlecrystal by Brillouin scattering. Preparation of the desired sample for Brillouin Spectroscopy was possible by using the focused ion beam technique. The chosen crystallographic plane is representative of the expected preferred orientation of antigorite crystals in a subduction zone environment and, therefore, our results put direct constraints on the expected seismic anisotropy emerging from a crystallographic preferred orientation (CPO) of antigorite in these regions. In addition, we performed direction-dependent high-pressure Raman Spectroscopy of the same single-crystal. We find anomalies in the pressure dependencies of both acoustic wave velocities and measured Raman shifts at a pressure of around 7 GPa consistent with previous observations.

Models describing the slip dynamics of a fault assume generally an isotropic interface giving rise to shear forces which are parallel to the slip direction. However, structured surfaces may be associated with an anisotropic frictional behavior, which is characterized by a dependence of the magnitude of friction force on slip direction and the presence of friction force components transverse to the slip direction. The arrangement of atoms in the crystal structure of the minerals composing the rock slab, the foliation and lineations originating from the rock deformation, and the presence of gouges formed as a consequence of rock erosion, contribute to the structuring of the surface on different length scales. Among these aspects, only the former is related to intrinsic properties of the fault, and its influence on the frictional properties is therefore independent on the rock history. In order to study the relation between frictional properties and crystal structure, we performed a nanotribological characterization of the surface of prominent mineral species with a scanning force microscope, where a micrometric tip mounted on an elastic cantilever is scanned by piezoelectric actuation along all directions of the sample surface on a nanoscopic area, while vertical deflection and lateral torsion of the cantilever are monitored in real-time, allowing for the quantitative determination of the friction force vectors. Antigorite, the high-temperature, high pressure polymorph of serpentine, is a mineral with a prominent role in defining the mechanical behavior of faulted regions. Indeed, there is general consensus that it forms in the forearc mantle wedge by hydration of olivine and pyroxenes through aqueous fluids released in the downgoing slab, upon increasing pressure and temperature. We asses frictional anisotropies as high as 100% of individual crystal domains of antigorite [1]. This anisotropy is related to the peculiar wavy arrangement of the TO-layer and the presence

The performance of CeO2-supported Pt-Ni and Pt-Co catalysts in the low temperature-Ethanol Steam Reforming (ESR) reaction has been evaluated studying the effect of the preparation method (impregnation/coprecipitation) and parameters such as dilution ratio, temperature, water-to-ethanol feed ratio and Gas Hourly Space Velocity (GHSV). The results show that impregnated samples perform better. In particular, the Pt/Ni/CeO2 catalyst starting from 350 C leads to a products distribution very close to the equilibrium calculations, with a low CO content that is ideal for fuel cells devices. In addition, the Co-based catalysts appear attractive in terms of hydrogen yield and coking tendency. For the Pt/Ni sample at 370 C, the analysis of products distributions vs contact time suggests a surface reaction path involving ethanol dissociative adsorption, dehydrogenation to acetaldehyde at short contact time (5-40 ms), followed by decomposition, reforming and CO-WGS reactions, to obtain H2, CH4, CO2 and CO. The last step is the methanation reaction, occurring at contact times higher than 100 ms, which drives the system to the equilibrium.

Canonite from Alfenza grows as crowded, radiating, acicular aggregates covering bismuthinite crystals. Individual crystals have a lozenge-shaped habit on {010}, the presumed cleavage plane of cannonite. Crystal structure refinements in the P21/c space group of two single crystals led to the following cell parameters: a = 7.7196(5) A ̊ , b = 13.8856(9), c = 5.6980(4), b = 109.174(1)o (R1 = 0.0424); and a = 7.7100(8), b = 13.8717(14), c = 5.6939(6), b = 109.155(2)o (R1 = 0.0438). Hydrogen atoms were also localized in the density-difference Fourier map and refined with soft restraints on the bond distances. Raman and IR spectroscopy confirm the presence of OH groups and the absence of molecular water, and deliver OH···O geometry wholly comparable with the structure refinement. Electron microprobe analyses revealed no significant levels of elements other than those expected in the ideal formula except fluorine which was present up to 0.14 a.p.f.u. The crystal structure can be described in terms of anion-centred OBi4 edge-sharing tetrahedra forming chains running parallel to z and strongly cemented along x by isolated SO4 tetrahedra. Each OBi4 tetrahedron is further connected along y by OH groups, making walls of composition Bi4O2(SO4)2(OH)4 parallel to (010). These walls are tied to each other along y by fewer Bi–O–S bridges and weaker OH···O bonds.

Earthquakes generated in subduction zones are caused by unstable movements along faults. This fault-slip instability is determined by frictional forces that depend on the temperature, pressure, morphology and deformation state of the fault rocks. Fault friction may also be influenced by preferred mineral orientations. Over-thrusting of rocks at the interface between a subducting slab and the overlying mantle wedge generates shear deformation that causes minerals to align, and this preferred mineral orientation affects the propagation of shear seismic waves. Here we use laboratory experiments to simulate fault slip in antigorite, the most abundant hydrous mineral phase within Earth's upper mantle. Using atomic force microscopy, we show that antigorite single crystals possess strong frictional anisotropy on their basal slip surface and that preferred mineral alignment extends this property to a regional scale. Depending on the alignment, fault movements can occur along a high-friction direction, creating stick-slip behaviour that generates earthquakes. In contrast, if movements occur along a low-friction direction, the mantle wedge will deform aseismically. Our results imply that mantle rocks in subduction-zone thrust faults can exhibit two opposite frictional behaviours, seismic and aseismic. © 2013 Macmillan Publishers Limited.

Two new hydrated bismuth sulfates have been discovered on a bismuth mineralization associated with quartziferous dikes intruding Pre-Triassic orthogneiss and garnet micaschist from Alfenza (Crodo), Antigorio Valley, Italian Alps (Capitani et al. 2011). The new minerals are found in “hortensia-like” aggregates of white, micrometre-sized crystallites or massive incrustations covering bismuthinite and quartz. At the SEM, they appear constituted of ~20 microns wide, few micron thick, randomly oriented, hexagonal platelets. EDS semi-quantitative analyses lead to the simplified chemical formula [Bi2SO4(OH)4]. IR spectroscopy confirms the presence of hydroxyls and the absence of water molecules. Due to the very small amount of material and the very small size of each crystallite, no X-ray diffraction analysis has been possible so far. On the contrary, transmission electron microscopy and diffraction, performed either by conventional zone orientation and automated diffraction tomography (Kolb et al 2007), reveals two distinct phases, both without any natural or synthetic analogue; one phase is monoclinic, with a = 17.2(4), b = 15.9(3), c = 20.3(4) Å, beta = 95(1), and P21 or P21/m symmetry; the other phase is hexagonal, with a = 9.6(2), c = 15.3(3) Å, and P ̄62c symmetry. The two phases can be recognized at the TEM not only by their unit cell geometry and symmetry, but also by their morphology and different beam sensitivity, whereas they are compositionally identical within the experimental error. The monoclinic phase has perfect basal cleavage that make its recognizable as (001) oriented platelets on TEM grids, show a strong diffuse scattering along the c* direction, and is very beam sensitive. The hexagonal phase is more resistant under the electron beam and does not show any preferential cleaving orientation. By morphological similarity, we draw the conclusion that the first phase is representative of the hexagonal platelets, while it was not possible to unambiguously

Being the high-pressure polymorph of serpentine, Antigorite is a candidate mineral to transport “water” into subduction zones. Its expected dehydration under elevated pressure-temperature conditions markedly influences physico-chemical properties and processes in these regions and is considered to trigger subduction zone seismicity. In order to map hydration in subduction zone environments, laboratory data on the (anisotropic) sound wave velocities of serpentine minerals are needed. Here, we present high-pressure acoustic wave velocities, measured within the a-c-plane of a natural antigorite single-crystal by Brillouin scattering. Preparation of the desired sample for Brillouin Spectroscopy was only possible by using the focused ion beam technique. The chosen crystallographic plane is representative of the expected preferred orientation of antigorite crystals in a subduction zone environment and, therefore, our results put direct constraints on the expected seismic anisotropy emerging from a crystallographic preferred orientation (CPO) of antigorite in these regions. In addition, we performed direction-dependent high-pressure Raman Spectroscopy of the same single-crystal. We find anomalies in the pressure dependencies of both acoustic wave velocities and measured Raman shifts at a pressure of around 7 GPa. This observation is consistent with previous Raman and x-ray diffraction studies and likely related to a stress-induced proton re-orientation.

The structure of antigorite (m = 17) has been studied by density functional theory from 0 to 30 GPa. The fourth-order Birch-Murnaghan equation of state fit of the thermally corrected LDA results yields an equilibrium volume (V0 = 2853.13 Å3), bulk modulus (K0 = 64.6 GPa), and its pressure derivative (K0′ = 6.94) in good agreement with experimental results. Two changes in compression mechanism occur at 6.1 and 20.5 GPa, individuating three pressure ranges: (1) in the low-pressure range, the antigorite wave flattens and the interlayer thickness decreases rapidly; (2) in the intermediate-pressure range, in-plane rotations of tetrahedra (ditrigonalization) and then wave-bending become the dominant compression mechanisms; (3) in the extreme-pressure range, the mechanism of wave-bending becomes prevalent. The first change reveals the origin of softening found experimentally near 6 GPa: the change in compression mechanism occurs after the minimal mismatch between T- and O-sheets is achieved and is accompanied by an apparent symmetry breaking: accidental degeneracies of structural parameters between short and long halfwaves are lifted, including T-sheet thicknesses and Si-O bond lengths. In the extreme-pressure range, Si-O-Si angles decrease below 122°, which may be the origin of amorphization found experimentally at similar pressure.

A flower-like mineralization of white, micrometre-sized crystallites, occurs on Bi sulfosalts from the Alfenza Mine (Crodo). With a standard optical stereomicroscope each single crystal is hardly resolvable. At the scanning electron microscope (SEM), the crystallites show perfect, pseudo-hexagonal, randomly oriented platelets forming a hemispheric aggregate up to several tens of microns in radius. The EDS spectra show distinct S, Bi, O peaks, pointing to a Bi sulfate (or Bi hydrated sulfate). As far as we know, only two natural phases, cannonite [Bi2O(OH)SO4] and riomarinaite [Bi(OH) SO4•H2O], have been reported with a composition that qualitatively match the observed one. However, available physical properties for these minerals (habit, color, cleavage...) do not match with those observed in our case. In order to solve the puzzle, few mineral aggregates were peaked up with a knife tip, dispersed in ethanol, ultrasonicated, and pipetted on a holey-carbon Cu grid for TEM analyses. Preliminary results show a Pmm projected symmetry down the pole of the pseudo-hexagonal platelets consistent with a monoclinic or orthorhombic system, 2D cell parameters a = 17.2(4) and b = 15.9(3) Å, (gamma = 90°), and systematic absences consistent with a 21 screw along the b-axis. A comparison of the observed electron diffractions with those simulated for (synthetic) cannonite and riomarinaite reveals substantial differences, excluding these two phases as possible and opening to the possibility for a new mineral. Deeper investigations are in progress.

In recent years, many efforts have been made on the synthesis of new self-assembled materials, and on scientific understanding of the formation mechanism from building molecules. With the aim to develop new functional materials, layered inorganic compounds recently attracted considerable interests for the preparation of nanoscale organic/inorganic hybrids. A large class of minerals presents inherently nanostructured morphologies. Within this group, we cite, to name a few, chrysotile, crocidolite, and montmorillonite, being examples of crystalline aggregates of nanotubes, nanorods, and nanosheets, respectively. On this basis, synthetic “geo-inspired” materials are being developed, with the aim to enhance their possibility to be employed as nanomaterials in technological applications. On the other hand, the surface of such minerals is rich of chemical groups which can bind covalently or electrostatically organic compounds capable to confer technological functionalities to the ultimate hybrid system. In order for a transition from bare mineral to functional material to be achieved, the development of processes for the deposition of highly-oriented thin film phases of such mineral nanoparticles are mandatory. Indeed, their marked anisotropic morphology influences also their physical properties, which may vary to a great extent depending on the probed direction. Here, we show the results of a scanning probe and electron microscopy, X-ray diffraction, and IR optical analysis of nanostructured thin films of natural and synthetic minerals obtained by liquid phase deposition of colloidal solutions. By assisting the deposition process with a static magnetic field, we demonstrate the possibility to induce a preferential orientation of the deposited nanoparticles, thanks to the anisotropy of their diamagnetic or paramagnetic susceptibilities. This effect is also studied in minerals deliberately doped with paramagnetic species such as Fe(II) and Fe(III) ions. Hybrid inorganic-or

We present a transmission electron microscopy (TEM) investigation of lillianite (Pb3Bi2S6) and heyrovskýite (Pb6Bi2S9), from Vulcano, Aeolian Islands, Italy. The minerals investigated represent the only naturally occurring Ag- and Cu-free sulfosalts in the lillianite homologous series (LHS). Three methods (crushing, ion-milling, and ultramicrotomy) were used to prepare TEM specimens. Selected area electron diffraction (SAED) patterns and high-resolution TEM (HRTEM) images indicate well-ordered crystals with only minor stacking faults and, more rarely, nanoscale intergrowths of lillianite and heyrovskýite. The latter were sometimes found to form an incommensurate structural modulation with an angle of ∼29° relative to b* in the (hk0) plane and a wavelength of ∼75 Å. This represents the first observation of such incommensurate modulations in heyrovskýite. Although considerable evidence points toward an artifact induced by the sample preparation technique (i.e., ion-milling), the possibility that the incommensurate modulation could be a primary feature of heyrovskýite itself cannot be completely ruled out. The modulation could derive from an ordering process of Pb and Bi cations over Me4 and Me5 sites within the PbS-like layer or from ordering of vacancies, naturally present or induced by Bi2S3 sublimation during ion-milling.

Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM) investigations were used to study a complex micro-structure of twin-related lamellae of 15R SiC polytype, in syntactic coalescence with 6H-SiC. The analysis of the local stacking microstructure by means of HRTEM revealed that the perfect structure of 15R polytype, (23)3 was locally interrupted by numerous adjoining stacking faults parallel to (0001) with stacking of the coupled (22) and (33) bilayers superimposed on the twin boundaries. HR images taken exactly across both the twin boundaries showed a zig-zag pattern (23) that switched to (32) by a twin coherent interfaces or, alternatively, a zig-zag pattern (32) which passed to (23) through an isolated (33), 6H like sequence. The selected area electron diffraction (SAED) patterns taken exactly above both the twin interfaces indicate classifying of the twin found in this study as a "Friedelian" reticular merohedric twinning. However, two indistinguishable twin operations matched the observed features: a reflection through rational plane (0001), and 180°-rotation around [0001]. Since individual Si and C atoms and even the SiC bilayer polarity could not be established from these HR images, the real twin law was deduced by taking into account that the coherent structural match at the interface could be guaranteed only by a 180°-rotation around [0001]. In order to explain the origin of the planar defects found in this sample, the growth mechanism and the influence of the low-energy stacking faults were considered. © 2010 Springer-Verlag.

Using theoretical methods, we calculate the full elastic constant tensor and equation of state of antigorite up to 10 GPa, a pressure range that encompasses its experimentally observed stability field. At ambient conditions, the elastic constant tensor reveals significantly large acoustic anisotropy(38% in VP; 35% in VS) compared to the dominant mantle phase olivine. The shear anisotropy is enhanced upon compression. Under compression, the full elastic constant tensor reveals anomaly at r ∼ 2.69 gm cm−3. At r < 2.69 gm cm−3, we find that the pressure‐volume results for antigorite are well represented by a third order Birch‐Murnaghan formulation, with K0 = 52 GPa, K0′ = 8.5 and V0 = 3037 Å^3(r0 ∼ 2.48 gm cm−3). Our results on elasticity and anisotropy 19 at conditions relevant to the mantle wedge indicates that a 10–20 km layer of antigorite might account for the observed shear polarization anisotropy with fast polarization parallel to the trench and associated large delay times of 1–2 sec observed in some subduction zone settings.

Transient (non-equilibrium) microstructures in crystals may arise in an order-disorder phase transition that generates lattice strain. A two-phase field can develop if fluctuations of the order parameter lead to nucleation of an ordered phase in a disordered matrix, as we describe here for columbite. Synchrotron X-ray diffraction and transmission electron microscopy show that ordering in columbite involves two discrete phases with different degree of order but the same composition. A highly unusual distribution of ordered rhombic-shaped domains within a disordered matrix establishes on a nanometer scale and remains relatively stable over a prolonged period of annealing. Progressive ordering takes place within the ordered domains and the disordered matrix but the domains maintain more or less constant shape and distribution. We speculate that a new family of such microstructures could develop in other oxide phases with cation ordering transitions that are strongly first order in character. Long-term stability of such microstructures and their dependence on strain could open up the possibility of engineering the properties of crystals containing a percolating disordered matrix with ordered nanodomains of controlled dimensions.

In this paper a single step process for the synthesis of composite thin films made of platinum nanoclusters embedded in a porous hydrocarbon matrix is proposed. The process consists of a simultaneous plasma enhanced-chemical vapour deposition of ethylene and sputtering of a platinum target. Films were characterized by means of Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy and transmission electron microscopy. Optical emission spectroscopy was used to investigate the plasma phase and to find correlations with the metal content of the film. The effect of radio frequency power, deposition time, and ethylene flow rate on the chemical composition and structure of the film is presented. Results show that the platinum content in the film can be controlled by tuning the power delivered to the plasma and the monomer flow rate, and that the metal aggregates in crystalline nanoclusters uniformly distributed in the material. Film morphology is characterized by columnar structures of variable diameter and orientation depending on the deposition conditions. ©2010 WILEY-VCH Verlag GmbH & Co. KGaA,.

The zero pressure structure of the m = 17 antigorite polysome [Mg48Si34O85(OH)62] has been recently investigated by first principle methods (Capitani et al. 2009). Here we present the results of the investigation of the antigorite structure up to 30 GPa making use of the same computational method. Basically, the 291-atom primitive cell was allowed to relax at different volumes using density functional theory and the plane wave pseudopotential method implemented in the VASP code (Kresse & Furthmuller,1996). Both, local density (LDA) and generalized (GGA) gradient approximations to exchange and correlation were used. Theoretical results are in good agreement with recent single crystal diamond anvil cell experiments (Nestola et al., 2009). As for most sheet silicates, LDA and GGA bracket the experimental data, so that LDA would provide a better match if phonon excitation is taken into account (Mookerjee and Stixrude, 2008). The third order Birch-Murnaghan (BM3) equation of state (EoS) yields a better agreement with experiments if data above ~ 17 GPa are excluded from fitting: V0 = 2806.8 Å3, K0 = 71.0, and K’ = 5.3 for LDA (static calculations), to be compared with V0 = 2914.1 Å3, K0 = 62.9, and K’ = 6.1 for the experiments (dataset up to ~ 6 GPa). The reason is an abrupt change in compressional behavior above ~ 17 GPa. A similar discontinuity – in that case related to antigorite softening – was also found in the experiments, even if at much lower pressure (6 GPa). In the present case, the change in compressibility is related to a change in axial anisotropy and unit cell contraction mechanism. While at low pressure the relevant axial anisotropy is Kb > Ka > Kc – alike experiments – with increasing pressure the a parameter becomes more compressible than c. At the same time, the contraction of the unit cell that is mainly accomplished by interlayer thinning and halfwave flattening at moderate pressure, with increasing pressure switches to an abrupt increase of the in-p

The crystal structure of tobelite 2M2, a dioctahedral mica with ideal composition (NH4,K)Al2(Si3Al)O10(OH)2, has been refined for the first time via single crystal XRD by Mesto et al. (2006). Those authors refined two single crystals of tobelite in the C2/c S.G., to R-values 6.0% and 7.2%. The average cell parameters were: a = 9.026, b = 5.203, c = 20.721, beta = 100.05°. However, residues in the Fourier density maps pointed to some kind of structure disorder. In the present work a HRTEM investigation was undertaken in order to elucidate the nature of the disorder. This turned out to be quite a difficult task due to the high sensitivity of the samples to electron beam damage (see also Nieto 2002, Fig. 5). After few seconds of exposure to the electron beam, even at very low magnification (i.e., low beam dose), “buttonhole” shaped features develop parallel to (001). They look like inflated pockets, probably due to NH4 + diffusion and accumulation, before volatilization, within the interlayer. In spite of these difficulties, numerous stacking faults parallel to (001) were detected at low magnification and some of them were characterized by high resolution imaging. Other kinds of inhomogeneities, such as intergrowths with other micas have not been observed in the studied samples at the present state of the investigation. Full stacking sequence characterization of micas by HRTEM requires structural images along two not equivalent direction of the [UW0] zone type of the same faulted area (Kogure 2002). For the samples considered here, this turned out as very demanding, because of the aforementioned instability of tobelite under prolonged exposure. This notwithstanding, assuming that stacking faults belonging to subfamily A are absent – a reasonable assumption since diffraction patterns taken along [-130] do not show any streaks or spot splitting along [31l]* rows – most of the near atomic resolution images of stacking faults taken along [010] can be interpreted as interru

The role of Agþ ions in the ultraviolet-driven photochemical synthesis of Au nanorods (NRs) in aqueous surfactant mixtures has been investigated in order to elucidate the mechanism that drives anisotropic nanoparticle growth. The samples, grown in the presence of varying amounts of Agþ ions for scheduled irradiation times, have been characterized by ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectroscopy, analytical transmission electron microscopy (ATEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and extended X-ray absorption fine structure (EXAFS) measurements. Moreover, the time evolution of size and shape distribution has been investigated by statistical analysis of the relevant TEM data. EXAFS measurements at the Ag K-edge have unambiguously disclosed the presence of Ag species in the final product, identifying their chemical state as well as the most probable lattice environment around them with a reasonably high level of confidence. The extensive sample knowledge gained by the combination of spectroscopic, structural, and morphological measurements has provided reliable information regarding the most relevant processes underlying the Agþ-assisted formation of Au NRs by the photochemical route. An induction period prior to occurrence of fast nanoparticle nucleation has been identified, which has been correlated to the slow accumulation of a critical concentration of Au(I)-surfactant species from reduction of their Au(III) parent precursors. The role played by Ag in directing Au growth toward the formation of NRs has been clarified through demonstration of preferential adsorption of zerovalent Ag species on {110} facets of the growing Au nanoparticles, which can be therefore responsible for restricting crystal development along the relevant crystallographic directions

X-ray diffraction topography (XRDT), using both synchrotron white-beam (SWBXT) and conventional MoKa1 radiation, -Raman spectroscopy and transmission electron microscopy (TEM) were used to study a complex syntactic coalescence of polytypes in a PVT-grown 6H–SiC sample. The combined use of these techniques has proven to be well suited to determine twin-related lamellae of polytype 15R. Each single technique used here was not fully adequate to establish the polytypic nature and the geometric relationship between the lamellae of foreign polytypes and the surrounding crystal bulk. X-ray diffraction topographies showing the whole slice, enabled the location of three lamellae: one lamella, labelled L1, and two micro-lamellae L2 and L3. Lamella L3 was enclosed in L1. The diffraction contrast analyses and the -Raman spectra indicated that the L1 and L2 lamellae were exactly iso-oriented and both consisted of the 15R polytype, whereas the polytype of the L3 lamella could not be clearly determined. Transmission electron microscopy (TEM) enabled a more thorough investigation of the actual polytype of this lamella. TEM analyses were carried out on a small fragment embedding the L3 lamella and these indicated that L3 was a 15R polytype which was twin-related by a rotation of 180° about the [0001] axis with a (0001) compositional plane.

Lattice parameters for gold nanocrystals, quartz, and vesuvianite have been determined by electron diffraction in routine transmission electron microscopy (TEM) work, with precision and accuracy near to 0.1%, after correction for elliptical distortion. The distortion, measured in three different microscopes, is constant for each microscope and may be easily eliminated.Variable camera constants have been avoided by positioning the oriented specimen on the eucentric plane and using parallel illumination. The current flowing in the first intermediate lens was kept fixed, assuring constant conditions of the TEM projecting system, with no further diffraction focus applied.Application of this method to micas from metamorphic rocks produced deviations between measured and expected values up to 0.8%. Although easy species distinction is still possible, minor crystal chemical differences within the sample may be lost. Likely causes of these deviations are the possible heterogeneous samples, as well as beam damage leading to cation loss with subsequent variation in basal spacings

The compressibility of antigorite has been determined up to 8.826(8) GPa, for the first time by single crystal X-ray diffraction in a diamond anvil cell, on a specimen from Cerro del Almirez. Fifteen pressure–volume data, up to 5.910(6) GPa, have been fit by a thirdorder Birch–Murnaghan equation of state, yielding V0 = 2,914.07(23) A ° 3, KT0 = 62.9(4) GPa, with K0 = 6.1(2). The compression of antigorite is very anisotropic with axial compressibilities in the ratio 1.11:1.00:3.22 along a, b and c, respectively. The new equation of state leads to an estimation of the upper stability limit of antigorite that is intermediate with respect to existing values, and in better agreement with experiments. At pressures in excess of 6 GPa antigorite displays a significant volume softening that may be relevant for very cold subducting slabs

We have investigated the antigorite m = 17 structure [Mg48Si34O85(OH)62] by density functional theory (DFT), with the aim to probe the method on such a large and low symmetry (Pm) system. We found a satisfactory match with the experiments using both LDA and GGA approximations to exchange-correlation, although the former performs slightly better here. Predicted cell constants are within 0.3% for LDA, and 0.5% for GGA, of the experimental values. Average atomic displacements after relaxation are within ~0.06 Å. All the fine structural details of antigorite are reproduced: apical Si-O bonds shorter than basal Si-O bonds; external Mg-O distances shorter than internal Mg-O distances; pronounced tetrahedral ditrigonal distortion. Where palpably biased bond distances were present in the experimental data because of disorder, theoretical methods promptly recover the structure into more reliable bond geometry. These findings let one envisage the employment of DFT calculations as a promising tool for refining or validating complex structures, whenever the experiments suffer of limitations due to the poor quality of the material being investigated.As an additional benefit, we also compare the total energies of two competing models for the m = 17 antigorite structure, the one refined by Capitani and Mellini (2004) and that proposed by Dódony et al. (2002). We found the former more stable, both as published (∆E = 1.1 kJ/mol·atom–1) and after full cell relaxation at constant volume (∆E = 0.5 kJ/mol·atom–1)

The serpentine mineral antigorite is known to be stable up to 6 GPa (200 km) at 620°C [1]. Under more extreme conditions, it breaks down to a dehydrated assemblage (Atg = Ol + Tlc (En) + H2O). Earthquakes at intermediate mantle depth may be triggered by dehydration embrittlement caused by antigorite breakdown. In spite of the importance of antigorite at subduction zones, little is known of its properties at high pressures. Knowledge of the elasticity of antigorite is important for the interpretation of seismological observations at subduction zones and for the detection of antigorite in this and other geological settings. This knowledge may be achieved, in principle, either experimentally or computationally. However, both routes are troublesome. Experimentally, one is faced with the high density of defects usually affecting single-crystal sized specimens, and with the difficulties in dealing with such a large unit cell and a DAC-shadowed reduced dataset. Computationally, the antigorite system is quite large and thus very costly. Moreover, a good guess of the crystal structure is mandatory to enhance the chances to successfully achieve any reliable result. We report here the results of an ab initio quantum mechanical investigation of the antigorite structures proposed in the recent literature. The structures investigated are the m = 17 and the m = 16 polysomes, XRD refined by Capitani & Mellini [2, 3] and a second m = 17 polysome proposed by Dódony & co-workers [4]. Basically, the first two structure models differ from the last by the presence of tetrahedral 8-reversals and the absence of octahedral offsets. The total free energy for the three models after structural relaxation are -6.983189, -6.986766, -6.977770 eV/atom, respectively. From these results it turns out that the models with tetrahedral 8-reversals and without octahedral offsets are energetically favoured with respect to the model excluding tetrahedral 8-reversals and including octahedral offsets. For c

The validity of the structural model of antigorite based on the occurrence of four- and eight-membered tetrahedral rings is confirmed. The quality of crystals, single-crystal X-ray and synchrotron diffraction, lattice geometry, HRTEM interpretations, and coordination chemistry still provide evi¬dence in favor of these refined structures. The arguments used to invalidate this model and support an alternative one based on the occurrence of tetrahedral six-reversals only and octahedral sheet offsets do not hold.

The Cerro del Almirez ultramafic massif offers an example of high pressure and high temperature antigorite serpentinites. A sharp antigorite-out isograd separates them from Chl-harzburgites, consisting of olivine + enstatite + chlorite. Antigorite is characterized by aluminium contents as high as 4 wt.% Al2O3. The microstructural study shows that, prior to the transformation antigorite is exceptionally ordered and consists of the polysome m = 17. No polysomatic defect occurs in antigorite forming most of the Cerro del Almirez serpentinites. Close to the antigorite-out isograd, limited disorder features may occur, mainly as (001) twins, reaction rims and reduction of m down to 14–15. Here, local recrystallization phenomena lead to sporadic growth of large antigorite and chlorite crystals.

The Cerro del Almirez ultramafic massif offers an example of high pressure and high temperature antigorite serpentinites. A sharp antigorite-out isograd separates them from Chl-harzburgites, consisting of olivine + enstatite + chlorite. Antigorite is characterized by aluminium contents as high as 4 wt.% Al2O3. The microstructural study shows that, prior to the transformation, antigorite is exceptionally ordered and consists of the polysome m = 17. No polysomatic defect occurs in antigorite forming most of the Cerro del Almirez serpentinites. Close to the antigorite-out isograd, limited disorder features may occur, mainly as (001) twins, reaction rims and reduction of m down to 14–15. Here, local recrystallization phenomena lead to sporadic growth of large antigorite and chlorite crystals.

Single-crystal structure determinations of the m = 17 and m = 16 antigorite polysomes confirmed the existence of two basic antigorite structures, i.e., “odd” antigorites, with m = 2n + 1 (m being the number of tetrahedra in a wavelength visible in the [010] projections), and “even” antigorites, with m = 2n. Both structures contain “8-reversals” and “6-reversals”. The “8-reversals” consist of 8-member rings of tetrahedra, where four tetrahedra point in one direction and four in the opposite direction, whereas “6-reversals” involve four tetrahedra and two tetrahedra, respectively. “Odd” antigorites have m tetrahedra and m − 1 octahedra along a wavelength (which coincides with the a translation) and Pm space group. Conversely, b/2 shifts every second 8-reversal cause the structure to be C-centered in “even” antigorites (C2/m space group). In the latter case, m tetrahedra and m − 1 octahedra occur within a wave, but two waves occur within an a periodicity. Transmission electron microscopy (TEM) of antigorite polysomes with m ranging from 15 to 18 is successfully interpreted on the base of previous X-ray data. In particular, [001] selected area electron diffraction (SAED) patterns show primitive or C-centered cells, for m = 2n or 2n + 1, respectively. In addition, [001] high-resolution (HR) images show 8-reversals aligned or offset by b/2 (i.e., stacking based on a primitive or C-centered lattice).

The crystal structure of a terrestrial 6H-SiC moissanite has been refined in the P63mc S.G. from area detector single crystal X-ray data, down to an R-index on the observed reflections of 0.0205. The cell parameters refined over all the collected reflections are a = 3.0810(2) and c = 15.1248(10) Å. The average Si-C bond lengths are 1.8898 Å, with average bonds along the stacking direction (1.8993 Å) slightly longer than those along the bilayer (1.8862 Å). The interlayer distances, defined as the distances along [0001] between Si-Si layers, which may occur either in cubic (c) or hexagonal (h) configurations, are maximal at the c-h interface (2.5270 Å) and minimal at the h-c interface (2.5165 Å), entailing that the h-bilayer is not equidistant from either c-bilayers. All the tetrahedral angles are identical within the experimental error and close to the ideal value of 109.47°, but those at the c-h interface, where a significant distortion of 0.15° is recorded. Finally, the anisotropic displacement factors are utterly very small, identical among different atoms within the experimental error, and significantly spherical. It thus appears that the 6H-SiC structure is affected by a slight relaxation along the [0001] stacking direction with respect to the ideal cubic structure, and that the relaxation is mainly accomplished at the c-h interface, i.e., at the twin-like boundary, where a bilayer in cubic configuration links a bilayer in antiparallel, hexagonal configuration. As far as we know this is the first crystal structure refinement of a natural 6H-SiC moissanite. Possible implications on the polytype stability in the light of these results are briefly discussed.

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.

Calcium sulphate dihydrate nanocrystals of 25–100 nm width have been synthesized in 100% purity and yield by means of a method—the cryovacuum process—consisting of rapid freezing of quasi-saturated solutions and subsequent vacuum assisted sublimation of water. Transmission electron microscopy reveals both curled nano-lamellae and smaller, irregular particles; electron diffraction patterns demonstrated that the particles are crystalline. This is a very powerful method for the ‘clean’ synthesis of moderately and completely water-soluble inorganic materials.

A practical, offline method for experimental detection and correction for projector lens distortion in the transmission electron microscope (TEM) operating in high-resolution (HR) and selected area electron diffraction (SAED) modes is described. Typical TEM works show that, in the simplest case, the distortion transforms on the recording device, which would be a circle into an ellipse. The first goal of the procedure described here is to determine the elongation and orientation of the ellipse. The second goal is to correct for the distortion using an ordinary graphic program. The same experimental data set may also be used to determine the actual microscope magnification and the rotation between SAED patterns and HR images. The procedure may be helpful in several quantitative applications of electron diffraction and HR imaging, for instance while performing accurate lattice parameter determination, or while determining possible metrical deviations (cell edges and angles) from a given symmetry.

Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS) have been used to refine the solid-state speciation of As in mine waste materials and stream-bed sediments from the Baccu Locci mine area in Sardinia, Italy. The combined use of TEM and XPS confirmed previous studies and pointed out that: (i) As is contained in plumbojarosite in substitution of S, (ii) As-bearing Fe(III) hydroxides are represented by arsenical 2-line ferrihydrites with most Fe=As molar ratios in the range of 1.6–3.2, (iii) scorodite often lacks long range order. Arsenatian plumbojarosite will tend to dissolve incongruently in ferrihydrite, releasing Pb and As into water. However, Pb is fated to precipitate as Pb(OH)2 or to be adsorbed onto ferrihydrite, whereas As mobility is favoured since the adsorptive capacity of ferrihydrite for As(V) is notably reduced under the neutral to alkaline pH conditions occurring in the Baccu Locci stream water. Arsenical ferrihydrites will tend to be converted into goethite or hematite with time, releasing As into the water as a consequence of the notably lower density of the adsorption sites of the crystalline Fe(III) phases. Also scorodite is relatively unstable under the Baccu Locci system conditions, and its stability is further decreased owing to the higher solubility of the amorphous form compared to the crystalline one.

Structural imaging of the “odd” m = 17 antigorite polysome is in keeping with the X-ray structure refinement, supplying direct evidence for the presence of 8-reversals. HRTEM also displays the polysomatic and stacking disorder features modeled during X-ray refinement to achieve convergence. Polysomatic faults occur in Mg159 antigorite as (100) lamellae, with wavelength differing from the matrix by only one serpentine module (m = 16 or 18 vs. 17). Mixed ordered polysomes may also occur, such as a regular alternation of m = 16 and m = 17 lamellae, producing a complex polysome with an a repeat distance of 84.5 Å. “Odd” and “even” antigorite polysomes differ in layer topology. “Odd” polysomes (m ≠ 2n, with n = integer) have a primitive cell with Pm space group, m tetrahedral and m-1 octahedral modules, and one 6-reversal and one 8-reversal per unit cell. Polysomes with m = 2n have C-centered cells (C2/m), m tetrahedral and m-2 octahedral modules, and two 6-reversals and two 8-reversals per cell.

The late crystallisation stages of Campiglia Marittima skarn are characterised by an increase in Mn-activity at the metasomatic front. Mn-pyroxenoids became stable with respect to johannsenite. The pyroxene-pyroxenoid textures indicate either pyroxenoid overgrowths on johannsenite or pyroxenoid pseudomorphs after johannsenite. TEM investigation of the replacement sites reveals that the pyroxene-to-pyroxenoid replacement mainly consists of a topotactic solid-state reaction involving the movement of a reaction front through the original johannsenite crystal (bulk reaction mechanism). Due to the fast kinetics of skarn formation, pyroxenoids are chemically heterogeneous and highly defective, with numerous chain periodicity faults. The crystallographic relationship (001)Pxd//(11–1)Jh is confirmed. Johannsenite-pyroxenoid solid-state transformations appear to occur at temperatures lower than 260°C. In contrast with previous assumptions, AEM data and crystal chemistry indicate that the maximum calcium content of pyroxmangite is 2/7, rather than 1/7. As a consequence, the AEM data plot along a normal and chemically collinear polysomatic trend. Moreover, it is suggested that minor concentrations of iron and magnesium in the system can affect the pyroxene pyroxenoid transformation, determining which pyroxenoid is stable in the replacement sequence.