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Byline: Fabrizio Nestola (1,2), Ross J. Angel (3), Jing Zhao (3), Carlos J. Garrido (4), Vicente Lopez Sanchez-Vizcaino (5), Giancarlo Capitani (6), Marcello Mellini (7) Keywords: Antigorite; High pressure; Softening; X-ray diffraction Abstract: 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 .sub.0 = 2,914.07(23) A.sup.3, K .sub.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. Author Affiliation: (1) Dipartimento di Geoscienze, Universita di Padova, Via Giotto 1, 35137, Padova, Italy (2) CNR-IGG, Sezione di Padova, Via Giotto 1, 35137, Padova, Italy (3) Crystallography Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061, USA (4) Facultad de Ciencias, Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC & UGR, 18002, Granada, Spain (5) Departamento de Geologia, Universidad de Jaen (Unidad Asociada al CSIC-IACT Granada), Escuela Politecnica Superior, Alfonso X El Sabio 28, 23700, Linares, Spain (6) Dipartimento di Scienze Geologiche e Geotecnologie, Universita degli Studi di Milano Bicocca, P.za della Scienza 4, 20126, Milan, Italy (7) Dipartimento di Scienze della Terra, Universita di Siena, Via Laterina 8, 53100, Siena, Italy Article History: Registration Date: 02/11/2009 Received Date: 20/03/2009 Accepted Date: 02/11/2009 Online Date: 19/11/2009 Article note: This paper is dedicated to the memory of Prof. Volkmar Trommsdorff whom we knew as a gentleman and great scientist. Communicated by M.W. Schmidt.

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. Keywords: Electron diffraction, lattice parameters, elliptical distortion, mica

The microstructural features of a biotite (Mg-rich annite) crystal from dacite rocks of Dzep, Serbia, were studied by X-ray diffraction topography (XRDT) and high-resolution and analytical transmission electron microscopy (HRTEM and ATEM, respectively). The average chemical composition, obtained by electron microprobe analysis (EMPA) and secondary-ion mass spectrometry (SIMS) data, is [([K.sub.0.87][Na.sub.0.05][Ca.sub.0.01]).sub.[summation]=0.93] [([Fe.sup.2+.sub.1.36][Mg.sub.1.25][Ti.sub.0.22] [Al.sub.0.14][Mn.sub.0.03][Li.sub.0.01].sub.[summation]=3.01] [([Si.sub.2.84][Al.sub.1.16]).sub.[summation]=4.00] [O.sub.10][[[(OH).sub.1.53][O.sub.0.35][F.sub.0.10] [Cl.sub.0.02]].sub.[summation]=2.00] Inhomogeneous regions from either different polytypes or twins were mapped by XRDT, and these images were used to guide sampling for TEM analysis. Three stacking arrangements, each belonging to subfamily-A of mica polytypes, were identified by selected area electron diffraction (SAED) patterns and HRTEM images: (1) dominant semi-random 1[M.sub.1]-n(120[degrees]), including an occasional repetition of long-period inhomogeneous stacking sequences belonging to the 3T structural series; (2) highly faulted 2[M.sub.2], sometimes with short-range-ordered long-period inhomogeneous sequences based on the 2[M.sub.1] structural series; and (3) perfectly ordered 3T stacking repeating over long distances (several micrometers). ATEM data revealed that the microchemical composition of 3T differs from that of the host matrix. The 3T is enriched in interlayer cations (K and Na), and depleted in Si and octahedral cations. No evidence of Li was detected. Compositional control on the stability of 3T stacking is suggested. Keywords: Polytypism, biotite, electron microscopy, chemical analysis

Dark-brownish, euhedral crystals of an 'allanite-like' mineral occur in a hematite-impregnated Mn-silicate rock at Kesebol, Vastra Gotaland, 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 Mossbauer spectroscopy and TEM investigation. In all the untreated crystals the mean distance indicates that [Me.sup.2+] (Me = Mn, Fe) prevails at this site ( in the range 2.169-2.180 [Angstrom]), in contrast with chemical data obtained by EPMA that yield a simplified formula Ca([REE.sup.3+.sub.2/3][[??].sub.1/3])[Me.sup.3+.sub.3](Si[O.sub.4]) ([Si.sub.2][O.sub.7]) 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[degrees]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 = ([Me.sup.3+], Al); M2 = (Al, [Me.sup.3+]); M3 = ([Me.sup.2+], [Me.sup.3+]). This assumption is also in agreement with the Mossbauer spectrum, which was fitted to two Lorentzian quadrupole doublets for [Fe.sup.3+] and one for [Fe.sup.2+]. Values of the isomer shifts (0.36 and 0.37 mm/s for [Fe.sup.3+]; 1.11 mm/s for [Fe.sup.3+]) and the quadrupole splitting (1.96 and 1.02 for [Fe.sup.3+]; 1.90 for [Fe.sup.2+]) show that [Fe.sup.2+] ([approximately equal to]12% of the total iron) is located in M3, while [Fe.sup.3+] 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 Si[O.sub.2] and likely in [H.sub.2]O. 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 CaREE[Fe.sup.3+] Al[Mn.sup.2+]([Si.sub.2][O.sub.7])(Si[O.sub.4])O(OH), which is related to ferriallanite-(Ce) by the substitutional vector [sup.M3]([MN.sup.2+] [right arrow] [sup.M3]([Fe.sup.2+]. Keywords: Epidote-group, allanite-subgroup, single-crystal X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy, metamictization

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 Mg 159 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 [Angstrom]. 'Odd' and 'even' antigorite polysomes differ in layer topology. 'Odd' polysomes (m [not equal to] 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 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 [([Mg.sub.2.673][Fe.sup.2+.sub.0.098][Fe.sup.3+.sub.0.015] [Al.sub.0.035][Cr.sub.0.007][Ni.sub.0.003][Mn.sub.0.002]).sub .[summation over]=2.823][([Si.sub.1.997][Al.sub.0.003]).sub.[summation over] =2[O.sub.5][(OH).sub.3.639. Lattice parameters [a = 43.505(6), b = 9.251(1), c = 7.263(1) A, [3 = 91.32(1)[degrees]] were determined using a single-crystal diffractometer equipped with an area detector. The structure was refined using 9242 independent reflections, obtaining a final [R.sub.4[sigma]] 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, values, ranging along the wave from 4 to 13.6[degrees]. The two half-waves have curvature radii of 99.4 and 110.9 [Angstrom]. 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.