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In the last decades many studies have been focusing on Carbon Capture and Storage(CCS) to find a possible remedy to reduce the large increase of anthropogenic carbon dioxide. Mineral Carbonation (MC) is a potential solution for almost irreversible chemical long-term CCS. It concerns the combination of CaO and MgO with carbon dioxide forming spontaneously and exothermically dolomite and magnesite. However, kinetic barriers pose sever limitations for the practical exploitation of this reaction. High fractions of MgO are available in silicates such as olivine, orthopyroxene, clinopyroxene and serpentine. To date, data reported that serpentine polymorphs, above all antigorite, is an excellent candidate for fixing the carbon dioxide as the reaction efficiency is approximately 92% compared to lizardite (40%) and olivine (66%). This is due to the surface reactivity of approximately 18.7 m^2/g for the dehydrated antigorite comparedto10.8 m^2/g for dehydrated lizardite and 4.6 m^2/g for olivine. The microwave assisted process for CCS is an innovative technology that can be employed to catalyze the reaction through thermal and non-thermal mechanisms. Some pioneering tests of direct carbonation by microwave hydrothermal equipment have been performed on olivine, lizardite and chrysotile powders [1] but not on antigorite. The structure of serpentine is characterized by corrugated stacked layers of silica and brucite. For this reason, MC involves dissolution of silica layers, dissolution/dehydration of brucite layers, and precipitation of magnesium carbonate. To address the chemical response of the single phases, experiments have been performed by both a local microwave-source acting locally on a specific crystal surface and a volume source interacting with an ensemble of grains on synthetic powders and single crystals of pure brucite and talc. In a second step, treatments have been extended to chrysotile, lizardite and antigorite. A characterization of the mechanism and kinetics were performed by scanning probe microscopy on the surface of single crystals phases, supported by Raman spectroscopy and by Scanning and Transmission Electron Microscopy study performed on micro- and nano-sized grains. [1] White, et al. Reaction mechanisms of magnesium silicates with carbon dioxide in microwave fields. Final Report to the U.S. Department of Energy, National Energy Technology Laboratory (2004)

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.

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.

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) Å, 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.

We investigated, by scanning and transmission electron microscopy (SEM, TEM), wavelength- and energy-dispersive spectroscopy (WDS, EDS), and electron diffraction tomography (EDT), several (Y-REE-U-Th)-(Nb-Ta-Ti) oxides from the Garnet Codera dike pegmatite (Central Italian Alps). These oxides have compositions in the samarskite-(Y) field and yield an amorphous response from the single-crystal X-ray diffractometer. Backscattered electron images reveal that the samples are zoned with major substitutions involving (U+Th) with respect to (Y+REE). At the TEM scale, the samples show a continuous range of variability both in terms of composition and in radiation damage, and the amount of radiation damage is directly correlated with the U-content. Areas with high U-content and highly damaged show crystalline, randomly oriented nanoparticles that are interpreted as decomposition products of the metamictization process. On the other hand, areas with lower U-content and radiation dose contained within 0.7×10 16 α-event/mg, although severely damaged, still preserve single-crystal appearance. Such areas, noticeably consisting of relicts of the original samarskite structure, were deeply investigated by electron diffraction techniques. Surprisingly, the retrieved crystal structure of untreated samarskite is consistent with aeschynite and not with ixiolite (or columbite), as believed so far after X-ray diffraction experiments on annealed samples. In particular, the resolved structure is a niobioaeschynite-(Y), with Pnma space group, cell parameters a = 10.804(1), b = 7.680(1), c = 5.103(1) A, and composition (Y 0.53 Fe 0.22 Ca 0.10 U 0.09 Mn 0.07 ) Σ=1 (Nb 1.07 Ti 0.47 Fe 0.34 Ta 0.07 W 0.06 ) Σ=2 O 6 . If this finding can be confirmed and extended to the other members of the group [namely samarskite-(Yb), calciosamarskite, and ishikawaite], then the samarskite mineral group should be considered no longer as an independent mineral group but as part of the aeschynite group of minerals. It is finally suggested that the rare crystalline sub-micrometric ixiolite domains, occasionally spotted in the sample by TEM, or the nanoparticles detected in highly metamict areas interpreted as decomposition product of the metamictization process, which may have in fact the ixiolite structure, act as seeds during annealing, leading to the detection of ixiolite peaks in the X-ray powder diffractograms.

We present a transmission electron microscopy (TEM) investigation of lillianite (Pb 3 Bi 2 S 6 ) and heyrovskýite (Pb 6 Bi 2 S 9 ), 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 ( hk 0) plane and a wavelength of ~75 A. 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 Bi 2 S 3 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.

Natural cordierite crystals with a range of Mg:Fe ratios have been annealed at a range of temperatures between 1200 and 1430 degreesC (under controlled f(O2)), after previously being annealed at 900 degreesC to drive off any volatiles contained within their channels. Complex microstructures were observed in these samples by transmission electron microscopy. No evidence was found for a transformation from orthorhombic symmetry to hexagonal symmetry in the bulk of the crystals. Rather, all the heat-treated samples contained pervasive glassy needles parallel to [001], each including a small bubble. These became larger and less regular in shape with increasing annealing temperature, and are interpreted in terms of early partial melting of cordierite at temperatures which are well below the macroscopic melting point. Larger glassy veins were also found, and these contained crystals of mullite. In two samples annealed at temperatures of similar to1400 degreesC or below, pockets of glass with prismatic spinel crystals were observed. Chemical analysis in the electron microscope showed that the glass phase was generally enriched in SiO2 and depleted in Al2O3 with respect to the cordierite matrix. Attempts to rationalise the chemical compositions of all the phases in terms of mass balance were not entirely successful, and it is concluded that these areas of more extensive melting probably occurred in regions of altered cordierite or around inclusions

We report about three bismuth sulfates from mineralized quartz dikes from Alfenza (Crodo, Italy), two new phases and a rare mineral, cannonite, all growing on bismuthinite. The first new phase occurs as white, "hortensia-like" aggregates of pseudo-hexagonal platelets, with perfect basal cleavage, similar to 20 mu m wide and few micrometers thick. The approximate composition is Bi2O2(SO4), and cell parameters and symmetry, as determined by automatic diffraction tomography, are a = 22.0(4), b = 16.7(3), c = 15.9(3) angstrom, beta = 102.9(5)degrees, space group Pc or P2/c. A major stacking disorder is detected by HR-SEM images and electron diffraction data.The second new phase was detected only by TEM. It can be distinguished by its random orientation on the TEM grid (i.e., absence of preferential parting), the higher resistance under the electron beam, and different cell parameters and structure, whereas the composition is similar (Bi/S similar to 2.2/1), apart for the presence of tellurium up to similar to 6 cations percents. The unit cell is hexagonal, space group P (6) over bar 2c, a = 9.5(2) and c = 15.4(3) angstrom. In this case, a structure model was obtained ab initio from electron diffraction data. Interestingly, the mineral has a porous structure with one dimensional porosity (diameter of the channel similar to 7 angstrom).Finally, within the same centimeter sized hand-specimens, we detected also cannonite. Its identification was done by automatic diffraction tomography. The measured cell parameters are a = 7.7(2), b = 13.9(3), c = 5.7(1) angstrom, beta = 109.8(5)degrees, the space group P2(1)/c. Cannonite at Alfenza forms radiating, acicular aggregates of colorless, transparent crystals with "scalpel-like" habit, elongated along c, up to 200 mu m in length.

The hydrated copper-aluminium sulphate cyanotrichite, ideally Cu4Al2(SO4)(OH)122H2O, often occurs in sky blue clumps or aggregates of sub-millimeter sized fibrous crystals. The problem of indistinguishable admixing of variable amounts of carbonate-cyanotrichite with cyanotrichite, the close association with other copper sulphates (chalcoalumite, brochantite) and the very small size of the acicular crystals hampered to date an ab initio structure determination from conventional X-ray diffraction. In light of these difficulties, we have taken advantage of the recent development of precessed automated electron diffraction tomography (ADT) combined with synchrotron powder X-ray diffraction to investigate the crystal structure of cyanotrichite. Through ADT investigation, two similar monoclinic cell were determined, corresponding to cyanotrichite (a = 10.16, b = 2.90, c = 12.64 Å and β = 92.4°) and carbonatecyanotrichite (a = 10.16, b = 2.91, c = 12.42Å and β = 98.4°). A structure model was obtained ab initio by direct methods in space group C2 from electron diffraction data and tested with the Rietveld method against X-ray powder diffraction profiles. All reflections in the powder pattern were indexed with the two cyanotrichite-like phases, according to electron diffraction data. The Rietveld analysis, consistently with electron diffraction investigations, indicates that the refined structural model has based on Al(OH)6 octahedra interconnected through common edges to build infinite columns running along b. Each Al-columns is coupled by sharing the remaining edges to two Cu-columns based on Cu distorted octahedra giving rise to ribbons along b. These ribbons are linked by SO4 tetrahedra to form corrugated layers.

A natural ferromagnesian cordierite which had been annealed at 1355°C has been found, by TEM, to contain submicroscopic cross hatched modulated microstructures (tweed) localised near Al 2 O 3 inclusions, while most of the sample, apart from some melting features, remained homogeneous and orthorhombic in symmetry. SAED patterns from regions with the tweed textures exhibited splitting parallel to [310] * and [110] * of the orthorhombic cell. Such features in cordierite have been interpreted in previous investigations as intermediate metastable states formed during the high-to-low transition associated with Al,Si ordering. Such metastable states appear to be favoured in K-bearing cordierites because the substitution Si 4+ + □ channel → K + + Al 3+ , which increases the number of the Al-O-Al bonds, hinders the development of long range Al,Si ordering and the high-to-low transition. In the present cordierite studied here, the mechanism of modulated structure formation is most probably similar but the chemical exchange involved is different and of the type R 2+ + Si 4+ → 2Al 3+ (where R 2+ is an octahedral cation). This entails the presence of some Al in octahedral coordination. It is suggested that octahedral Al would stabilise the hexagonal form of cordierite.

Clinopyroxenes from the Campiglia Marittima skarn range from hedenbergite to johannsenite (Jh 7 to Jh 100 ). Twenty single-crystals were selected for structural refinements and electron microprobe analyses. (100) twins were present and treated using a halved data set. End-member johannsenite is best approximated by the crystal Jh7GJ2, with a formula Na 0.01 Ca 0.96 Mn 0.99 Mg 0.02 Fe 0.01 Al 0.01 Si 2.00 O 6 , which was refined to R obs = 0.024. The structural change along the johannsenite-hedenbergite join is interpreted using the concept of the I-beam module. For instance, the variation of lattice parameters with composition is markedly anisotropic, being minimal along c (the I-beam direction), intermediate along a (perpendicular to the bases of adjacent I-beams) and maximal along b (parallel to the base of the I-beam). The replacement of manganese for iron expands and deforms the M1 polyhedron, and determines further coupling effects in the M2 and T polyhedra. These effects mostly consist of a more deformed M2 geometry, with longer M2-O bonds, and larger differences between the bridging and non-bridging T-O bond distances. Both the compositional and structural data indicate a complete solid solution between johannsenite and hedenbergite. All the different crystal structure parameters (lattice constants, bond distances, distortion parameters) vary continuously. Their trends suggest small values for the thermodynamic excess functions, implying limited deviations from the ideal solid solution along the johannsenite-hedenbergite join.

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 of reversals occurring along the a-axis of the tetrahedral sheets of antigorite. If we associate the known strain-induced lattice preferred orientation (LPO) of serpentine rocks to our nanoscopic observation, a strong anisotropic frictional behavior of sliding surfaces of serpentinized rocks in subduction zones can be inferred. The results obtained with other phyllosilicates such as mica muscovite and phlogopite will be also presented for comparison. Impressive phenomena can be envisaged if frictional anisotropy is introduced in the dynamical description of faults such as mechanical instabilities giving rise to a strong declination of the slip vector with respect to the plate convergence direction, explaining seismic degree of partitioning otherwise to be considered anomalous. On the other hand, for specific LPO mechanisms, a strain softening behavior of the serpentinized mantle wedge can be inferred, providing a physical explanation to its postulated aseismic character. [1] Campione M., and Capitani G. C., Subduction-zone earthquake complexity related to frictional anisotropy in antigorite, Nature Geoscience, in press, 2013.

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 ( V = 2853.13 A3), bulk modulus ( K = 64.6 GPa), and its pressure derivative ( K ′ = 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.

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 associate the second phase with a specific morphology in SEM images. Structure solution for both the phases has been attempted on the basis of automated diffraction tomography data. The orthorhombic phase, since beam sensitive and affected by stacking disorder, gave only partial results. On the contrary, the hexagonal phase gave a sound and reproducible structure. This new phase is built up by laterally offset, stacked pairs of Bi3O3 (001) ditrigonal rings, capped by SO4 tetrahedra, interleaved and further cemented by additional (001) BiO layers.

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-organic colloids are synthesized by reacting chrysotile nanotubes with ionic derivatives of metal-phthalocyanines, organic molecules which display semiconducting character in the solid state. The structure, morphology and optical properties of the magnetically driven ordered deposits of hybrid nanoparticles are analyzed and compared with those of the bare minerals. Some conclusive remarks for their technological application and perspectives are drawn.

Experimental petrology suggested that phlogopite has a peculiar mineral chemistry at high pressure: excess of Si coupled with a decrease in IVAl and deficiency in K+Na. This K-edenite exchange, □XIISi(K+Na)-1Al-1, where □XII is a vacancy in the interlayer, should imply that phlogopites incorporate significant amount of talc component. At high pressure conditions, however, in a fluid saturated system, a talc component might translate into a 10 Å phase component, being the latter phase the product of the reaction talc + H2O = 10 Å phase occurring at P = 4-5 GPa and at T = 600-700 °C. We aim to study the structure of natural and synthetic phlogopites showing this peculiar mineral chemistry. Samples were analysed by single-crystal X-ray diffraction, powder X-ray diffraction with full profile fitting, EMP, SIMS, TEM and Mössbauer spectroscopy. The natural samples were recovered from spinel and garnet wedge peridotites of the Ulten Zone Eastern Italian Alps, (Italy), whereas synthetic ones were obtained from high pressure synthesis in a K-doped lherzolite system. The single crystal X-ray refinements of natural phlogopites, together with the EMP analysis, showed two coupled substitutions: Tschermak (Si + Mg = IVAl + VIAl) and talc exchanges (□ + Si = K + Al), with no octahedral vacancies. The Rietveld refinement of the synthetic samples confirmed the ipersilicic character and K deficiency found with EMPA and SIMS analysis. The c lattice parameter of both synthetic and natural phlogopites positively correlates with the increase of vacancies at the interlayer site, pointing toward the c lattice parameter of the 10Å phase. The tetrahedral α rotation angle of natural phlogopites was about 9°, lower than that expected by Tschermak substitution. The present data indicate that the 10 Å phase component, instead of the talc component, stabilizes the phlogopite structure by reducing the α rotation under high pressure condition. This increases the pressure conditions needed to reach the upper limit value for the tetrahedral rotation.

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.

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 third-order Birch–Murnaghan equation of state, yielding V 0 = 2,914.07(23) A3, K T0 = 62.9(4) GPa, with K′ = 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.

In recent years, the study of the main polytypes of Silicon Carbide and their synthetic homologous have received renewed interest in the material science field due to the physical and electrical properties that enable SiC to be employed in high temperature, high power and high frequency electronic devices. The main focus of numerous previous studies has been the characterization of the grown-in structural defects that can damage device performance [1]. However, from a crystallographic point of view, several questions such as polytype stability, stacking fault and twinning formation, foreign polytype inclusions and interface between polytypes in syntactic coalescence, need still to be resolved taking into account the low stacking fault energy that characterizes the most important SiC polytypes. Hence, in this work an original contribute has been offered in analysing by means of HRTEM the local stacking microstructure of twinning, macrosteps on the twin boundaries and stacking faults found in a 15R twin –related lamellae [2]. HR images revealed that the perfect structure of 15R polytype (point group R3m), (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. Instead, the twin boundaries showed a zig-zag pattern (32) which passed to (23) through an isolated (33), 6H like, sequence. The electron diffraction 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) that generates the ’m2’ twin point group, and 180°-rotation around [0001] with composite symmetry 6’mm’. 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. From a geometrical point of view, the stacking sequence will match that which was experimentally observed if a 180° rotation around [0001] is visualized in terms of only some bilayers rather than considering the rotation of the whole sequence of the 15R polytype. This type of operation produces the insertion of (33), a one cell thick 6H like sequence, while preserving the nearest-neighbour geometric relationship between adjacent SiC bilayers and their polarity. Similarly, the stacking faults can be explained by imaging a 180°-rotation around [0001] of a single bilayer along a slip plane. Hence, it follows that the formation of both, stacking faults and twins, should rely on similar processes. Actually, the growth mechanism was achieved by adding material to the self-perpetuating steps formed at the sites of a single or multiple cooperating screw dislocation and a lateral step flow [1]. The local increase of nitrogen dopant content in the growth surface modified the velocity of the steps, determining step bunching and the formation of macrosteps by the coalescence of multiple SiC growth steps. When the steps were too high, the danger of step bunching increased, macro-steps formed and local super-saturation changes occurred resulting in stacking fault and twinning formation.

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-plane tetrahedral rotation angle (ditrigonalization) and wavelength shortening and curling, which in addition occur differently in the short and long halfwaves.

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 interruptions of the alternating ± 60° rotations of the 2M2 stacking sequence resulting from the occurrence of 180° rotations – i.e. nanotwins given by 180°-rotations along c*. These findings can explain the presence of residues in the Fourier density maps in single crystal structure refinements and hopefully would help to improve the disorder modelling for future structural refinements

8 páginas, 6 figuras, 7 tablas., 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., M.M. is grateful to the Secretaría de Estado de Universidades y Investigación del Ministerio de Educacion y Ciencia de España for the invitation to spend a sabbatical term at Universidad de Granada (Grant SAB2005-0191), where this study originated. F.N. is grateful to the financial support by Research Project no. CGL2007-66744 (Ministerio de Educacion y Ciencia de España).

Dark-brownish, euhedral crystals of an “allanite-like” mineral occur in a hematite-impregnated Mn-silicate rock at Kesebol, Västra Götaland, Sweden, associated with gasparite-(Ce), chernovite-(Y), rhodonite, andradite, manganoan calcite, and quartz. A structural study was carried out on single crystals—untreated, heated in air, and heated under inert atmosphere—combined with Mössbauer spectroscopy and TEM investigation. In all the untreated crystals the mean distance indicates that Me2+(Me = Mn, Fe) prevails at this site ( in the range 2.169–2.180 Å), in contrast with chemical data obtained by EPMA that yield a simplified formula Ca(REE3+⅔[]⅓)Me33+(SiO4)(Si2O7)O(OH), when normalized to Si = 3.00 apfu. Moreover, when a crystal is heated in air, all geometrical and structural variations indicate the development of an oxidation-dehydrogenation reaction, thus confirming that M3 is occupied by divalent cations before heating. The corresponding dehydrogenation is confirmed by a dramatic lengthening of the donor-acceptor distance. A crystal was annealed under inert atmosphere to verify possible effects of radiation damage on the polyhedral volumes. After prolonged annealing at 700 °C, a slight decrease of the unit-cell parameters is observed, suggesting restoring of crystallinity from a “partially metamict” state. Nonetheless, even in the annealed crystal, the distance is still consistent with a dominance of divalent cations at the M3 site. For all the examined crystals, structural data point to an octahedral cation population as follows: M1 = (Me3+, Al); M2 = (Al, Me3+); M3 = (Me2+, Me3+). This assumption is also in agreement with the Mössbauer spectrum, which was fitted to two Lorentzian quadrupole doublets for Fe3+ and one for Fe2+. Values of the isomer shifts (0.36 and 0.37 mm/s for Fe3+; 1.11 mm/s for Fe2+) and the quadrupole splitting (1.96 and 1.02 for Fe3+; 1.90 for Fe2+) show that Fe2+ (~12% of the total iron) is located in M3, while Fe3+ occupies M1 and, to lesser extent, M2. TEM-EDS investigations have revealed chemical heterogeneities related to different degree of radiation damage. In particular, areas showing poor crystallinity are relatively enriched in Si and O with respect to the highly crystalline areas, thus suggesting that EPMA chemical data are biased by the presence of metamict areas enriched in SiO2 and likely in H2O. EPMA data were therefore corrected for the excess of silica. The cation population after correction is in keeping with the structural and spectroscopic data. Disregarding minor substitutions, the ideal chemical formula for the epidote-group mineral from Kesebol is CaREEFe3+AlMn2+(Si2O7)(SiO4)O(OH), which is related to ferriallanite-(Ce) by the substitutional vector M3(Mn2+) → M3(Fe2+).

The study of SiC has been recently developed significantly because the physical and electrical properties of its basic polytypes, making it suitable as promising semiconductor material for electronic devices. Nevertheless, the device performance is often compromised by the grown-in structural defects such as dislocations, micropipes, grain boundaries, misoriented areas, etc. [1]. Moreover, due to the low staking fault energy, it is difficult to obtain single polytype material during bulk crystal growth and, consequently, it is common to find the coalescence of 6H, 4H, 15R and 3C polytypes. These polytypes differ among them for different ordered stacking sequences of SiC bilayers. Different stacking of bilayers can also generate twins but, unfortunately, works that investigate the twins of SiC are quite rare. Although several improvements have been recently obtained on the reduction of the micropipes and of the density of dislocations [2], the presence of several structural defects in the SiC crystals remains a problem that has not yet been solved. In this contribution it has been reports the results a twin micro-lamella of 15R, embedded in a macro-lamella of 15R enclosed in 6H-SiC wafer. The wafer, grown by PVT method, shows a complex syntactic coalescence of polytypes that was previously investigated by means of X-Ray Diffraction Topography, using both White-Beam-Synchrotron-Radiation-Source and MoKα1 conventional source and μ-Raman Spectroscopy. This earlier multi-analytical study permitted to localize and to identify the twin lamella that, subsequently, was prepared for TEM analyses. The electron diffraction patterns (SAPDs) taken exactly above the interface of the micro-lamella show the presence of twinning. In order to determine the twin law, comparisons among SAPDs and simulations of the diffraction pattern introducing all the possible twin operations were carried out. Moreover, taking into account also the XRDT and μ-Raman data previously obtained, it has been found that the twin law consists of π-rotation around [0001] with (0001) as composition plane. Additionally, the analysis of SAPDs permits also to determine that the twin index is 3 and the obliquity is zero. High Resolution (HRTEM) images of the twin boundaries reveal that the interpretation of the contrasts is not trivial because of the presence of numerous adjoining stacking faults parallel to (0001)with interleaving (33) and (22) lamellae. The Fourier Transform calculated across the boundary (33)/(22) does not shows any splitting and thus this sequence introduce no twinning within the 15R but consists of 4H and 6H like sequences. On the contrary, HRTEM images taken across the boundaries can be interpreted with reference to the primitive oblique 2D cell of the 15R polytype as a zig-zag pattern (23) that switches to (32) by a twin coherent interface or, alternatively, as zig-zag pattern (32) which pass to (23) trough an isolated (33) lamella. In other words, the twin interface can be also considered as the interposition of a 6H lamella and thus the origin of the twin lamella is strictly related to the stacking faults. Consequently, it can be concluded that the low energy of stacking faults in 15R SiC [3] favours also the growth of twin lamellae.

Natural crystals of tourmaline are characterized by striations parallel to the c axis on prismatic faces. Crystals without striations are really rare, so it is important to understand the origin and the meaning of striations. Different hypotheses were made about the origin of the striations and the most important of which are: 1) juxtaposition of prismatic facets, that make curved prism surfaces, 2) multiple parallel twinning. A different conclusion was made in a study of tourmaline crystals from pegmatite pockets of island of Elba (Italy) [1]: the origin of striations was attributed to a particular type of growth mechanism. Two growth stages were identified in the quoted study from both defect analysis and crystalchemical characterization of the samples. The former stage was the pegmatitic, corresponding to the inner greenish-yellow region dislocation-free, that was characterized by growth bands and growth sector boundaries. Therefore, 3D nucleation occurred and the bulk tourmaline crystals grew either by 2D mechanism or by direct adsorption of growth units, or both these mechanisms at once. The latter stage was hydrothermal and regarded the colorless overgrowth characterized by screw dislocations. During this last growth stage striations were formed. Small crystal grains, in fact, developed parallel to the c axis by spiral growth mechanism. The imperfect juxtaposition of prismatic facets of each crystal grain caused the striations. Nevertheless, it should be emphasized that the absence of striations on prismatic facets of core during the pegmatitic growth stage has not been found yet. In order to deeply investigate the origin of striations, in this study other samples of tourmaline from island of Elba with a greenish-yellow core and colorless overgrowth have been analyzed. In particular, sample fragments with missing external overgrowth have been investigated. Hence it has been possible to observe optically that the prismatic facets of inner core did not show striations. XRDT studies of basal slices confirm that the core is dislocation-free, while the colorless overgrowth is characterized by a high density of dislocations. Therefore these observations confirm the conclusions of Agrosì et al. [1]. In addition, to investigate the interface between core and rim, smaller fragments of tourmalines have been studied by TEM. Preliminary HRTEM results show a perfect structural continuity between inner core and external rim. Moreover, the interface does not correspond to a integer surface but correspond to a transition region characterized by a high concentration of impurities.

The mechanism of the intracrystalline cation exchange reaction in a natural columbite sample from Kragero, with composition (Mn0.85Fe0.15)(Nb1.8Ta0.2)O6, has been characterised by means of TEM and HR synchrotron-radiation powder diffraction. In columbite, cations may occupy two different octahedral sites, with divalent cations preferring the A site and pentavalent cations preferring the B site. When both the octahedral sites are randomly occupied, columbite adopts a α-PbO2-type crystal structure; ordering of divalent cations on A sites and pentavalent cations on B sites gives rise to an ...ABBABB… sequence along the a direction resulting in a tri-α-PbO2-type structure in which the Pbcn symmetry is maintained. The completely ordered scheme represents the only thermodynamically stable state. Nevertheless, partially disordered distributions of cations among the sites are often found in natural samples. High temperature treatment of these samples leads to the completely ordered state [1, 2]. Two single crystals with different degrees of order obtained by quenching experiments were prepared for TEM study. They revealed a pervasive presence of nano-sized domains with rhombic shape and sides parallel to (310) planes. Diffraction contrast images and high resolution images clearly show that rhombic domains are ordered and dispersed in a disordered matrix. In situ high temperature powder diffraction data collected at ESRF ID31 starting from untreated powders (degree of cation order Q ~ 0.15) and until completion of the ordering process shows that the sluggish cation ordering process in columbite is more complicated than the simple picture which is deducible from laboratory X-ray diffraction studies. The diffraction patterns show split reflections and require analysis using two distinct columbite phases with different degrees of order. The phase characterised by a higher Q value has slightly broader peak profiles. This is consistent with TEM observations of some small ordered domains in a largely disordered material. Furthermore the size of these ordered domains in the starting material, as derived by peak-profile analysis, is also consistent with TEM observations. Preliminary Rietveld analyses exclude a nucleation and growth model followed by coarsening of the domains, and indicate two stages for the cation ordering mechanism: a first stage in which the degree of order increases and the domains size remains nearly constant and a second one in which the growth of the domains accompanies the achievement of the complete cation order. PDF (Pair Distribution Function) analysis of patterns collected at different Q will allow to give further insights and a better understanding of the factors controlling the columbite ordering process.

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 comparison, the total energy difference between the two m = 17 antigorite models (5.42 meV/atom) is close to the energy difference between andalusite and kyanite. Moreover, has highlighted by the largest atomic displacements during relaxation, the energy excess is almost entirely due to the octahedral offsets, where Mg atoms at the centres of triangular prisms entail unfavourable crystal-chemical bonding. Future calculations will be focused to evaluate the antigorite compressibility.

The late crystallisation stages of Campiglia Marittima skarn are characterised by an increase in Mn-activity at the meta-somatic 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 //(111) 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.

The nature of the amorphous regions and their recovery processes in two natural metamict zircon samples from Sri Lanka have been studied by high resolution and analytical transmission electron microscopy. Samples untreated and annealed at different temperatures were investigated. Nanoprobe analyses on untreated samples and samples annealed at 1000 °K show that within experimental uncertainties, no chemical segregation occurred. In samples annealed at higher temperatures (>= 1100 K) recovery occurs in a two stage process and leads to different microstructures, which depended on the initial amount of metamictization. In highly amorphized samples, recrystallisation starts at 1200 K. Randomly oriented ZrO2 grains embedded in a silica rich matrix are detected. At higher temperature (16 hours at 1600 K), the assemblage transforms into a polygonal texture of small zircon grains. Some untransformed zirconia grains and pockets of silica-rich glass are still present, however. In partially metamict samples, recovery starts at 1100 K. The small surviving oriented zircon domains grow at the expense of the surrounding amorphous material. At 1200 K, new zirconia grains nucleate with random orientations. After 1 hour annealing at 1400 K, the zircon structure is restored and the microstructure coarse-grained. The proportion of crystalline zirconia and silica-rich glass has dramatically decreased.

Johannsenite, along with hedenbergite and ilvaite, occurs in a skarn of the Campiglia Marittima Cu-Pb-Zn metasomatic ore deposit (central Italy). The skarn formed at low pressure, under decreasing temperature and in redox conditions which changed from oxidising to reducing. The clinopyroxenes are arranged in spheroidal bodies consisting of radiating crystals tens of centimetres long which formed under fast crystallisation rates. Optical and scanning electron microscopy (SEM) revealed two types of lamellar features. In one case the lamellae are parallel to [001], the morphological elongation of the crystal, but in other cases they are almost perpendicular. Single-crystal X-ray diffraction patterns of samples with lamellae parallel to [001] contain reflections forbidden for the C2/c space-group and compatible with either (100) or (10¯2) twin planes. High Resolution Transmission Electron Microscopy (HRTEM) confirms that (100) is the twin plane. HRTEM also shows the presence of a translational component necessary to achieve the twin structure. At the level of the crystal structure, the twin interface appears as an out of step boundary, with the two opposite portions related by a b-glide. The lamellar features almost perpendicular to [001] also derive from twinning, but in this case (001) is the twin plane. In contrast with the ubiquitous (100) twins, (001) twins are restricted to manganese-rich crystals with more equidimensional habit. The two twin types never occur together in the same crystal. Both (100) and (001) twins are obtained by reticular pseudo-merohedry (or alternatively, by twin-lattice quasi-symmetry, TLQS). They are interpreted as primary growth features.