Wadsleyite is a spinelloid and is based on a nearly an acceleration voltage of 15 kV, a 10 nA probe current beam diam- cubic-close-packed oxygen sub-lattice.
Anhydrous wadsleyite is orthorhombic space and Mg-ka and Kakanui augite standard USNM was used group Imma , whereas hydrous varieties can be slightly monoclinic for Ca-ka and Al-ka. The mineral compositions listed in Table 1 are ; Ye et al, , but the unit cells and structures are approx- the averages of six individual analyses. Of these, olivine shows imately the same. Olivine oxidation produces mary beam to generate negative secondary ions: H—, D—, and 30Si—. Although olivine—ringwoodite topotaxy has been described previ- ously Kerschhofer et al.
Additionally, Couvy et al and Demouchy et al have discussed shear defor- mation mechanisms in wadsleyite and effects of shear on the oliv- ine—wadsleyite transformation. An understanding of observed topotaxial relations between the two phases is potentially impor- tant in studies of crystal-preferred orientation and elastic anisot- ropy in the transition zone Mainprice et al. Experimental In order to measure H and Fe partitioning among nominally Fig.
Backscatter electron image of the capsule showing the bright Pt capsule anhydrous phases at the km discontinuity, we have conducted material, and the distribution of the quenched melt phase Mlt , clinoenstatite P , experimental syntheses of phases coexisting with hydrous melt in a olivine Ol , and wadsleyite. Author's personal copy J. Chemical compositions weight percent oxide of co-existing phases as determined by The regions with higher Fe contents contain exsolution lamellae of electron microprobe chemical analysis and H by ion microprobe SIMS.
The higher TiO2 0. Coherent wadsleyite lamellae within olivine have NiO 0. In the vicinity of the wadsleyite Total Its presence is consistent with hydration of wadsleyite by Ni 0. The beam current and raster size were then reduced to pA and 5 lm, respectively for the measurement. A measurement con- sisted of 40 cycles through the masses with count times of 1 sec- ond H , 40 seconds D , and 1 second 30Si. The measurements were calibrated using an experimental ring- woodite sample that had been measured by FTIR.
Uncertainties in our water contents, which include the statistical uncertainties Fig. Backscattered electron image of exsolution texture of wadsleyite bright in measurement and background and the uncertainty in the sensi- from olivine darker. The wadsleyite bright rounded grains and lamellae are richer in Fe and H relative to the darker olivine. In order to determine the crystallographic orientations between phases, a portion of an optical thin section was selected, removed from the glass slide and thinned using Ar-ion milling.
High resolu- tion TEM images were recorded under defocused conditions near the Scherzer defocus 67 nm. For chemical analysis across wads- leyite lamellae, energy dispersive X-ray spectroscopy EDXS using scanning beam mode was performed. The chemical data were eval- uated with calibrated k-factors for oxygen, magnesium and iron against silicon by using a pyrope-almandine mineral standard van Cappellen, and a thickness-absorption correction based on electron neutrality van Cappellen and Doukhan, Results and discussion 3.
Petrographic relations Fig. Crossed-polars optical image of wadsleyite lamellae and coarse grains blue- The SEM electron backscatter image of Fig. The image is about crystals near the center of the capsule that contain coherent lamel- micrometers across. The optical thin section was cut from near the center of the lae of wadsleyite.
The backscatter image shows variable contrast in capsule Fig 1A. Author's personal copy 88 J. There are several large grains of wadsleyite. Several orientations of wadsleyite inclusions are visible within A 30 lm thick petrographic thin section was prepared of the the olivine grain. The inclusions are rounded blebs, several of sectioned capsule. A crossed-polars optical image is given in which appear to be near extinction and are possibly in optical con- Fig.
This shows a single olivine grain with a strong interference tinuity. There are also the prominent coherent lamellae of wads- color gradient probably due largely to thickness, but also to com- leyite that appear only in the Fe-rich regions of the olivine grain.
In this orientation relation, transformation between the phases preserves hexagonal close-packed and cubic close-packed layers of oxygen, respectively.
Cpx: clinoenstatite. High resolution lattice images of the two types of interfaces between olivine a and wadsleyite lamellae b. A Type 1, where [0 0 1] olivine and [0 1 0] wadsleyite zone axes are parallel, B Type 2, [0 0 1] olivine and [1 0 0] wadsleyite zone axes The Type 1 in the image a displays a sharp interface, but the Type 2 in Fig.
In reo plots of axes and plane directions in Appendix Fig. A1 , and the the same olivine grain, clinopyroxene lamellae are also present and the 1 0 0 plane is also parallel to the 1 0 0 of olivine upper right inset in Fig. In all the relations, the close-packed layers in oliv- ine and wadsleyite, as well as clinopyroxene, are preserved in the same orientation normal to the lamella walls. High resolution lat- tice images of the two-types of interfaces between olivine and wadsleyite indicate a sharp interface for Type 1 and a broader interface for Type 2, respectively Fig.
Moreover, a textural transition from semi-coherent lamellae to incoherent blebs is observed in wadsleyite Fig. Such textures appear to indicate that the coherent wadsleyite lamellae act as nucleation sites for incoherent wadsleyite grains that have much higher growth rates, as observed previously for the oliv- ine—ringwoodite transformation by Kerschhofer et al.
Our interpretation of the texture is that the sample initially equilibrated with olivine, wadsleyite, clinoenstatite and melt being distributed according to the thermal gradient in the capsule. In the Fig. Such textures suggest that incoherent wadsleyite grains nucleate on the narrow lamellae, as observed for the olivine—ringwoodite transformation by Kerschhofer et al.
The data point indicated by the arrow is [1 0 0] edge dislocations. Author's personal copy 90 J. The olivine grew into a previously wadsleyite re- 3. The new olivine then approached chemical equilibrium tinuity as suggested by Bercovici and Karato In the exper- by exsolving wadsleyite and clinoenstatite lamellae Figs. The higher iron content of the olivine that contains wadsleyite wt.
H2O, the olivine about ppm, and the clinoenstatite about lamellae is seen in the electron backscatter contrast in Fig. Kinetics can play an important role in the crystallization of molecules and can give rise to polymorphism, the tendency of molecules to form more than one crystal structure.
Current computational methods of crystal structure prediction, however, focus almost exclusively on identifying the thermodynamically stable polymorph. Kinetic factors of nucleation and growth are often neglected because the underlying microscopic processes can be complex and accurate rate calculations are numerically cumbersome. In this work, we use molecular dynamics computer simulations to study simple molecular models that reproduce the crystallization behavior of real chiral molecules, including the formation of enantiopure and racemic crystals, as well as polymorphism.
A significant fraction of these molecules forms crystals that do not have the lowest free energy. We demonstrate that at high supersaturation crystal formation can be accurately predicted by considering the similarities between oligomeric species in solution and molecular motifs in the crystal structure.
For the case of racemic mixtures, we even find that knowledge of crystal free energies is not necessary and kinetic considerations are sufficient to determine if the system will undergo spontaneous chiral separation. Our results suggest conceptually simple ways of improving current crystal structure prediction methods. Additional methods, simulation results, and data analysis PDF. Such files may be downloaded by article for research use if there is a public use license linked to the relevant article, that license may permit other uses.
This book written by Paul-Henri Haumesser and published by Elsevier which was released on 29 July with total pages We cannot guarantee that Nucleation and Growth of Metals book is available in the library, click Get Book button to download or read online books. Join over Nucleation and Growth of Metals: From Thin Films to Nanoparticles explores how nucleation and growth phenomena condition the morphology and related characteristics of metallic thin films and nanoparticles to help control the functional properties of these objects.
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