Gian Carlo Capitani

The 6H-SiC structure model: Further refinement from SCXRD data from a terrestrial moissanite

Abstract 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 reß ections 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.

The crystal structure of a second antigorite polysome (m = 16), by single-crystal synchrotron diffraction

Abstract A model for the modulated crystal structure of an antigorite polysome with m = 16 (where m is related to the number of tetrahedra spanning a wavelength along a) was refined by single-crystal synchrotron diffraction data in C2/m, using crystals coexisting with the m = 17 polysome from Val Malenco, Italy, which was previously determined structurally. Lattice parameters [a = 81.664(10), b = 9.255(5), c = 7.261(5) Å, β = 91.409(5)°] were determined using a single-crystal diffractometer equipped with an area detector at the Desy synchrotron (Hamburg). The structure was solved by direct methods, and the model refined using 19 222 symmetry-related reflections. The final R4σ factor was 0.0951, calculated for 7246 reflections. The structure of the m = 16 antigorite polysome strongly resembles that of the m = 17 polysome. A continuous, wavy octahedral sheet is linked to a tetrahedral sheet, reversing its polarity through sixfold tetrahedral and eightfold tetrahedral rings. The half-wave has a curvature radius of 80.1 Å. Polyhedral geometry, ditrigonalization angles, and interlayer O-O distances are similar in the two polysomes. The only differences concern the number of tetrahedra for the m = 16 polysome (an even number which leads to symmetric half-waves) and the periodic b/2 shift involving the eightfold rings (to produce the doubling of the a parameter and a C-centered cell).

The Bi sulfates from the Alfenza Mine, Crodo, Italy: An automatic electron diffraction tomography (ADT) study

Abstract 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, ~20 μm 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) Å, β = 102.9(5)°, 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 ~ 2.2/1), apart for the presence of tellurium up to ~6 cations percents. The unit cell is hexagonal, space group P62c, a = 9.5(2) and c = 15.4(3) Å. 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 ~7 Å). 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) Å, β = 109.8(5)°, the space group P21/c. Cannonite at Alfenza forms radiating, acicular aggregates of colorless, transparent crystals with “scalpel-like” habit, elongated along c, up to 200 μm in length.

First-principles energetics and structural relaxation of antigorite

Abstract 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 exchangecorrelation, 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 (ΔE = 1.1 kJ/mol·atom-1) and after full cell relaxation at constant volume (ΔE = 0.5 kJ/mol·atom-1).

A first-principle investigation of antigorite up to 30 GPa: Structural behavior under compression

Abstract 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.

Structural insights and elasticity of single-crystal antigorite from high-pressure Raman and Brillouin spectroscopy measured in the (010) plane

Abstract We report high-pressure Raman and Brillouin spectroscopy results measured in the (010) plane of a natural antigorite single crystal. We find that structural changes at >6 GPa lead to (1) an intensity crossover between Raman modes of the Si-O-Si bending vibrations, (2) changes of the compression behavior of Raman modes related to the SiO4 tetrahedra, (3) changes of the pressure derivative of the Raman shifts associated with OH stretching vibrations, (4) the emergence of a new Raman band in the OH spectral region, (5) a softening of the elastic constants c33 and c11, and (6) a directional change of the slowest compressional wave velocity in the a-c plane. In addition to the structural insights at high-pressure, the unique characteristics of our single-crystal sample allows for first direct measurements of the acoustic velocity anisotropy in a plane perpendicular to the basal a-b plane. Comparison to previously published data indicates that the elastic anisotropy of antigorite strongly depends on the FeO and/or Al2O3 content. In contrast, it seems not to be affected by increasing temperature as inferred from an additional high-temperature experiment performed in our study. These constraints are important for the interpretation of seismic anisotropy observations in subduction zone environments.

Transmission electron microscopy investigation of Ag-free lillianite and heyrovskýite from Vulcano, Aeolian Islands, Italy

Abstract 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 wellordered 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

The johannsenite-hedenbergite complete solid solution: clinopyroxenes from the Campiglia Marittima skarn

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.

Strain modulation around inclusions in an annealed natural cordierite

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.

“Early partial melting” in annealed natural cordierites

Natural cordierite crystals with a range of Mg:Fe ratios have been annealed at a range of temperatures between 1200 and 1430°C (under controlled f o2 ), after previously being annealed at 900 °C 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 ∼ 1400 °C 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 SiO 2 and depleted in Al 2 O 3 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.