Incommensurately modulated structures and subsolidus phase relations of intermediate plagioclase feldspars

2020 ◽  
Author(s):  
Huifang Xu ◽  
Shiyun Jin
Keyword(s):  
Cooling Rate ◽  
Group Symmetry ◽  
Contrast Imaging ◽  
Subsolidus Phase ◽  
Plagioclase Feldspar ◽  
Modulated Structure ◽  
Modulated Structures ◽  
Density Modulation ◽  
Z Contrast ◽  
Rich Side

<p>Plagioclase feldspars are the most abundant mineral in the Earth’s crust. Intermediate plagioclase feldspars commonly display incommensurately modulated structure or aperiodic structure. Both fast-cooled and slow-cooled plagioclase feldspars display the incommensurately modulated structures. The ordering pattern in the incommensurately modulated structures of e-plagioclase (characterized by the satellite diffraction peak called e-reflections) are the most complicated and intriguing. The modulated structure has a superspace group symmetry of <em>X</em>-1(αβγ)0 with a special centering condition of (½ ½ ½ 0), (0 0 ½ ½), (½ ½ 0 ½), and the <em><strong>q</strong></em>-vector has components (i.e., δh, δk, δl) along all three axes in reciprocal space. Displacive modulation, occupational modulation, and density modulation are observed in slowly cooled labradorite feldspars. Z-contrast images show both Ca-Na ordering and density modulation. Local structure of lamellae domains has<em> I</em>1 symmetry. The neighboring lamellae domains are in inversion twinning relationship.  The results from Z-contrast imaging and low-temperature single XRD provide consistent structure with density modulation.  The amplitudes of the modulation waves are new parameters for quantifying the ordering state of plagioclase feldspars. Iridescent labradorite feldspars display exsolution lamellae with average periodicity ranging from ~ 150 nm to ~350 nm. Compositional difference between the lamellae is about 10 to 15 mole % in anorthite component. Areas or zones with red iridescent color (i.e., long lamellae periodicity) always contain more Ca (~ 1 to 3 mole %) than the areas with blue (or green) iridescent color within the same labradorite crystal.  We proposed that the solvus for Bøggild intergrowth has a loop-like shape ranging from ~An<sub>44</sub> to ~ An<sub>63</sub>. The Ca-rich side has higher temperature than the Na-rich side. The shapes of satellite peaks, the distances between e-reflections (modulation periods), and even the intensity of c-reflections may also be used evaluate the ordering state or cooling rate of the plagioclase feldspar. Both modulated structure and the exsolution lamellae can be used as proxies for quantifying cooling rate of a labradorite and its host rock.</p>

scholarly journals Modulated Structures, Microstructures and Subsolidus Phase Relations of Labradorite Feldspars

2021 ◽  
Vol 6 (1) ◽  
pp. 1
Author(s):  
Huifang Xu ◽  
Shiyun Jin ◽  
Seungyeol Lee
Keyword(s):  
Cooling Rate ◽  
Group Symmetry ◽  
Subsolidus Phase ◽  
Plagioclase Feldspar ◽  
Modulated Structure ◽  
Modulated Structures ◽  
Coupled Substitution ◽  
Density Modulation ◽  
Q Vector ◽  
Rich Side

The coupled substitution between Na+Si and Ca+Al, in the plagioclase solid solution, results in a continuous variation in the Al/Si ratio of the composition, which is the reason for the complicated ordering patterns in the intermediate plagioclase feldspars such as labradorite. Both fast-cooled and slow-cooled labradorite feldspars display the incommensurately modulated structures. The ordering pattern in the incommensurately modulated structures of e-plagioclase (characterized by the satellite diffraction peak called e-reflections) is the most complicated and intriguing. The modulated structure has a super-space group symmetry of X(αβγ)0, with a special centering condition of (½ ½ ½ 0), (0 0 ½ ½), (½ ½ 0 ½), and the q-vector has components (i.e., δh, δk, δl) along all three axes in reciprocal space. Displacive modulation, occupational modulation, and density modulation are observed in slowly cooled labradorite feldspars. No density modulation was observed in fast cooled (volcanic) labradorite feldspars. The amplitudes of the modulation waves are new parameters for quantifying the ordering state of labradorite. Iridescent labradorite feldspars display exsolution lamellae with an average periodicity ranging from ~150 nm to ~350 nm. Compositional difference between the lamellae is about 12 mole % in anorthite components. Areas or zones with red (or yellow) iridescent color (i.e., long lamellae periodicity) always contain more Ca (~1 to 3 mole %) than the areas with blue (or green) iridescent color within the same labradorite crystal. We proposed that the solvus for Bøggild intergrowth has a loop-like shape, ranging from ~An44 to ~An63. The Ca-rich side / zone has higher exsolution temperature than the Na-rich side / zone. The shapes of satellite peaks, the distances between e-reflections (modulation periods), and even the intensity of e-reflections may also be used to evaluate the ordering state or cooling rate of the plagioclase feldspar. Both modulated structure and the exsolution lamellae can be used as proxies for quantifying cooling rate of a labradorite and it’s host rock.

Structure of KNbOB2O5 – a commensurately modulated structure

2005 ◽  
Vol 61 (4) ◽  
pp. 361-366 ◽  
Author(s):  
Siegbert Schmid ◽  
Trixie Wagner
Keyword(s):  
Single Crystal ◽  
Diffraction Data ◽  
Group Symmetry ◽  
X Ray Diffraction ◽  
Modulated Structure ◽  
Space Group ◽  
X Ray ◽  
Average Structure ◽  
Modulated Structures ◽  
First Time

Members of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds can be described as modulated structures with a single superspace group and very similar modulation functions. Single-crystal X-ray diffraction data (Mo Kα radiation) are used to solve and refine the structure of KNbOB2O5 in these terms for the first time. The average structure is solved and refined in the space group Pmn21. Subsequently, the atomic modulation functions are determined using JANA2000 and superspace-group symmetry Pmn21(0,0.375,0)s. The commensurately modulated structure is finally refined as a superstructure in the space group Pbn21 using SHELXS97 converging to R 1 = 0.024.

Complementary Z-Contrast Imaging and Digital Image Processing

1985 ◽  
Vol 43 ◽  
pp. 304-305
Author(s):  
K. N. Colonna ◽  
G. Oliphant
Keyword(s):  
Image Processing ◽  
Heavy Metal ◽  
Digital Image Processing ◽  
Digital Image ◽  
Biological Tissue ◽  
Biological Imaging ◽  
Tissue Preparation ◽  
Contrast Imaging ◽  
Imaging Tool ◽  
Z Contrast

Harmonious use of Z-contrast imaging and digital image processing as an analytical imaging tool was developed and demonstrated in studying the elemental constitution of human and maturing rabbit spermatozoa. Due to its analog origin (Fig. 1), the Z-contrast image offers information unique to the science of biological imaging. Despite the information and distinct advantages it offers, the potential of Z-contrast imaging is extremely limited without the application of techniques of digital image processing. For the first time in biological imaging, this study demonstrates the tremendous potential involved in the complementary use of Z-contrast imaging and digital image processing.Imaging in the Z-contrast mode is powerful for three distinct reasons, the first of which involves tissue preparation. It affords biologists the opportunity to visualize biological tissue without the use of heavy metal fixatives and stains. For years biologists have used heavy metal components to compensate for the limited electron scattering properties of biological tissue.

Intergrowth of modulated structures in high Tc Bi-Sr-Ca-Cu-O superconductors

1990 ◽  
Vol 48 (4) ◽  
pp. 76-77
Author(s):  
A.Q. He ◽  
G.W. Qiao ◽  
J. Zhu ◽  
H.Q. Ye
Keyword(s):  
Twin Structure ◽  
Modulated Structure ◽  
High Tc ◽  
One Dimensional ◽  
Lattice Constants ◽  
Superconducting Phases ◽  
Modulated Structures ◽  
Layer Structures

Since the first discovery of high Tc Bi-Sr-Ca-Cu-O superconductor by Maeda et al, many EM works have been done on it. The results show that the superconducting phases have a type of ordered layer structures similar to that in Y-Ba-Cu-O system formulated in Bi2Sr2Can−1CunO2n+4 (n=1,2,3) (simply called 22(n-1) phase) with lattice constants of a=0.358, b=0.382nm but the length of c being different according to the different value of n in the formulate. Unlike the twin structure observed in the Y-Ba-Cu-O system, there is an incommensurate modulated structure in the superconducting phases of Bi system superconductors. Modulated wavelengths of both 1.3 and 2.7 nm have been observed in the 2212 phase. This communication mainly presents the intergrowth of these two kinds of one-dimensional modulated structures in 2212 phase.

Atomic resolution Z-contrast imaging of bulk crystal surfaces in Scanning Reflection Electron Microscopy

1990 ◽  
Vol 48 (4) ◽  
pp. 414-415
Author(s):  
Z. L. Wang ◽  
J. Bentley
Keyword(s):  
Electron Microscopy ◽  
Elastic Scattering ◽  
Dark Field ◽  
Contrast Imaging ◽  
Crystal Lattices ◽  
Small Probe ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Image Position ◽  
Z Contrast

The success of obtaining atomic-number-sensitive (Z-contrast) images in scanning transmission electron microscopy (STEM) has shown the feasibility of imaging composition changes at the atomic level. This type of image is formed by collecting the electrons scattered through large angles when a small probe scans across the specimen. The image contrast is determined by two scattering processes. One is the high angle elastic scattering from the nuclear sites,where ϕNe is the electron probe function centered at bp = (Xp, yp) after penetrating through the crystal; F denotes a Fourier transform operation; D is the detection function of the annular-dark-field (ADF) detector in reciprocal space u. The other process is thermal diffuse scattering (TDS), which is more important than the elastic contribution for specimens thicker than about 10 nm, and thus dominates the Z-contrast image. The TDS is an average “elastic” scattering of the electrons from crystal lattices of different thermal vibrational configurations,

High-resolution z-contrast imaging of semiconductor interfaces

1989 ◽  
Vol 47 ◽  
pp. 468-469
Author(s):  
S. J. Pennycook
Keyword(s):  
High Resolution ◽  
Phase Contrast ◽  
Contrast Imaging ◽  
Compositional Modulation ◽  
Semiconductor Interfaces ◽  
Complex Figure ◽  
Thin Region ◽  
Stem Image ◽  
Annular Detector ◽  
Z Contrast

Using a high-angle annular detector on a high-resolution STEM it is possible to form incoherent images of a crystal lattice characterized by strong atomic number or Z contrast. Figure 1 shows an epitaxial Ge film on Si(100) grown by oxidation of Ge-implanted Si. The image was obtained using a VG Microscopes' HB501 STEM equipped with an ultrahigh resolution polepiece (Cs ∽1.2 mm, demonstrated probe FWHM intensity ∽0.22 nm). In both crystals the lattice is resolved but that of Ge shows much brighter allowing the interface to be located exactly and interface steps to be resolved (arrowed). The interface was indistinguishable in the phase-contrast STEM image from the same region, and even at higher resolution the location of the interface is complex. Figure 2 shows a thin region of an MBE-grown ultrathin super-lattice (Si8Ge2)100. The expected compositional modulation would show as one bright row of dots from the 2 Ge monolayers separated by 4 rows of lighter Si columns. The image shows clearly that strain-induced interdiffusion has occurred on the monolayer scale.

Quantitative structure determination of interfaces through Z-contrast imaging and electron energy loss spectroscopy

1996 ◽  
Vol 54 ◽  
pp. 104-105
Author(s):  
S. J. Pennycook ◽  
P. D. Nellist ◽  
N. D. Browning ◽  
P. A. Langjahr ◽  
M. Rühle
Keyword(s):  
Grain Boundaries ◽  
Cuprate Superconductors ◽  
Structure Refinement ◽  
3D Structure ◽  
Real Space ◽  
Contrast Imaging ◽  
Coordination Polyhedra ◽  
Burgers Vector ◽  
Z Contrast ◽  
Contrast Image

The simultaneous use of Z-contrast imaging with parallel detection EELS in the STEM provides a powerful means for determining the atomic structure of grain boundaries. The incoherent Z-contrast image of the high atomic number columns can be directly inverted to their real space arrangement, without the use of preconceived structure models. Positions and intensities may be accurately quantified through a maximum entropy analysis. Light elements that are not visible in the Z-contrast image can be studied through EELS; their coordination polyhedra determined from the spectral fine structure. It even appears feasible to contemplate 3D structure refinement through multiple scattering calculations.The power of this approach is illustrated by the recent study of a series of SrTiC>3 bicrystals, which has provided significant insight into some of the basic issues of grain boundaries in ceramics. Figure 1 shows the structural units deduced from a set of 24°, 36° and 65° symmetric boundaries, and 24° and 45° asymmetric boundaries. It can be seen that apart from unit cells and fragments from the perfect crystal, only three units are needed to construct any arbitrary tilt boundary. For symmetric boundaries, only two units are required, each having the same Burgers, vector of a<100>. Both units are pentagons, on either the Sr or Ti sublattice, and both contain two columns of the other sublattice, imaging in positions too close for the atoms in each column to be coplanar. Each column was therefore assumed to be half full, with the pair forming a single zig-zag column. For asymmetric boundaries, crystal geometry requires two types of dislocations; the additional unit was found to have a Burgers’ vector of a<110>. Such a unit is a larger source of strain, and is especially important to the transport characteristics of cuprate superconductors. These zig-zag columns avoid the problem of like-ion repulsion; they have also been seen in TiO2 and YBa2Cu3O7-x and may be a general feature of ionic materials.

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