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1.
It has been shown that a unit cell of parawollastonite in a matrix of triclinic wollastonite may be generated by (1) a single stacking fault and (2) by twinning. A wide domain of parawollastite may be produced by stacking faults in every second triclinic unit cell, whereas a twin lamella may be produced by faults in every adjacent triclinic unit cell.  相似文献   

2.
The non-ferroic triclinic to triclinic \(I\bar 1 - P\bar 1\) phase transition in anorthite is described in terms of the spontaneous onset of an order parameter η. A triclinic to triclinic phase transition can be driven by order parameters (representations) arising from the Γ, Z, X, U, V, R, Y, and T points of symmetry of the Brillouin zone. Each point leads to a set of two inequivalent representations and thus there is a total of sixteen inequivalent order parameters. However, only the R 1 + representation is consistent with the change from the body-centered to primitive cell (increase of primitive cell size of two) and also with the origin of the two space groups (inversion center) being at the same position. The R 1 + order parameter of the high symmetry triclinic phase \(P\bar 1_0\) (or equivalently \(I\bar 1\) ) causes a reciprocal lattice change and, in terms of the lower symmetry reciprocal lattice, the order parameter corresponds to the b* point. This is consistent with experimentally observed x-ray diffuse scattering. Using induced representation theory, microscopic distortions compatible with the R 1 + order parameter are obtained. Assuming a distortion in an arbitrary direction at the general 2(i) Wyckoff position (x0,y0,z0) of \(P\bar 1_0\) (the higher symmetry phase) induced representation theory demands an opposite displacement at the position (x0, y0, z0), an opposite displacement at (x0+1,y0+1,z0+1), and the same displacement at ( \(\bar x\) 0+1, \(\bar y\) 0+1, \(\bar z\) 0+1) of \(P\bar 1_0\) . This is also consistent with experiment. The presence of the weak c-type reflections above the transition is attributed to the fluctuating lower symmetry antiphase domains related by the translation (1/2, 1/2, 1/2).  相似文献   

3.
Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R 1=±1/3 [010], R 2=±1/6 [310] or R 3=±1/6 [3 \(\bar 1\) 0]. Structural considerations indicate that a stacking fault R 1, R 2 or R 3 is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults.  相似文献   

4.
The structures of kyanite, yoderite and staurolite are all built up of alternating layers one of which is the Al2SiO5 layer characteristic of the kyanite structure. All the lattice defects found by transmission electron microscopy in these minerals (dislocations, conservative and nonconservative stacking faults, twins) do not affect these layers. In the case of staurolite two types of (010) nonconservative faults are found. Their displacement vectors are [0 1/3 0] and [1/4 1/n 1/4] with n～10 and they respectively cause local regions to transform from the staurolite matrix to kyanite and spinel microdomains. These faults suggest the occurrence of topotactic transformations relating the corresponding phases.  相似文献   

5.
Deformation-induced stacking defects in dolomite have been characterised following examination at the cation sublattice level using high-resolution electron microscopy at 500kV. Slip on c (≡{0001}) is observed to produce stacking faults, often de-localised laterally, which are terminated by partial dislocations with Burgers vectors of the form 1/3 [1 \(\overline 1 \) 00]: a model for the faulted dolomite lattice has been constructed which agrees with the image appearance. Slip on f (≡{10 \(\overline 1 \) 2}) produces long planar faults which are established as not being stacking faults, in the normal sense, since there appear to be no offsets of the cation sublattice across the faults, nor any general indication of any terminating partial dislocations: it is proposed that the contrast arises from rotational disorder in CO3 groups which has resulted from the prior passage of partial dislocations during deformation.  相似文献   

6.
The polymorphism (2M1 or 3T modification) of phengites coexisting with Jadeite+quartz+K-feldspar in gneisses of granitic composition from the internal part of the Sesia Zone correlates with their Mg/(Al+Fe+Mg) ratio. For values 0.015 the 2M1 modification, for values 0.025 the 3T modification is formed. For Mg/(Al+Fe+Mg) values between these limits both polymorphs coexist as distinct grains; less than 1% of a large number of separated grains show an intergrowth of both modifications. In these cases the predominant phase boundaries are (hkO) planes, rational at least with respect to the 2M1 lattice; boundaries parallel (001) occur as subordinate steps in (hkO) boundaries. Extensive intergrowth parallel (001) is not observed. As far as revealed by optical microscopy, the stacking sequence once established in a nucleus is retained during growth of the crystal.  相似文献   

7.
Two crystals of natural chalcopyrite, CuFeS2, experimentally deformed at 200° C have been studied by means of transmission electron microscopy (TEM). The activated glide planes are (001) and {112}. The dislocations in (001) have the Burgers vector [110] and a predominating edge character. They are split into two colinear partials b=1/2[110] and can cross split into {112}. The dislocations in {112} consist of straight segments along low index lattice lines. They are often arranged in dipoles generating trails of loops. Few dislocations with b=1/2[ \(\overline {11} \) 1] and [1 \(\bar 1\) 0] are present and dislocations with b=[0 \(\bar 2\) 1] occur in low angle subgrain boundaries. From weak beam contrasts it is presumed that most of the dislocations gliding in {112} have b=1/2〈3 \(\overline {11} \) 〉. They are dissociated into up to four partials. Microtwins and different types of stacking faults in {112} also occur. Models of the dissociation of dislocations are discussed.  相似文献   

8.
The mineral ussingite, Na2AlSi3O8(OH), an interrupted tectosilicate, has strong hydrogen bonding between OH and the other nonbridging oxygen atom in the structure. Infrared spectra contain a strongly polarized, very broad OH-stretching band with an ill-defined maximum between 1500 and 1800 cm–1, and a possible OH librational bending mode at 1295 cm–1. The IR spectra confirm the orientation of the OH vector within the triclinic unit cell as determined from X-ray refinement (Rossi et al. 1974). There are three distinct bands in the 1H NMR spectrum of ussingite: a predominant band at 13.5 ppm (TMS) representing 90% of the structural hydrogen, a second band at 15.9 ppm corresponding to 8% of the protons, and a third band at 11.0 ppm accounting for the remaining 2% of structural hydrogen. From the correlation between hydrogen bond length and 1H NMR chemical shift (Sternberg and Brunner 1994), the predominant hydrogen bond length (H...O) was calculated to be 1.49 Å, in comparison to the hydrogen bond length determined from X-ray refinement (1.54 Å). The population of protons at 15.9 ppm is consistent with 5–8% Al–Si disorder. Although the ussingite crystal structure and composition are similar to those of low albite, the bonding environment of OH in low albite and other feldspars, as characterized through IR and 1H NMR, is fundamentally different from the strong hydrogen bonding found in ussingite.  相似文献   

9.
High temperature X-ray investigations of enstatite from Bamble shows that the proto (PE) to ortho (OE)/clino (CE) inversion is most probably triggered by preformed single stacking faults acting as nuclei and completed by a subsequent rapid growth process compatible with the martensitic nature of the transformation. New stacking faults are created during the transformation. From a detailed analysis of line profiles we deduce the formation of out-of-phase OE domains which are intergrown with well ordered CE sequences. Single layer faults and PE type faults can be ruled out. A transformation mechanism similar to that proposed by Sadanaga et al. (1969) fits well with our results. Inelastic neutron scattering experiments show that the origin of the transformation is not an elastic shear instability. A transient increase of the elastic constant c 55 is interpreted as a secondary effect caused by elastic strains in the progress of the transformation.Work was supported by the BMFT under 03-SC1LMU3  相似文献   

10.
High-resolution electron microscopy of an intermediate microcline (Or93) from a granodiorite in southeastern Australia reveals anen echelon arrangement of triclinic lens-shaped domains, twinned on the albite law. The domains are tabular on (010), are only a few unit cells wide, but extend 20 or 30 unit cells alongx, until they merge into a zone of monoclinic cells roughly aligned in the rhombic section. The domains are longer and less clearly terminated alongz. Strain calculations show that the energy released by Al/Si ordering, producing the orthoclase-microcline inversion, is equal to the strain energy developed when triclinic domains are forced to retain the original monoclinic crystal shape. This balance of strain energies thus explains the metastable persistence of intermediate microcline into the region of maximum microcline stability. Shearing along faults during deformation of the granodiorite released the strain in some of these feldspars, allowing maximum microcline to develop, and so giving rise to a bimodal distribution of triclinicities throughout the pluton. The value of measured for the intermediate microcline is the average of a range of values throughout each domain, and may be considerably closer to 90° than from an unstrained crystal with the same degree of Al/Si order.  相似文献   

11.
Dissociated dislocations have been observed for the first time by transmission electron microscopy in the perovskite-structure compound CaGeO3. Dislocations with Burgers vectors \(\left[ {1\bar 10} \right]\) and [001] (in pseudo-cubic index) are dissociated into collinear partials on the (110) plane: $$\left[ {1\bar 10} \right] = {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {1\bar 10} \right] + {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}\left[ {1\bar 10} \right]$$ and [001] = 1/2[001] + 1/2[001]. The partials react to form octagonal extended nodes. The stacking fault ribbons with displacement vector \(\left[ {1\bar 10} \right]\) have a width of 350 A, which corresponds to a stacking fault energy of 35 erg/cm2 (or mJ/m2).  相似文献   

12.
Widely extended, cation stacking faults in experimentally deformed Mg2GeO4 spinel have been studied using transmission electron microscopy (TEM). The faults lie on {110} planes. The displacement vector is of the form \(\frac{1}{4}\left\langle {1\bar 10} \right\rangle \) and is normal to the fault plane. The partial dislocations which bound the stacking fault have colinear Burgers vectors of the form \(\frac{1}{4}\left\langle {1\bar 10} \right\rangle \) which are normal to the fault plane.  相似文献   

13.
We present here a numerical modelling study of dislocations in perovskite CaTiO3. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSF are determined from first-principles calculations using the VASP code. The dislocation properties such as collinear, planar core spreading and Peierls stresses are determined for the following slip systems: [100](010), [100](001), [010](100), [010](001), [001](100), [001](010), and All dislocations exhibit lattice friction, but glide appears to be easier for [100](010) and [010](100). [001](010) and [001](100) exhibit collinear dissociation. Comparing Peierls stresses among tausonite (SrTiO3), perovskite (CaTiO3) and MgSiO3 perovskite demonstrates the strong influence of orthorhombic distortions on lattice friction. However, and despite some quantitative differences, CaTiO3 appears to be a satisfactory analogue material for MgSiO3 perovskite as far as dislocation glide is concerned.  相似文献   

14.
Zusammenfassung Die Kristallstruktur des neuen Minerals Warikahnit, Zn3[(H2O)2|(AsO4)2], wurde mit Diffraktometerdaten bestimmt und bis zuR=0,038 für 3428 unabhängige Reflexe verfeinert.Warikahnit ist triklin, , mita=6,710(1),b=8,989(2),c=14,533(2) Å, =105,59(1), =93,44(1), =108,68(1)°,Z=4.Die Kristallstruktur des Warikahnits enthält sechs unterschiedliche Koordinationspolyeder des Zinks mit den Koordinationszahlen 6, 5 und 4 und mit fünf verschiedenen Ligandenkombinationen. Die Wasserstoffbrückenbindungen werden mit Hilfe der Ladungsbilanz und des IR-Spektrums diskutiert.
The crystal structure of warikahnite, Zn3[(H2O)2|(AsO4)2]
Summary The crystal structure of the new mineral warikahnite, Zn3[(H2O)2|(AsO4)2], was determined from diffractometer data and refined toR=0,038 for 3428 observed independent reflections.Warikahnit is triclinic, , witha=6.710(1),b=8.989(2),c=14.533(2) Å, =105.59(1), =93.44(1), =108.68(1)°,Z=4.The crystal structure of warikahnite contains 6 different coordination polyhedra of zinc with the coordination numbers 6,5 and 4 and with 5 different combinations of ligand. The hydrogen bonds are discussed on the basis of charge balance and IR spectra.

Mit 3 Abbildungen  相似文献   

15.
The atomic scale structure and chemistry of (111) twins in MgAl2O4 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 Mg2+ ions are replaced with Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (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   

16.
In situ high-pressure synchrotron X-ray diffraction and infrared absorption experiments for gibbsite were performed at room temperature up to 53 and 25 GPa, respectively. A phase transition was confirmed at about 2.5 GPa. The high-pressure phase is indexed as an orthorhombic structure, rather than a triclinic structure as reported in previous studies. The compressibility of gibbsite and its high-pressure polymorph were studied, and their bulk moduli K0 were determined to be 49 and 75 GPa, respectively with K0 as 4. The in situ high-pressure infrared absorption spectra revealed the gradual disordering of hydrogen substructure above 15 GPa in quasihydrostatic compression.  相似文献   

17.
The high-grade metamorphic basement of the Modum Complex, South Norway,exhibits retrogradation and alteration due to late stage fluid infiltration.Extensive alteration zones of albite-and calcite-rich veining occur especiallywithin and around numerous metagabbros. The gabbros, intruded at 1224±15Ma, are now partly altered to amphibolites due to the subsequent high-grade metamorphism.Two generations of albite-rich rocks have been recognized: (1) a fine-grained, foliated type;(2) a coarse-grained, crosscutting type. Both types show a typical greenschist facies mineralassemblage; albite ± actinolite ± chlorite ± talc. The calcite veins/dykesrepresent a younger generation of veins than both albite-rich types. U–Pb data for spheneof type (1) yielded an age of 1080±3 Ma, determining a point on the retrogradeP-T-t path of the Modum Complex. Increasing albitisation of themetagabbros leads to a decrease in(143ND/144Nd)oand an increase in (87Sr/86)o.Albite- and calcite-rich samples show negative Nd and positive Sr, suggesting that fluids which interacted with the metagabbros originated from a crustal reservoir. The Nd and Sr isotopic data show disequilibrium at the microscale as well as at the macroscale. Negative Sm–Nd model ages of the albite-rich rocks demonstrate that rare-earth elements (REEs) were mobile and fractionated during albitisation.  相似文献   

18.
The effect of the Si/Al distribution in the tetrahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the structural formula (Mg4)(Mg12)(Si8Al8)O40(OH)8·24(H2O) per unit cell were fully optimized. The models differ by the T···Mg2+···T coordination of the interlayer Mg2+ cations by two central cations from the adjacent tetrahedral sheets of the 2:1 vermiculite layers (T,T=Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg2+ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the vermiculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg2+ cations.  相似文献   

19.
From a total of 335 olivine crystal grains, crystallographically orientated platelets and, where possible, parallelepipeds were prepared, chemically analysed by electron microprobe, examined under the polarisation microscope, and studied by polarised FTIR microscope-absorption-spectrometry in the OH vibrational range, 3,000–3,800 cm–1. The 335 crystal grains were extracted from 174 different specimens of Yakutian upper mantle material, including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the Siberian shield. The other specimens were mega- and macrocrysts or inclusions in diamonds and garnets. Analysis of the polarised OH-spectra allowed distinction between hydroxyl in non-intrinsic separate inclusions, NSI, and in isolated local or condensed extended defects, intrinsic to the olivines, ILD or CED, respectively. As the two latter types cannot be distinguished by vibrational spectroscopy, and as they are presumably interconnected by intracrystalline condensation reactions of the type n [ILD][CED]n, we propose to symbolise them as [ILD/CED]. Of the total of 70 polarised OH-bands that were found in the whole set, 17 are caused by NSI, 53 by [ILD/CED]. Total mean integrated OH-band intensities, (̄int)total with ̄int=(||a+||b+||c)int/3, were determined from the spectra. They yielded the contents of structurally unallocated water, using the recent calibration of the IR-method (Bell et al. 2003). The range is 0<wt. ppm (H2O)total<419. Olivines included in diamonds were found to be free of hydroxyl (detection limit of the single crystal IR-spectrometry, ca. 1 wt. ppm water). The total water contents of the different types of olivines increase in the sequence groundmass crystals < megacrysts < macrocrysts. NSI are: (1) Serpentine plus talc with OH in the range 3,704–3,657 cm–1, either polarised along a of the olivine matrix (Pbnm setting) or unpolarised. Approximately 232 olivines out of the 335 contain such NSI. Serpentine and talc occur mostly together, in rare cases one of them alone and if so, mostly talc. (2) Mg-edenite or Mg-pargasite occur rarely and with OH at 3,709–3,711 cm–1. NIS types (1) and (2) are presumably formed by metasomatic alterations of the host olivines, the orientated ones probably in the mantle, the unorientated ones during later stages. (3) The spectra of 23 olivine crystals, displayed specifically a OH-band, polarised c>a>b, at 3,327–3,328 cm–1, an energy typical of OH in hydrous wadsleyite. We assume this phase to be present as NIS in the respective olivines, possibly as relic phase. (4) Weak bands between 3,175 and 3,260 cm–1 polarised along c, are tentatively assigned to molecular water NSI with relatively strong hydrogen bonds to the matrix. We did not find larger clusters of molecular water, i.e. liquid-like water with its characteristic broad band centred at ca. 3,400 cm–1. We did also not find any humite minerals as an NSI. Of the 53 OH-bands intrinsic to olivine, the 29 most abundant and strong ones were subject to further analysis in terms of OH–-bearing structural defects [ILD/CED]. Nearly all these bands are strongly polarised along a. Two bands at 3,672 and 3,535 cm–1 are assigned to boron-related defects, [ILD/CED]B. Five bands at 3,573, 3,563, 3,541, 3,524 and 3,512 cm–1 are intensity-correlated and are assigned to Si-depleted titan-clinohumite-like defects, [ILD/CED](thl). The other, so far unidentified OH of [ILD/CED] are suggested to originate from OH– in different types of (Mg, Fe)-depleted defects recently predicted and discovered by TEM. These are called [ILD/CED](KWK). Eight mostly strong bands of them occur at energies higher than 3,573 cm–1, [ILD/CED](KWK)-H, 13 strong to medium strong bands occur below 3,500 cm–1, [ILD/CED](KWK)-L. Such intrinsic defects may occur alone, [ILD/CED](thl) and [ILD/CED](KWK)-H, or in different combinations with each other, [ILD/CED](KWK)-H+[ILD/CED](thl), [ILD/CED](KWK)-H+[ILD/CED](KWK)-L and [ILD/CED](KWK)-H+[ILD/CED](thl)+[ILD/CED](KWK)-L. Though there are indications that the occurrences of such types and combinations of the intrinsic OH–-bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks, straightforward and simple correlations do not exist. The reasons for this and also for the great number of varieties of intrinsic [ILD/CED] are discussed.Editorial responsibility: J. Hoefs
S. S. MatsyukEmail:
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20.
The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have Nd values at 2700 Ma that range between +3.8 and +6.8, 1 (initial 238U/204Pb) of 7.5 and K 1 (initial 232Th/238U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, Nd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in 207Pb/204Pb with 1 of about 8.0 and k 1 of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, Nd at 2700 Ma of +1.5 to +8, and 1 of about 8 and k 1 of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in 207Pb/204Pb with a 1 about 8.5 and Nd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the 207Pb/204Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.  相似文献   

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