István Dódony

STRUCTURAL RELATIONSHIP BETWEEN PYRITE AND MARCASITE

Abstract We studied pyrite spherules built of radiating crystals to establish the relationships among microstructure, composition, and macroseopic appearance. Results of high-resolution transmission electron microscopy (HRTEM) imaging and selected-area electron diffraction (SAED) show that many pyrite crystals contain planar faults perpendicular to one of the [001] axes. A comparison between HRTEM micrographs and images simulated for defect model structures indicates that the faults can be interpreted as single (101) layers of marcasite that disrupt the regular sequence of (002) layers in pyrite. Such a marcasite layer can be described as a boundary between two pyrite crystals that are related by a 21 screw axis parallel to [100]. Energy-dispersive X-ray spectrometry (EDS) indicates that such disordered pyrite crystals contain about 3 at% As. However, the distribution of As is uniform over heavily faulted and fault-free regions, indicating the As content is not related to the occurrence of marcasite lamellae in pyrite.

Revised structure models for antigorite: An HRTEM study

Abstract We have revised the structure model of antigorite so that they conform to observations made using high-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction. The new models retain the original half-wave configuration proposed by Kunze (1956). The Kunze model, and all subsequent research, assumes the occurrence of four- and eightmembered silicate rings in one of the two places where there are reversals of tetrahedron orientations in the tetrahedral sheets. However, TEM images at sufficiently high resolution show no traces of such rings and only half the number of octahedral-sheet offsets as occur in the Kunze model. Using our measurements and models, we generated atom positions for antigorite unit cells having various modulation lengths and then calculated the corresponding images, which provide good matches with our experimental HRTEM images. We also characterized and described antigorite structures with different modulation wavelengths and stacking sequences. Depending on the number of polyhedra in a unit cell and the presence or absence of b/3 shifts between adjoining tetrahedral and octahedral sheets, the antigorite crystals have monoclinic or triclinic symmetry, which we call antigorite-M and antigorite-T, respectively. By resolving the tetrahedral and octahedral positions, we were able to make a direct estimate of the compositions of specific antigorite samples.

Serpentines close-up and intimate: An HRTEM view

High-resolution transmission electron microscopy (HRTEM) affords a close look at the complex structures and intergrowths of the serpentine minerals. All contain alternating sheets of cations in tetrahedral and octahedral coordination. Lizardite, the flat species, forms in sufficiently large and well-ordered crystals to permit reliable X-ray structure determinations, and it is the reference mineral for estimates of the structures of antigorite and chrysotile. However, even lizardite forms in a wide variety of polytypes, only some of which have been explored. It also forms polygonal serpentine, a roughly cylindrical variety that typically consists of 15 or 30 sectors, each of which consists of lizardite layers. HRTEM images of the structure at sector boundaries show offsets of fringes that we interpret as the result of inversions of the tetrahedral sheets. Using lizardite as the basis for an estimate of the curled chrysotile structure, we obtained atomic coordinates and used them to calculate fiber-axis X-ray and electron-diffraction patterns for polytypes. HRTEM images obtained viewing down the fiber axis show no ordering between layers. Chisholm (1988) reported 2mm symmetry for fibers when viewed perpendicular to their length, but most of our measured fibers show no symmetry for such orientations. Employing Fourier transforms of HRTEM images, we found a new one-layered orthorhombic chrysotile polytype with mirror symmetry perpendicular to the fiber axis. Antigorite is notable for its conspicuous, modulated structure. We observed the waves to be asymmetrical and infer that the asymmetry results from an inhomogeneous distribution of hydrogen bonding between the layers. This distribution helps with a long-standing problem by explaining some apparently anomalous features of HRTEM images. The abundant (001) faults in antigorite are produced by boundaries of lamellae having different modulation profiles. HRTEM images show the relations of serpentine minerals to each other as well as their host materials. Areas exist where layers of each of the serpentine minerals grade continuously and free of faults from one variety to another. The resulting intermediate or partial structures defy categorization into simple mineral types.

Protoanthophyllite from three metamorphosed serpentinites

Abstract This is the first report of a natural Mg-rich protoanthophyllite. It is common in metamorphosed serpentinites from three Japanese ultramafic complexes, and some crystals contain anthophyllite (Pnma) lamellae. The Mg/(Mg + Fe) ratios of the Hayachine, Tari-Misaka, and Takase protoanthophyllites are 0.90, 0.92, and 0.91, respectively. The samples have identical optical properties: X = a, Y = b, Z = c, and 2Vx = 64 ± 5°. Their space group is Pnmn (or Pn2n), as revealed by systematic extinctions in selected-area electron-diffraction patterns. The protoanthophyllite and anthophyllite have similar compositions and orthorhombic symmetry. They are difficult to distinguish using optical, microanalytical, and powder X-ray diffraction measurements. This problem raises the possibility that some of the published data on geological and synthetic anthophyllite samples may be of misidentified materials, potentially leading to errors in the published stability relations of anthophyllite. We provide a method to identify protoanthophyllite and differentiate it from its polymorphs using selected-area electron diffraction and high-resolution transmission electron microscopy methods.

The real structure of ε-Ga2O3 and its relation to κ-phase

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called e phase; i.e. they exhibited hexagonal P63mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna21 space group symmetry, referred to as κ-Ga2O3. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P63mc space group symmetry usually referred to as e-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna21 space group and previous observations.

Does antigorite really contain 4- and 8-membered rings of tetrahedra?

Abstract Recent studies of the structure of antigorite by Capitani and Mellini (2004, 2005) and by us (Dódony et al. 2002; Dódony and Buseck 2004a) produced contradictory results. The main point of contention is whether 4- and 8-membered rings of tetrahedra occur at the positions where the tetrahedra in the tetrahedral sheets reverse their orientation. We analyzed electron diffraction patterns and transmission electron microscopy images in the paper of Capitani and Mellini (2005) and found no evidence for 4- and 8-membered rings of tetrahedra. On the contrary, we show that their TEM data confirm our antigorite model (Dódony et al. 2002) for the m = 16 structure. The significance of this debate goes beyond the subtleties of the structure of antigorite and highlights ambiguities in interpretation of HRTEM images as well as problems that can arise during image processing.

Lizardite-chlorite structural relationships and an inferred high-pressure lizardite polytype

Abstract We determined details of the layer stackings in lizardite (variety: baltimorite) and chlorite in a splintery sample from a massive serpentinite. Based on high-resolution transmission electron microscopy (HRTEM) and image simulations, we localized the positions of projected columns of tetrahedral (T) and octahedral (O) cations that allow determination of the nature of the transformation of lizardite to chlorite and the structural formula of the chlorite. The lizardite stacking is of an unexpected type because adjacent layers are in non-hydrogen-bonded placements, a configuration that permits easy, strain-free transformation to chlorite. We conclude that the lizardite-chlorite transformation was isochemical and at constant volume. A consequence is that the product chlorite consists of neutral brucite- and talc-type layers and so can be regarded as a 1:1 interstratification of brucite and talc. Both the T and the O sheets in talc-type layers contain M3+ (M = Al, Fe, Cr, ...) cations. An implication of our results combined with high-pressure studies of chlorite and TO silicates is that lizardite stacking sequences can indicate whether a given crystal has been subjected to high pressures. Lizardite in non-hydrogen-bonded sequences presumably has this configuration because of its prior exposure to elevated pressures and can thus be added to the list of indicators of past high pressure. Calibration of this new geobarometer remains for the future.`

Crystal structure of protoanthophyllite: A new mineral from the Takase ultramafic complex, Japan

Abstract Protoanthophyllite, (Mg, Fe)7Si8O22(OH)2, is a newly discovered amphibole species from the Takase ultramafic complex in Japan. It occurs as prismatic crystals up to 5 mm in length in a thermally altered serpentinite that experienced contact metamorphism. The protoanthophyllite is associated with forsterite, talc, serpentine minerals, chlorite, chromian spinel, magnetite, pentlandite, and calcite. Some protoanthophyllite crystals contain minute lamellae of anthophyllite, other pyriboles, or both. Protoanthophyllite is biaxial negative, with refractive indices nα = 1.593(2), nβ (calc.) = 1.609, nγ = 1.615(2), and 2Vx = 64(5)°. Electron microprobe analyses give an empirical formula of (Mg6.31Fe0.61Na0.06Mn0.01Ni0.01)Σ7.00(Si7.90Al0.14)Σ8.04O22(OH)2. It is orthorhombic with space group Pnmn. The unit-cell dimensions are a = 9.3553(8), b = 17.9308(15), and c = 5.3117(4) Å: with V = 891.0(3) Å3 and Z = 2. A single-crystal X-ray structure determination shows that, following the convention of Thompson (1981), protoanthophyllite has an (X) configuration. The topology of the silicate tetrahedral chains is similar to that of the anthophyllite A-chains. Silicate tetrahedral chains are O-rotated in protoanthophyllite, whereas those in protoferro-anthophyllite are S-rotated. Iron atoms are concentrated in the 4-coordinated M4 sites. The unit-cell volume is ~1.5% larger than the equivalent volume of anthophyllite with Mg/(Mg + Fe) = 0.885, suggesting a high-temperature or low-pressure stability relative to anthophyllite, assuming that protoanthophyllite is not metastable.

Amorphous and partly ordered structures in SiO2 rich volcanic glasses. An ED study

Glass structures of obsidian and pumice samples were measured using electron diffraction. Amorphous, partly ordered, and nanocrystalline regions were distinguished and analysed separately. The deconvoluted atomic distances obtained from experimental diffraction patterns through total pair-distribution functions are consistent with distances for ideal SiO 4 tetrahedra. Partly ordered structures in pumices are composed of plate-like fragments of tridymite/cristobalite layers, whereas obsidian contains quartz nanocrystals with abundant moganite-like planar faults. The validity of the structure-model of Goodman for silica glasses is discussed and an alternative interpretation is proposed for silicic volcanic glasses.

Report on the natural occurrence of a silica-clay nanocomposite

A natural occurrence of a silica/clay nanocomposite material was investigated by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). High-resolution images show that this nanocomposite material consists of 5–20 nm thick slabs of smectite and tridymite/cristobalite layers with coincident normals. In spite of the brittle glass-like appearance of the nanocomposite material its colloidal properties are similar to those of pure smectite but partial loss of expansion capacity was detected upon glycerol solvation. The structural relationship between smectite and silica is interpreted based on the smectite structure model of Edelman and Favejee (1940) which supposes reversed tetrahedra in the SiO4 layer of the TOT structure. This structure model explains the presence of silica impurities in bentonites used as raw material and several geological standard montmorillonites.