Scholarly Works, Geosciences

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Stability of zircon u-pb systematics in a greenschist-grade mylonite - an example from the rockfish valley fault zone, central Virginia, USA
Wayne, D. M.; Sinha, A. K. (University of Chicago Press, 1992-09)
The mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone (RVFZ) of central Virginia cuts the Grenville-aged Pedlar River Charnockite Suite (PRCS) and contains zircons that underwent brittle failure during ductile deformation. Electron microprobe analyses and scanning electron microscope (SEM) backscattered electron (BSE) imaging show that zircons from the protolith PRCS are concentrically zoned (with alternating U-Hf-rich and U-Hf-poor bands), and contain numerous radial microcracks. Zircons from the RVFZ mylonite are unzoned, fragmented, show no internal microfractures, and have low U and Hf concentrations relative to the PRCS zircons. U-Pb isotopic studies of zircons from the mylonites and from the charnockitic protolith demonstrate that no preferential Pb loss occurred in the zircons from the mylonite, and that the Pb-207/Pb-206 ages of the mylonite zircons are identical to those of the protolith zircons. The loss of primary zoning from the zircons of the RVFZ ultramylonites can be explained by the physical removal of microfractured, U-rich, alpha-damaged zircon domains as the result of brittle failure and disaggregation during mylonitization. Mechanically resistant (low-U) portions of zircon grains tended to remain intact in the mylonite. Thus, it may not always be possible to predict whether or not the zircon U-Pb system has been disturbed by mylonitization by using physical criteria (e.g., grain size reduction, obliteration of primary zoning textures) alone. Evidently, fluids present during mylonitization accomplished the hydration of primary mineral assemblages, but did not chemically interact with zircons, and their primary U-Pb and Pb-Pb ages were preserved.
Structure of the subducting Nazca plate beneath Peru
Norabuena, E. O.; Snoke, J. A.; James, D. E. (American Geophysical Union, 1994-05)
Arrival times from intermediate-depth (110-150 km) earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east-central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern Peru. Observed P wave arrival times for coastal stations in Peru, some 3-6-degrees from the epicenters, are up to 4 s early relative to predicted arrival times based on the best fit velocity-depth model used for hypocenter locations. These large negative time residuals appear to be the result of propagation paths which have long segments in the colder, higher-velocity subducting plate. P wave travel times were modeled for the effects of the slab using three-dimensional (3-D) ray tracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6% lower within the uppermost slab (assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick) and 8% higher within the cold uppermost mantle of the slab. Ray tracing for this plate model shows that P wave ''shadow zones'' can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the clip angle of the slab changes significantly. Such geometries exist for seismic waves propagating to some coastal stations from intermediate-depth earthquakes located east of the Andes. Observed first-arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for ''direct'' paths at similar distances. Modeling such arrivals as internally reflected waves propagating through the high-velocity part of the plate produces a significant improvement in the travel time residuals. For the slab velocities given above, we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction.
First-principles study of several hypothetical silica framework structures
Teter, D. M.; Gibbs, Gerald V.; Boisen, Monte B. Jr.; Allan, D. C.; Teter, M. P. (American Physical Society, 1995-09-15)
Several hypothetical silica structures have been generated using a simulated-annealing strategy with an ab initio based covalent-bonding potential. First-principles total-energy pseudopotential methods have been used to examine several. promising hypothetical structures and to compare their structural parameters, cohesive energies, and bulk moduli with those of low quartz;, low cristobalite, silica sodalite, and stishovite. The cohesive energies of these hypothetical structure types are found to be equivalent to those of low quartz, low cristobalite, and silica sodalite, and significantly lower than that of stishovite.
Lithospheric structure of the Chaco and Parana Basins of South America from surface-wave inversion
Snoke, J. A.; James, D. E. (American Geophysical Union, 1997-02)
Surface-wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Parana Basins of central South America. The upper-mantle velocity structure beneath the Parana Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface-wave analysis used ;broadband recordings from a subset of a 14-station array deployed in a roughly east-west sawtooth arrangement along 20 degrees S latitude, with a total EW aperture of similar to 1,400 km. Results from receiver-function analysis, as well as direct P-wave regional travel-time data, were used in the inversions to help constrain Moho depths and crust and upper-mantle velocities. S-wave structure for the intracratonic Parana Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Parana Basin and one event with a path across the western Parana Basin. The average Moho depth in the eastern Parana Basin is similar to 42 km. The high-velocity upper-mantle lid has a maximum S-wave velocity of 4.7 km/s, with no resolvable low-velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Parana Basin despite emplacement of the Parana plume. The limited data from the western Parana Basin are consistent with a homogeneous upper-mantle structure throughout the Parana Basin. Waveform inversion of fundamental-mode and first-higher-mode Rayleigh waves from a single subandean event was used to obtain estimates for pure-path dispersion along propagation paths through the Chaco Basin and the western half of the Parana Basin. The data were partitioned to isolate the partial-path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain: a velocity-depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (''asthenospheric'') upper-mantle S-wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3 km/s at 150 km depth.
Location of the southern edge of the Gorda slab and evidence for an adjacent asthenospheric window: Results from seismic profiling and gravity
Beaudoin, B. C.; Hole, John A.; Klemperer, S. L.; Trehu, A. M. (American Geophysical Union, 1998-12)
As the Mendocino Triple Junction migrates northward along the California margin it is widely presumed to leave a "slab-free" or "asthenospheric" window in its wake. A 250-km-long south-north seismic refraction-reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the North American margin before and after it is modified by passage of the Mendocino Triple Junction. From the seismic data we have determined that (1) the crust is laterally homogeneous in velocity to a depth of 20 km (interpreted by us as Franciscan complex), (2) below 20 km depth the crust is characterized by velocities of greater than or equal to 7.0 km/s for the southern half of the profile and by velocities of less than or equal to 7.0 km/s for the northern half, (3) regions of high reflectivity in the crust occur below similar to 20 km depth throughout the profile, and (4) the North American crust is thickest (similar to 35 km) in the center of the profile and thins to similar to 25 km at either end. From the gravity data we have determined that (1) asthenospheric densities (3.2 g/cm(3)) occur subjacent to the North American crust in the center of the profile, and (2) a wedge of lithospheric mantle density material (greater than or equal to 3.2 g/cm(3)) is required on the southern end of the profile. We interpret these combined results to indicate that our profile crosses the southern edge of the Gorda plate and that directly adjacent to this edge is an asthenospheric window with overlying mafic rocks in the crust. These mafic rocks and a reforming lithospheric mantle increase in thickness southward.
Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons
Nguuri, T. K.; Gore, J.; James, D. E.; Webb, S. J.; Wright, C.; Zengeni, T. G.; Gwavava, O.; Snoke, J. A.; Kaapvaal Seismic Group (American Geophysical Union, 2001-07-01)
The formation of Archean crust appears to involve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa re veal significant differences in the nature of the crust and the crust-mantle boundary between Archean and post-Archean geologic terranes. With the notable exception of the collisional Limpopo belt, where the crust is thick and the Moho complex, the crust beneath undisturbed Archean craton is typically thin (similar to 35-40 km), unlayered, and characterized by a strong velocity contrast across a relatively sharp. Moho. This crustal structure contrasts markedly with that beneath post-Archean terranes and beneath Archean regions affected by large-scale Proterozoic events (the Bushveld complex and the Okwa/Magondi belts), where the crust tends to be relatively thick (similar to 46-50 km) and the Moho is complex.
Crustal thicknesses in SE Brazilian Shield by receiver function analysis: Implications for isostatic compensation
Assumpcao, M.; James, D.; Snoke, A. (American Geophysical Union, 2002-01)
[1] The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of Sao Paulo and Carnegie Institution, 1992-1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect 1000 km long and 300 km wide, covers different geological provinces: the Precambrian Sao Francisco craton, the adjacent Brasiliano (700-500 Ma) fold belts, and the Parana basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P-to-S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 (Vp/Vs = 1.70) was used for the Sao Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Parana basin and coastal belt. Crustal thicknesses ranged from 35-47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Parana basin, have thicker crust (40-47 km) compared with the high elevation areas of the craton and fold belt (37-43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30-40 kg/m(3), in the lithospheric mantle under the Archean block of the Sao Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Parana basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Parana basin, as inferred from other studies.
Constraints on the S wave velocity structure in a continental shield from surface wave data: Comparing linearized least squares inversion and the direct search Neighbourhood Algorithm
Snoke, J. A.; Sambridge, M. (American Geophysical Union, 2002-05)
[1] In their study of upper mantle structure beneath the Parana Basin of SE Brazil, Snoke and James [1997] concluded, on the basis of a linearized least squares inversion (LLSI) of surface wave dispersion data, that a strong (5% contrast) low-velocity zone (LVZ) beginning at a depth less than 150 km was not required to fit the data. They were unable to establish a quantitative estimate, however, on the maximum depth at which such a LVZ could be resolved by their data. Sambridge [1999a, 1999b] has introduced the Neighbourhood Algorithm (NA), a direct search method for nonlinear inversion which can be tuned to extract information from an ensemble of models in addition to finding a single best fit model. Applying NA to the Brazilian dispersion data quantifies the statistics of the ensemble of models classified as "acceptable'' based on a data misfit criterion and a smoothness constraint. The NA best fit model is not significantly different from the LLSI best fit model, but the analysis of the ensemble of models provides new insights regarding how well constrained the model is. Synthetics runs show that for this data set, our modeling procedures could resolve a strong LVZ that began at a depth of 120 km but could not rule out such an LVZ beginning at a depth of 180 km.
Fluorinert as a pressure-transmitting medium for high-pressure diffraction studies
Varga, T.; Wilkinson, A. P.; Angel, R. J. (AIP Publishing, 2003-10)
Fluorinert is a liquid pressure-transmitting medium that is widely used in high-pressure diffraction work. A systematic study of five different fluorinerts was carried out using single-crystal x-ray diffraction in a diamond-anvil cell in order to determine the pressure range over which they provide a hydrostatic stress state to the sample. It was found that none of the fluorinerts studied can be considered hydrostatic above 1.2 GPa, a lower pressure than reported previously. (C) 2003 American Institute of Physics.
General rules for predicting phase transitions in perovskites due to octahedral tilting
Angel, Ross J.; Zhao, J.; Ross, Nancy L. (American Physical Society, 2005-07-08)
Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that dT(c)/dP < 0, contrary to a general rule previously proposed for such zone-boundary transitions. We show that the negative slope of the phase boundary is a consequence of the octahedra in these perovskites being more compressible than the extra-framework cation sites. Conversely, when the octahedra are stiffer than the extra-framework cation sites, the phase transition temperatures increase with increasing pressure, dT(c)/dP > 0.
Cell adhesion of Shewanella oneidensis to iron oxide minerals: Effect of different single crystal faces
Neal, Andrew L.; Bank, Tracy L.; Hochella, Michael F. Jr.; Rosso, Kevin M. (American Institute of Physics, 2005-12-30)
The results of experiments designed to test the hypothesis that near-surface molecular structure of iron oxide minerals influences adhesion of dissimilatory iron reducing bacteria are presented. These experiments involved the measurement, using atomic force microscopy, of interaction forces generated between Shewanella oneidensis MR-1 cells and single crystal growth faces of iron oxide minerals. Significantly different adhesive force was measured between cells and the (001) face of hematite, and the (100) and (111) faces of magnetite. A role for electrostatic interactions is apparent. The trend in relative forces of adhesion generated at the mineral surfaces is in agreement with predicted ferric site densities published previously. These results suggest that near-surface structure does indeed influence initial cell attachment to iron oxide surfaces; whether this is mediated via specific cell surface-mineral surface interactions or by more general interfacial phenomena remains untested. (C) 2005 American Institute of Physics.
Increased susceptibility to repeated freeze-thaw cycles in Escherichia coli following long-term evolution in a benign environment
Sleight, Sean C.; Wigginton, Nicholas S.; Lenski, Richard E. (2006-12-05)
Background In order to study the dynamics of evolutionary change, 12 populations of E. coli B were serially propagated for 20,000 generations in minimal glucose medium at constant 37°C. Correlated changes in various other traits have been previously associated with the improvement in competitive fitness in the selective environment. This study examines whether these evolved lines changed in their ability to tolerate the stresses of prolonged freezing and repeated freeze-thaw cycles during adaptation to a benign environment. Results All 12 lines that evolved in the benign environment for 20,000 generations are more sensitive to freeze-thaw cycles than their ancestor. The evolved lines have an average mortality rate of 54% per daily cycle, compared to the ancestral rate of 34%. By contrast, there was no significant difference between the evolved lines and their ancestor in mortality during prolonged freezing. There was also some variability among the evolved lines in susceptibility to repeated freeze-thaw cycles. Those lines that had evolved higher competitive fitness in the minimal glucose medium at 37°C also had higher mortality during freeze-thaw cycles. This variability was not associated, however, with differences among lines in DNA repair functionality and mutability. Conclusion The consistency of the evolutionary declines in freeze-thaw tolerance, the correlation between fitness in glucose medium at 37°C and mortality during freeze-thaw cycles, and the absence of greater declines in freeze-thaw survival among the hypermutable lines all indicate a trade-off between performance in minimal glucose medium at 37°C and the capacity to tolerate this stress. Analyses of the mutations that enhance fitness at 37°C may shed light on the physiological basis of this trade-off.
Climate model boundary conditions for four Cretaceous time slices
Sewall, Jacob O.; van de Wal, R. S. W.; van der Zwan, K.; van Oosterhout, C.; Dijkstra, H. A.; Scotese, C. R. (Copernicus Publications, 2007)
General circulation models (GCMs) are useful tools for investigating the characteristics and dynamics of past climates. Understanding of past climates contributes significantly to our overall understanding of Earth's climate system. One of the most time consuming, and often daunting, tasks facing the paleoclimate modeler, particularly those without a geological background, is the production of surface boundary conditions for past time periods. These boundary conditions consist of, at a minimum, continental configurations derived from plate tectonic modeling, topography, bathymetry, and a vegetation distribution. Typically, each researcher develops a unique set of boundary conditions for use in their simulations. Thus, unlike simulations of modern climate, basic assumptions in paleo surface boundary conditions can vary from researcher to researcher. This makes comparisons between results from multiple researchers difficult and, thus, hinders the integration of studies across the broader community. Unless special changes to surface conditions are warranted, researcher dependent boundary conditions are not the most efficient way to proceed in paleoclimate investigations. Here we present surface boundary conditions (land-sea distribution, paleotopography, paleobathymetry, and paleovegetation distribution) for four Cretaceous time slices (120 Ma, 110 Ma, 90 Ma, and 70 Ma). These boundary conditions are modified from base datasets to be appropriate for incorporation into numerical studies of Earth's climate and are available in NetCDF format upon request from the lead author. The land-sea distribution, bathymetry, and topography are based on the 1 degrees x 1 degrees (latitude x longitude) paleo Digital Elevation Models (paleoDEMs) of Christopher Scotese. Those paleoDEMs were adjusted using the paleogeographical reconstructions of Ronald Blakey (Northern Arizona University) and published literature and were then modified for use in GCMs. The paleovegetation distribution is based on published data and reconstructions and consultation with members of the paleobotanical community and is represented as generalized biomes that should be easily translatable to many vegetation-modeling schemes.
Structural refinement of neutron powder diffraction data of two-stage martensitic phase transformations in Ti50.75Ni47.75Fe1.50 shape memory alloy
Sitepu, H. (Cambridge University Press, 2007-09)
Transformation behaviors of the technologically important polycrystalline Ti50.75Ni47.75Fe1.50 shape memory alloy were investigated using differential scanning calorimeter (DSC) and powder diffraction techniques. DSC revealed that there are two-stage (i.e., cubic -> trigonal -> monoclinic) martensitic phase transformations on cooling and a one-step transformation (monoclinic -> cubic) on heating. In situ structural refinement of cubic -> trigonal -> monoclinic on cooling was carried out using the D1A high-resolution neutron powder diffractometer at the Institut Laue-Langevin Neutrons for Science in Grenoble, France. Results showed that the phases involved during the phase transition are consistent with the differential scanning calorimeter cooling curve, and the refined crystal structure parameters obtained from Rietveld refinements with the generalized spherical harmonic description agreed reasonably well with X-ray single-crystal data. Subsequently, a combined neutron and synchrotron structural refinement for each phase was conducted because the trial refinements initially using only the synchrotron data of trigonal phase yielded a false minimum with a somewhat high goodness-of-fit chi(2). Results obtained from the combined neutron and synchrotron data of the cubic, trigonal, and monoclinic phases show that the same minimum goodness-of-fit indices were always obtained. (C) 2007 International Centre for Diffraction Data.
In situ structural and texture analyses of monoclinic phase for polycrystalline Ni-rich T49.86Ni50.14 alloy from neutron diffraction data
Sitepu, H. (Cambridge University Press, 2008-03)
Phase transformation temperatures of a polycrystalline Ni-rich Ti49.86Ni50.14 shape memory alloy were investigated using a differential scanning calorimeter. In situ structural and texture analyses of the monoclinic Ti49.86Ni50.14 were investigated using neutron powder diffractometer technique. Differential scanning calorimeter results showed that this Ni-rich alloy has a one-step cubic to monoclinic martensitic phase transformation on cooling and a one-step monoclinic to cubic transformation on heating. In situ high-resolution neutron powder diffraction data of the monoclinic phase from low temperatures to room temperature on heating are consistent with the differential scanning calorimeter's heating results. In addition, the refined monoclinic crystal structure parameters for all neutron diffraction data sets agree satisfactorily with single-crystal X-ray diffraction results. The multiple-data-set capabilities of the GSAS Rietveld refinement program, with a generalized spherical-harmonics description was used successfully to extract the texture description directly from a simultaneous refinement using 52 time-of-flight monoclinic neutron diffraction patterns, taken from a polycrystalline sample held in 13 orientations inside the diffractometer. (c) 2008 International Centre for Diffraction Data.
Pressure-induced phase transition in PbSc0.5Ta0.5O3 as a model Pb-based perovksite-type relaxor ferroelectric
Mihailova, B.; Angel, R. J.; Welsch, A. M.; Zhao, J.; Engel, J.; Paulmann, C.; Gospodinov, M.; Ahsbahs, H.; Stosch, R.; Guttler, B.; Bismayer, U. (American Physical Society, 2008-07-04)
We report pressure-induced structural changes in PbSc0.5Ta0.5O3 studied by single-crystal x-ray diffraction and Raman scattering. The appearance of a soft mode, a change in the volume compressibility, broadening of the diffraction peaks, and suppression of the x-ray diffuse scattering show that a phase transition occurs near p(c)similar to 1.9 GPa. The critical pressure is associated with a decoupling of the displacements of the B site and Pb cations in the existing polar nanoregions, leading to the suppression of B-cation off-center shifts and enhancement of the ferroic distortion in the Pb-O system.
Evidence for a change in Milankovitch forcing caused by extraterrestrial events at Massignano, Italy, Eocene-Oligocene boundary GSSP
Reid, Rachel E. B.; Koeberl, Christian; Montanari, Alessandro; Bice, David (2009-04-01)
High-resolution spectral analyses of four climate proxies from Massignano, Italy (Eocene-Oligocene boundary global stratotype section and point [GSSP]) indicate that the deposition of this rhythmically bedded sedimentary sequence was controlled by Milankovitch orbital cycles. An inverse relationship between the magnetic susceptibility record and the co-varied calcium carbonate, δ18O, and δ13C records is indicative of a climate model in which limestones represent dry/cold periods, while marly limestones represent warm/wet periods. Through pattern matching of band-pass fi ltered signals with the La2004 eccentricity curve, we propose an astrochronological calibration for this important time period. Constrained by three radioisotopically dated volcanic ashes and based on a band-pass version of eccentricity that exhibits expected amplitude modulations, our astrochronology yields a refi ned age for the Eocene-Oligocene boundary of 33.91 ± 0.05 Ma. Orbital forcing is less pronounced in the lower portion of the Massignano section (meter levels 0–15), which contains evidence of several impact events and a 2.2-m.y.-long comet/asteroid shower. We propose that substantial, nonperiodic climate alterations caused by this period of enhanced extraterrestrial activity mask the Milankovitch climate cycles. Possible mechanisms for the exaggeration of impact-related climatic changes include the ice-albedo feedback or the combined effect of impact-related atmospheric alterations with ongoing dust-particle loading associated with the comet/asteroid shower.
The worm turned, and the ocean followed
Lyons, Timothy W.; Gill, Benjamin C. (National Academy of Sciences, 2009-05-19)
Effect of Ba incorporation on pressure-induced structural changes in the relaxor ferroelectric PbSc0.5Ta0.5O3
Welsch, A. M.; Maier, B. J.; Engel, J. M.; Mihailova, B.; Angel, R. J.; Paulmann, C.; Gospodinov, M.; Friedrich, A.; Stosch, R.; Guttler, B.; Petrova, D.; Bismayer, U. (American Physical Society, 2009-09-28)
Pressure-induced structural changes in the canonical relaxor Pb0.78Ba0.22Sc0.5Ta0.5O3 were studied with both in-house and synchrotron single-crystal x-ray diffraction as well as Raman spectroscopy at pressures up to 9.8 GPa. The results reveal that the substitution of Ba for Pb in ABO(3) perovskite-type structures, i.e., the substitution of a two-valent element with an isotropic electron shell for an isovalent element with a stereochemically active lone pair, leads to a "diffuse pressure-induced phase transition," a structural transformation over a broad pressure range, without a well-defined critical pressure point. The smeared phase transition of the average structure results from the existence of local structural deformations in the vicinity of A-positioned Ba cations.
Texture and structural refinement using neutron diffraction data from molybdite (MoO3) and calcite (CaCO3) powders and a Ni-rich Ni50.7Ti49.30 alloy
Sitepu, H. (Cambridge University Press, 2009-12)
Preferred orientation or texture is a common feature of experimental powder patterns. The mathematics of two commonly used models for preferred orientation-the March-Dollase and the generalized spherical-harmonic models-is reviewed. Both models were applied individually to neutron powder data from uniaxially pressed molybdite (MoO3) and calcite (CaCO3) powders in Rietveld analyses, as well as the as-received powders. The structural refinement results are compared to single-crystal structures. The results indicate that reasonable refinement of crystal structures can be obtained using either the March model or generalized spherical-harmonic description. However, the generalized spherical-harmonic description provided better Rietveld fits than the March model for the molybdite and calcite. Therefore, the generalized spherical-harmonic description is recommended for correction of preferred orientation in neutron diffraction analysis for both crystal structure refinement and phase composition analysis. Subsequently, the generalized spherical-harmonic description is extended to crystal structure refinement of annealed and the aged polycrystalline Ni-rich Ni50.7Ti49.30 shape memory alloys. (C) 2009 International Centre for Diffraction Data. [DOI: 10.1154/1.3257906]

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