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The Monte Fico lizardite crystals have an internal skeletal spongy microstructure, formed by two micrometric domains having different optical reliefs. This intracrystalline microstructure parallels the previously reported intercrystalline arrangement, consisting of lizardite prisms within a chrysotile plus polygonal serpentine matrix. In the high-wavenumber region, the larger and more abundant domains (that represent approximately 87ĝ€¯% of the total field view) produce μ-Raman spectra characterized by two major peaks at 3686 and 3705ĝ€¯cm-1. The smaller, less abundant domains present a wide band confined between these wavenumbers. These features are interpreted as lizardite and chrysotile, respectively. Raman results are confirmed by TEM, which emphasizes the presence of well-recognizable polygonal serpentine too. Tight crystallographic control exists between lizardite and this first serpentine generation. A second serpentine generation occurs perpendicularly to the first one. The lizardite crystals grew up with a skeletal habit, whereas chrysotile fibres and polygonal serpentine filled the voids, growing epitactically on the lizardite crystals, with fast crystal growth in a fluid-rich environment.

This study provides the first characterization of Fe-rich antigorite (FeOtot up to 12 wt%), a rock-forming mineral occurring in ophicarbonate rocks from different low-temperature/high-pressure meta-ophiolitic suites: Acceglio (Traversiera Valley, external Piemonte Zone, NW Italy), Macedonia and Verias (Thessaloniki and Vurinos-Kozani ophiolitic complexes, NE Greece), Tinos (Tinos Island, Cyclades Archipelago, Greece). Fe-rich antigorite has been characterized through optical and transmission electron microscopy (TEM), and its mineral chemistry has been investigated by means of wavelength-dispersive and TEM-based energy dispersive spectrometry. In thin section, Fe-rich antigorite is characterized by a strong, peculiar pleochroism (alfa = green-dark green; gamma = bright-yellow-orange). It occurs in both mesh and bastite microstructures, and it is locally associated with relics of lizardite and/ or chrysotile. The modulated lattice parameters of disordered Fe-rich antigorites have been determined by electron diffraction in the transmission electron microscope. The values are highly variable, even within each ophicarbonate sample. Verias dominantly has a superlattice parameter a clustering around 43.5 Å (corresponding to the m = 17 polysome); Tinos and Macedonia have around 35.4 Å (m = 14); Acceglio may even go down to 29 Å (m = 12). Globally, the disorder features (i.e., reduced size of crystals, polysomatic faults, wobbling, misalignment among sublattice and superlattice reflections, etc.) increase from Macedonia to Verias, Tinos and Acceglio, respectively. The analyzed Fe-rich antigorites accommodate up to 12 wt% FeOtot, with XFe values (XFe = Fetot/[Mg + Fetot]) in the range 0.10-0.16 for Macedonia, 0.05-0.17 for Acceglio, 0.10-0.12 for Tinos and 0.05-0.10 for Verias. The intensity of the pleochroism seems to be directly correlated with the Fe content, with the Fe-richer samples showing the deeper absorption colours. Mineral relationships and TEM observations suggest that Fe-rich antigorite replaces former mesh and bastite microstructures consisting of lizardite ± chrysotile, only locally preserved as relict phases. The thermodynamic modelling approach (i.e., P/T-X(CO2) pseudosection) qualitatively shows that the stability of Fe-rich antigorite is compatible with low-temperature, high-pressure conditions (i.e., blueschist-facies metamorphic conditions), and is enhanced by the occurrence of CO2 in the fluid, consistent with the systematic occurrence of this mineral in meta-ophicarbonate rocks.

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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 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 modulated crystal structure of an antigorite polysome with m = 17 was refined by single crystal X-ray diffraction in the Pm space group, using highly ordered single crystals from Val Malenco, Italy. The chemical composition is 007Ni0.003Mn0.002) ∑=2.823(Si1.997Al0.003)∑=2O5(OH)3.639. Lattice parameters [a = 43.505(6), b = 9.251(1), c = 7.263(1) Å, β = 91.32(1)°] were determined using a single-crystal diffractometer equipped with an area detector. The structure was refined using 9242 independent reflections, obtaining a final R4σ factor of 0.0577. A continuous, wavy octahedral sheet is linked to a tetrahedral sheet with tetrahedral apices alternatively pointing +c and -c. This sheet is located on the concave side of the octahedral-sheet wave. The octahedral sheet shows normal thickness for a serpentine of this composition, and does not have any internal offset. The tetrahedral sheet inverts its polarity through six- and eight-membered tetrahedral rings (6- and 8-reversals). Between reversals, 6-membered rings are distorted toward ditrigonal configuration, with tetrahedral rotation, α values, ranging along the wave from 4 to 13.6°. The two half-waves have curvature radii of 99.4 and 110.9 Å. Variable interlayer O-O distances occur, indicating the absence of homogeneous, continuous hydrogen bonding. The bond geometry, very similar to that of lizardite, suggests common crystal chemical and geochemical properties. The larger stability field of antigorite compared to lizardite is interpreted to arise from the occurrence of three-dimensionally connected chemical bonds

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.