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1.
Phase-transformation-induced twins in pressureless-sintered lanthanum gallate (LaGaO 3) ceramics have been analysed using the transmission electron microscopy (TEM). Twins are induced by solid state phase transformation upon cooling from the rhombohedral
to orthorhombic ( o, Pnma) symmetry at ∼145°C. Three types of transformation twins {101}
o
, {121}
o
, and {123}
o
were found in grains containing multiple domains that represent orientation variants. Three orthorhombic orientation variants were distinguished from the transformation domains converged into a triple junction. These twins are the reflection type as confirmed by tilting experiment in the microscope. Although not related by group–subgroup relation, the transformation twins generated by phase transition from rhombohedral to orthorhombic are consistent with those derived from taking cubic
aristotype of the lowest common supergroup symmetry as an intermediate metastable structure. The r→ o phase transition of first order in nature may have occurred by a diffusionless, martensitic-type or discontinuous nucleation and growth mechanism. 相似文献
2.
Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001] 90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470=" k.=" mutually=" perpendicular=" {110}=" and="> 90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.A National Science Foundation Science and Technology Center 相似文献
3.
The atomic scale structure and chemistry of (111) twins in MgAl 2O 4 spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods
of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries
were investigated in crystallographic projections
and
Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel
can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This
operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present
in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy
(EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin
boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative
analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg 2+ ions are replaced with Be 2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl 2O 4 (chrysoberyl) and BeMg 3Al 8O 16 (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins
in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable
formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The
twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces
a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary. 相似文献
4.
Moderate to strong biaxiality (2V = 10 °–45 °) in experimentally deformed calcite (in single crystals and in marble) is attributed to overlap between one or two thin {01¯12} twin lamellae and the enclosing host. A perfectly centered conoscopic figure (section normal to [0001]) is perceptibly asymmetric about the trace of the optic axial plane. This asymmetry is pronouned in thick sections (> 0.04 mm) and completely distrupts the biaxial configuration of the figure if the overlapping lamella exceeds about 0.0025 mm in thickness. In sections somewhat oblique to [0001] and cut at 20 ° or less to the plane of twinning the conoscopic figure may appear to be perfectly biaxial-expecially in thin sections ( 0.02 mm) enclosing thin ( 0.001 mm) but still visible twins.Similar values of 2V recorded for natural calcite likewise are attributed to twinning on a visible scale. 相似文献
5.
This paper deals with the incidence and geometry of lattice bending in calcite of naturally strained marble and possible relations in time to associated {01¯12} twinning, as revealed by microscopic examination with a universal stage. Microscopic technique and graphic treatment of data with a view to identifying the relation in time of bending to twinning, and possible glide mechanisms involved in the bending process are reviewed as a basis for present and subsequent investigation of this general topic.In most instances development of surviving thin e twins postdates bending of the host crystal (such twins everywhere are rationally oriented). Rarely (in contrast with experimental experience) some thin twins are slightly irrational (L e lamellae) and apparently developed in the final stages of bending. Thick twins (a few tens of microns wide) on the other hand rather commonly predate or are synchronous with bending.The geometry of bending (external rotation) — especially for rotation less than 25° —commonly is compatible with models (based on experimental experience) of simple translation in the strained domains. The commonest translation system so identified is on {10¯11}, with sense of shear, where recognizable, either negative or positive. Translation on {02¯21} is relatively rare. Unexpectedly there is rather strong but not conclusive evidence of translation on {0001} parallel to an a axis. This mechanism, hitherto never identified with certainty, is now here documented unequivocally in a crystal of calcite experimentally strained at 300° C, 5 kb. 相似文献
6.
Lead chloride formation constants at 25°C were derived from analysis of previous spectrophotometrically generated observations
of lead speciation in a variety of aqueous solutions (HClO 4–HCl and NaCl–NaClO 4 mixtures, and solutions of MgCl 2 and CaCl 2). Specific interaction theory analysis of these formation constants produced coherent estimates of (a) PbCl +,
\text PbCl20 {\text{PbCl}}_{2}^{0} , and PbCl 3− formation constants at zero ionic strength, and (b) well-defined depictions of the dependence of these formation constants
on ionic strength. Accompanying examination of a recent IUPAC critical assessment of lead formation constants, in conjunction
with the spectrophotometrically generated formation constants presented in this study, revealed significant differences among
various subsets of the IUPAC critically selected data. It was found that these differences could be substantially reduced
through reanalysis of the formation constant data of one of the subsets. The resulting revised lead chloride formation constants
are in good agreement with the formation constants derived from the earlier spectrophotometrically generated data. Combining
these data sets provides an improved characterization of lead chloride complexation over a wide range of ionic strengths:
|
log \text Cl b 1 = 1. 4 9 1- 2.0 4 I 1/ 2 ( 1+ 1. 5 I 1/ 2 ) - 1 + 0. 2 3 8 I log \text Cl b 2 = 2.0 6 2- 3.0 6 I 1/ 2 ( 1+ 1. 5 I 1/ 2 ) - 1 + 0. 3 6 9 I log \text Cl b 3 = 1. 8 9 9- 3.0 6 I 1/ 2 ( 1+ 1. 5 I 1/ 2 ) - 1 + 0. 4 3 9 I. \begin{gathered} {\log}\,{}_{\text{ Cl}} \beta_{ 1} = 1. 4 9 1- 2.0 4\,I^{ 1/ 2} \left( { 1+ 1. 5\,I^{ 1/ 2} } \right)^{ - 1} +\,0. 2 3 8\,I \hfill \\ {\log}\,{}_{\text{ Cl}} \beta_{ 2} = 2.0 6 2- 3.0 6\,I^{ 1/ 2} \left( { 1+ 1. 5\,I^{ 1/ 2} } \right)^{ - 1} +\,0. 3 6 9\,I \hfill \\ {\log}\,{}_{\text{ Cl}} \beta_{ 3} = 1. 8 9 9- 3.0 6\,I^{ 1/ 2} \left( { 1+ 1. 5\,I^{ 1/ 2} } \right)^{ - 1} +\,0. 4 3 9\,I. \hfill \\ \end{gathered} 相似文献
8.
Centimeter- to decimeter-thick reaction bands occur at hornblendite/marble interfaces in Val Fiorina in the granulite facies metamorphic Ivrea zone. From hornblendite to marble the reaction bands show a consistent succession of sharply bounded mineral layers comprising a monomineralic clinopyroxene layer, a garnet-clinopyroxene layer and a scapolite-clinopyroxene layer. Reaction band formation occurred as a response to gradients in the chemical potentials of calcium and magnesium as defined by the hornblendite assemblage and the marble matrix. The metasomatic corona primarily replaced the hornblendite, and only minor amounts of marble were consumed. The reaction band behaved as an open system with net transfer of calcium from the marble into the reaction band, and a net transfer of iron and magnesium in the opposite direction. Mass balance considerations allow us to constrain a range of feasible mass balance scenarios for which major element fluxes across the boundaries of the reaction band may be quantified. Modeling of layer growth as a steady diffusion process yields ratios of the phenomenological diffusion coefficients for Si, Al, Mg, and Ca of ${{L_{SiSi} } \over {L_{CaCa} }}> 2.5,{\kern 1pt} {\rm }{{L_{AlAl} } \over {L_{CaCa} }}<10,{\rm }{{L_{MgMg} } \over {L_{CaCa} }}> 1.${{L_{SiSi} } \over {L_{CaCa} }}> 2.5,{\kern 1pt} {\rm }{{L_{AlAl} } \over {L_{CaCa} }}<10,{\rm }{{L_{MgMg} } \over {L_{CaCa} }}> 1. . The relative diffusivities are primarily constrained by the sequence of mineral layers of the reaction band and by the relative thickness of the layers. The results of steady-state diffusion modeling are relatively insensitive with respect to variations in the major element boundary fluxes. 相似文献
9.
To investigate high-temperature creep and kinetic decomposition of nickel orthosilicate (Ni 2SiO 4), aggregates containing 3 vol% amorphous SiO 2 have been deformed in uniaxial compression at a total pressure of one atomsphere. Twenty-three samples with grain sizes (d) from 9 to 30 m were deformed at temperatures (T) from 1573 to 1813 K, differential stresses () from 3 to 20 MPa, and oxygen fugacities ( f
o
2) from 10 -1 to 10 5 Pa. At temperatures up to 1773 K, the steady-state creep rate () can be described by the flow law |
相似文献
10.
Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), \(\left( 10\bar{1} \right)\), \(~\{110\}\), \(\left\{ 212 \right\},\) and type two (irrational) twin planes with rational shear directions in \([0\bar{1}\bar{1}]\) and \([\bar{1}\bar{1}0]\). SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a 207Pb/ 206Pb age of 2010?±?15 Ma (2 σ, n?=?9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259?±?5 Ma (2 σ, n?=?7), which is consistent with previous impact age estimates of 255?±?3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts. 相似文献
11.
A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin
planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in
a previous paper. This particular type of twinning is derived from the combination of cyclic
twinning with lamellar
twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample
from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices
that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron
Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes:
and They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic
relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region
and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that
the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar
growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from
Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped
morphology of the face close to the twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic
fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal
effect.
相似文献
12.
Sogdianite, a double-ring silicate of composition
( \text Zr0. 7 6 \text Ti0. 3 84 + \text Fe0. 7 33 + \text Al0.13 ) \Upsigma = 2 ( \square 1. 1 5 \text Na0. 8 5 ) \Upsigma = 2 \text K[\text Li 3 \text Si 1 2 \text O 30 ] ( {\text{Zr}}_{0. 7 6} {\text{Ti}}_{0. 3 8}^{4 + } {\text{Fe}}_{0. 7 3}^{3 + } {\text{Al}}_{0.13} )_{\Upsigma = 2} \left( {\square_{ 1. 1 5} {\text{Na}}_{0. 8 5} } \right)_{\Upsigma = 2} {\text{K}}[{\text{Li}}_{ 3} {\text{Si}}_{ 1 2} {\text{O}}_{ 30} ] from Dara-i-Pioz, Tadjikistan, was studied by the combined application of 57Fe M?ssbauer spectroscopy and electronic structure calculations. The M?ssbauer spectrum confirms published microprobe and
X-ray single-crystal diffraction results that indicate that Fe 3+ is located at the octahedral A-site and that no Fe 2+ is present. Both the measured and calculated quadrupole splitting, Δ E
Q, for Fe 3+ are virtually 0 mm s −1. Such a value is unusually small for a silicate and it is the same as the Δ E
Q value for Fe 3+ in structurally related sugilite. This result is traced back to the nearly regular octahedral coordination geometry corresponding
to a very symmetric electric field gradient around Fe 3+. A crystal chemical interpretation for the regular octahedral geometry and the resulting low Δ E
Q value for Fe 3+ in the M?ssbauer spectrum of sogdianite is that structural strain is largely “taken up” by weak Li–O bonds permitting highly
distorted LiO 4 tetrahedra. Weak Li–O bonding allows the edge-shared more strongly bonded Fe 3+O 6 octahedra to remain regular in geometry. This may be a typical property for all double-ring silicates with tetrahedrally
coordinated Li. 相似文献
13.
Spatial factor analysis (SFA) is a multivariate method that determines linear combinations of variables with maximum autocorrelation at a given lag. This is achieved by deriving estimates of auto-/cross-correlations of the variables and calculating the corresponding eigenvectors of the covariance quotient matrix. A two-point spatial factor analysis model derives factors by the formation of transition matrix U comparing auto-/cross-correlations at lag 0, R
0, with those at a specified lag d, R
d, expressed as U
d= R
0
–1
R d. The matrix U
d can be decomposed into its spectral components which represent the spatial factors. The technique has been extended to include three points of reference. Spatial factors can be derived from the relationship: |
相似文献
14.
Deformed, synthetic quartz containing a dislocation density of 2.9 ± 1.9 × 10 8/cm 2 and abundant bubbles and small inclusions was shocked to peak pressures of 12 and 24 GPa. The resultant material was inhomogeneously deformed and extremely fractured. The 12 GPa sample contained large regions lacking apparent shock deformation, suggesting that the original microstructure of a quartz target may be distinguished in low-stress shocks with minimal annealing. No change in dislocation density was caused by shock loading except in regions containing shock lamellae, where the dislocation density was lowered. Generally the same types of microstructures were induced by shock of deformed quartz as by shock of relatively defect-free as-grown crystals. Glass-filled veins were abundant, especially at lower stresses, and contained angular fragments of quartz welded together. Microfaults formed on \({{\{ 10\bar 11\} } \mathord{\left/ {\vphantom {{\{ 10\bar 11\} } {\{ 01\bar 11\} }}} \right. \kern-0em} {\{ 01\bar 11\} }}\) and \({{\{ 11\bar 22\} } \mathord{\left/ {\vphantom {{\{ 11\bar 22\} } {\{ 1\bar 212\} }}} \right. \kern-0em} {\{ 1\bar 212\} }}\) , inclined close to 45° to the shock propagation direction. Curviplanar features occurred in groups, with contrast indicating Moiré patterns, twins, and stacking faults or related structures; most were interpreted as fractures, possibly welded together with glass. Regions containing shock lamellae sets were present. Lamellae sets were uncommon at 12 GPa, but distributed every few microns at 24 GPa. Lamellae occurred in a spectrum of habits ranging from 35–1500 Å in thickness, from 35 Å upward in spacing, and from closely-packed parallel sets to networks of diverse orientations; some lamellae were not parallel-sided, but wedge-shaped with basal and \({{\{ 10\bar 13\} } \mathord{\left/ {\vphantom {{\{ 10\bar 13\} } {\{ 01\bar 13\} }}} \right. \kern-0em} {\{ 01\bar 13\} }}\) edges, Thick lamellae were connected to glassy veins, and the wedge-shaped type generally narrowed away from veins; they also subdivided and merged along their length. Lamellae were dominantly basal at 12 GPa, and at 24 GPa on \({{\{ 10\bar 12\} } \mathord{\left/ {\vphantom {{\{ 10\bar 12\} } {\{ 01\bar 12\} }}} \right. \kern-0em} {\{ 01\bar 12\} }}\) , with poles normal to the shock direction. We propose that they are not shear features, but rather glass-filled tensile fractures. Vitrification was widespread, especially at 24 GPa, apparently more so than in shock of as-grown material. This suggests that index of refraction is not an appropriate shock paleopiezometer, as it depends on the defect structure of the starting material. Neither lamella width nor spacing was correlated with shock stress; however, the criterion of multiple glass lamellae sets as indications of shock deformation and its intensity are consistent with our measurements. Dislocation density was lowered in lamellae-containing and glassy areas, possibly removed by nucleation of and absorption by lamellae. No high-pressure phases were observed. Based on the complete set of observed features, it appears that shock deformation in quartz is primarily brittle-melt deformation, with an important role played by hot, fluid silica. 相似文献
15.
We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies ( T = 300–400°C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphiné twinning developed pervasively
during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal
to the compression direction ( z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment
of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction ( r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r}
planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows
the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphiné twins induced a switch in the dominant active slip systems during plastic deformation, from
basal <a> (regions without twinning) to {π} and {π′} <a> (pervasively twinned regions). Dynamic recrystallization is localized
along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part
of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for
the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress
of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphiné twinning could
have a potential use as palaeopiezometer in quartz-bearing rocks. 相似文献
17.
Experiments were conducted to determine the water solubility of alkali basalts from Etna, Stromboli and Vesuvius volcanoes,
Italy. The basaltic melts were equilibrated at 1,200°C with pure water, under oxidized conditions, and at pressures ranging
from 163 to 3,842 bars. Our results show that at pressures above 1 kbar, alkali basalts dissolve more water than typical mid-ocean
ridge basalts (MORB). Combination of our data with those from previous studies allows the following simple empirical model
for the water solubility of basalts of varying alkalinity and fO 2 to be derived:
\text H 2 \text O( \text wt% ) = \text H 2 \text O\textMORB ( \text wt% ) + ( 5.84 ×10 - 5 *\text P - 2.29 ×10 - 2 ) ×( \text Na2 \text O + \text K2 \text O )( \text wt% ) + 4.67 ×10 - 2 ×\Updelta \text NNO - 2.29 ×10 - 1 {\text{H}}_{ 2} {\text{O}}\left( {{\text{wt}}\% } \right) = {\text{ H}}_{ 2} {\text{O}}_{\text{MORB}} \left( {{\text{wt}}\% } \right) + \left( {5.84 \times 10^{ - 5} *{\text{P}} - 2.29 \times 10^{ - 2} } \right) \times \left( {{\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}}} \right)\left( {{\text{wt}}\% } \right) + 4.67 \times 10^{ - 2} \times \Updelta {\text{NNO}} - 2.29 \times 10^{ - 1} where H 2O MORB is the water solubility at the calculated P, using the model of Dixon et al. ( 1995). This equation reproduces the existing database on water solubilities in basaltic melts to within 5%. Interpretation of
the speciation data in the context of the glass transition theory shows that water speciation in basalt melts is severely
modified during quench. At magmatic temperatures, more than 90% of dissolved water forms hydroxyl groups at all water contents,
whilst in natural or synthetic glasses, the amount of molecular water is much larger. A regular solution model with an explicit
temperature dependence reproduces well-observed water species. Derivation of the partial molar volume of molecular water using
standard thermodynamic considerations yields values close to previous findings if room temperature water species are used.
When high temperature species proportions are used, a negative partial molar volume is obtained for molecular water. Calculation
of the partial molar volume of total water using H 2O solubility data on basaltic melts at pressures above 1 kbar yields a value of 19 cm 3/mol in reasonable agreement with estimates obtained from density measurements. 相似文献
18.
Five specimens deformed at differential stresses between 220 and 980 bar were selected for a transmission electron microscopy study from the suite of Yule Marble specimens of Heard and Raleigh (1972). The electron micrographs show twinning, curved free dislocations, sessile dislocation loops, and small angle boundaries. Both free dislocations and dislocation boundaries contain dislocation reactions which most likely involve the Burgers vectors 1/61¯102 and 1/121¯210 reacting to form 1/1210¯14. The dislocation densities of specimens deformed at the higher stresses were greater than the starting density and fit the relation
. However, the dislocation densities of specimens deformed at the lower stresses decreased only partially with respect to the starting density during the deformation experiments and thus are larger than predicted by this relation. 相似文献
19.
The principle of maximum entropy can be used to determine the shear strain in natural shear zones. When the margin of a shear zone is assumed, the principle leads to the truncated exponential distribution of the shear strain. If x is the distance remote from the shear zone center, which possesses the maximum shear strain, the shear strain ( x) is given by |
相似文献
20.
The deformation-related microstructure of an Indian Ocean zircon hosted in a gabbro deformed at amphibolite grade has been
quantified by electron backscatter diffraction. Orientation mapping reveals progressive variations in intragrain crystallographic
orientations that accommodate 20° of misorientation in the zircon crystal. These variations are manifested by discrete low-angle
(<4°) boundaries that separate domains recording no resolvable orientation variation. The progressive nature of orientation
change is documented by crystallographic pole figures which show systematic small circle distributions, and disorientation
axes associated with 0.5–4° disorientation angles, which lie parallel to rational low index crystallographic axes. In the
most distorted part of the grain (area A), this is the [100] crystal direction. A quaternion analysis of orientation correlations confirms the [100] rotation axis
inferred by stereographic inspection, and reveals subtle orientation variations related to the local boundary structure. Microstructural
characteristics and orientation data are consistent with the low-angle boundaries having a tilt boundary geometry with dislocation
line [100]. This tilt boundary is most likely to have formed by accumulation of edge dislocations associated with a 〈001〉{100}
slip system. Analysis of the energy associated with these dislocations suggest they are energetically more favorable than
TEM verified 〈010〉{100} slip. Analysis of minor boundaries in area A indicates deformation by either (001) edge, or [100](100) and [001](100) screw dislocations. In other parts of the grain, cross slip on (111), and (112) planes seems likely. These data provide the first detailed microstructural analysis of naturally deformed zircon
and indicate ductile crystal-plastic deformation of zircon by the formation and migration of dislocations into low-angle boundaries.
Minimum estimates of dislocation density in the low-angle boundaries are of the order of ∼3.10 10 cm −2. This value is sufficiently high to have a marked effect on the geochemical behavior of zircon, via enhanced bulk diffusion
and increased dissolution rates. Therefore, crystal plasticity in zircon may have significant implications for the interpretation
of radiometric ages, isotopic discordance and trace element mobility during high-grade metamorphism and melting of the crust. 相似文献
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