Partial interlayers in phyllosilicates studied by transmission electron microscopy

Interstratification of complete layers has long been recognized in phyllosilicates. Interlayering on a fine scale with a host containing partial layers of a second phase has only recently been recognized by using TEM. Considering similarities in structural units of phyllosilicates this type of interlayering should be expected. Alteration samples from Butte, Montana, show extensive development of partial interlayers in kaolinite, sericite, and pyrophyllite. Kaolinite found in plagioclase sites always grows from smectite and in the growth process incorporates unit cell thick lenses of the smectite. The interlayers show up as increased spacing in 001 fringes and cause considerable strain in the surrounding kaolinite structure. Pyrophyllite studied was found to have abundant interlayers and inclusions of muscovite. This muscovite was identified from diffuse 10 Å reflections and microanalysis. The muscovite inclusions are usually less than 5 layers thick and extend from 50 Å to 1,000 Å in the ab plane. In sericite fine interlayering is subtle for the interlayers vary only slightly in unit cell size and composition from the host; however, structural changes are significant enough to cause imagable strain contrast in the host. Zones of fine interlayers are thought to mark healed subgrain boundaries which originally were zones of compositional heterogeneity.     

Transformation mechanisms of tridymite to cristobalite studied by transmission electron microscopy

Available kinetic data on the transformation of tridymite to cristobalite imply that several different mechanisms can operate. Optical microscopy, and transmission electron microscopy (TEM), are here used to establish the mechanisms in specimens of industrial refractory brick heated experimentally at 1545° C. Two mechanisms are distinguished, in both of which cristobalite grows directly, in the solid state, from well-ordered tridymite. Cristobalite with lamellar morphology develops topotactically, with strict crystallographic orientation derived from tridymite. In competition with this mechanism, a massive transformation produces cristobalite with no crystallographic relation to the tridymite. Massive cristobalite nucleates at grain boundaries whereas lamellar cristobalite nucleates within tridymite grains. Together with a third mechanism, involving melting as an intermediate step, these processes can account for the different transformation-rate behaviours of different tridymite specimens.     

Morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy

In pelitic schists composed mainly of quartz and albite grains, the morphology of intergranular pores, which were filled with water, was studied by transmission electron microscopy (TEM). Although some pores are defined by crystallographic planes (F-face), most of their form has an ideal shape determined by interface tensions between grains and fluid. High-resolution TEM observations demonstrate that pore-free regions at grain boundaries are tight even at the nanometer scale, showing that the wetting angle is larger than 0° in this rock. The pore distribution in two-grain junctions can be compared to a "necklace microstructure" developed by instability of a fluid film along the boundary induced by microcracking. Wetting angles for pores located at grain edges of quartz and albite decrease in the order albite/albite, quartz/quartz, and quartz/albite. The quartz/quartz wetting angle in a calcite-free sample is smaller than that in a calcite-containing sample. This angle also changes due to grain misorientation. Our results confirm that solid-solid and solid-fluid interfacial energies control the geometry of intergranular fluid in natural rocks.     

Microfibrous quartz varieties: characterization by quantitative X-ray texture analysis and transmission electron microscopy

Quantitative X-ray texture analysis was used to identify the fiber elongation direction of microfibrous quartz varieties in materials that contain narrow bands of closely spaced, radially divergent spherulitic fiber bundles. The statistical approach of X-ray texture analysis makes it possible to determine the fiber axis direction even when the maximum divergence angle of fibers within spherulitic fiber bundles diverge ±20° from a common growth direction. Representative samples of chalcedony <11&2macr;0>, quartzine [0001], and pseudochalcedony were selected for additional study with transmis-sion electron microscopy. All varieties of microfibrous quartz were found to contain high concentrations of polysynthetic Brazil twins and domains characterized by a modulated microstructure. Trace element profiles of aluminum and iron concentration were measured by electron microprobe analysis for an agate sample that contains alternating bands of chalcedony <11&2macr;0> and quartzine fibers. The elemental concentration profiles were measured in a direction parallel to the growth direction of microfibrous quartz. Although fibers within the chalcedony bands initially displayed cooperative helical twisting, the fibers became untwisted during the later stages of growth within any one band. The trace element profiles revealed that higher concentrations of aluminum are associated with quartzine and non-twisted <11&2macr;0> fibers, rather than twisted chalcedony <11&2macr;0> fibers. Except for the presence of iron-rich inclusions between quartzine and chalcedony bands, trace concentrations of iron were below the detection limit for the different varieties of microfibrous quartz analyzed. Received: 6 January 1997 / Accepted: 4 September 1997     

High-pressure behaviour of germanate olivines studied by X-ray diffraction and X-ray absorption spectroscopy

Germanate olivines Mg₂GeO₄, Ca₂GeO₄ and CaMgGeO₄ have been studied by high-pressure X-ray Diffraction and high-pressure X-ray Absorption Spectroscopy. The three compounds were compressed, in the 0–30 GPa pressure range, at room temperature in a diamond-anvil cell, silicon oil being used as the pressure transmitting medium. Values of K₀ are 166 ± 15, 117 ± 15 and 152 ± 14 GPa for Mg₂GeO₄, Ca₂GeO₄ and CaMgGeO₄ respectively. These olivines all exhibit compression anisotropy, the a axis being the least compressible. Crystal to crystal phase transitions have been observed in Mg₂GeO₄ and Ca₂GeO₄ above 12 GPa and 6 Gpa respectively. The nature of these structural changes remains unclear yet. The onset of amorphization has been observed in Mg₂GeO₄ and Ca₂GeO₄ at pressures above about 22 and 11 GPa respectively. These phase transitions and amorphization processes do not involve any detectable increase in the coordination number of germanium atoms. At higher pressure (P >23 GPa), we report the onset of a transition from a phase with fourfold coordinated germanium to a phase with higher germanium coordination number in CaMgGeO₄.     

Study of peristerites using transmission electron microscopy and energy dispersive X-ray analysis

Peristerite feldspars from Seiland in Norway have been studied by a combination of transmission electron microscopy and energy dispersive X-ray analysis. The different types of microtextures have been correlated with the local mean composition, and it is found that the miscibility gap between An₄ and An₁₇ extends into the ternary Or—Ab—An region. The orientation of the more common type of exsolution lamellae varies from (08¯1) to about (¯1, 21, ¯2) with increasing potassium content.     

Investigation of superstructures in mullite by high resolution electron microscopy and electron diffraction

Superstructures in synthetic mullite, Al_4+2x Si_2?2x O_10?x , prepared using the zone melting technique, are studied by high resolution electron microscopy (HREM), electron diffraction, and in situ energy dispersive X-ray spectroscopy. For x=0.40 composition HREM images indicate only short-range order. Near x=0.5 composition mullite has an antiphase domain structure with antiphase boundaries (APB's) oriented parallel to (100). For higher Al-content the APB's run in non-rational orientations which are slightly inclined against the {601}-planes. We propose models for the superstructures in which oxygen vacancies are arranged in channels parallel to the crystal b-axis. The models are supported by a one-to-one match between observed and computer-simulated images. An Al-rich limit of x=2/3 for mullite is deduced on the basis of stereochemical considerations. Different ordering schemes for the tetrahedral cations are believed to result in the orientation change of the APB's near x=0.5 composition.     

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes.     

Twin structure in coesite studied by high resolution electron microscopy

A synthetic crystal of coesite, SiO₂, has been found to contain microtwins. The (100) reflection twin has been made evident on incoherent (010) planes. The twin structure is directly imaged by high resolution electron microscopy at 2,5 Å resolution, allowing an atomic model to be proposed. This twin could be incoherent on large surfaces and the dislocation density is predicted. The X-ray crystallographic data are analyzed critically in view of the presence of such defects in single crystals. The mutual orientation between a silicon matrix and coesite precipitates in Czochralski-silicon annealed specimens is proposed to be non unique as a consequence of the easy twinning of coesite.     

Deformation of shock-loaded andalusite studied with X-ray diffraction techniques

The structural deformation of an andalusite single crystal, shockloaded up to 400 kb with shock wave direction approximately parallel to c, was investigated by means of X-ray powder (Guinier) and single crystal techniques (Weissenberg, precession). Exposure to the dynamic pressure revealed a fracturing of the crystal into lattice blocks, with a mean size >1,000 Å. No change of the lattice constants could be observed after pressure release. From the streaks of X-ray reflection spots measured within the hk0, h0l, 0kl, and hhl planes the shock-induced lattice deformation is interpreted in terms of rotational gliding and/or microfracturing. The distortion mode is highly structure controlled. It follows preferrably two different structural motion systems: (1) Gliding parallel to (001) occurs, which produces lamellae parallel to (001), mainly arranged in two sublattices with common c-axis. The stacking sequence of lamellae along c is irregular. The lamellae-type structure may also result from an orientated transformation into a high pressure phase of lower symmetry and subsequent inversion into the original phase after pressure release. (2) Gliding parallel to (100) occurs. In this case the deformation mode is asymmetrical with respect to the undistorted crystal. The common direction b of the (001) and (100) deformation planes is probably the main direction of the shock-induced lattice deformation.     

Observations on coexisting pyrrhotite phases by transmission electron microscopy

Electron optical observations of coexisting pyrrhotite phases in a specimen of bulk composition near Fe₇S₈ are described. The implications of the coexistence of FeS and Fe₇S₈ are discussed in terms of the thermodynamic stability of the intermediate pyrrhotites and it is concluded that the intermediate pyrrhotites represent metastable phases at room temperatures.     

Relationships among zirconosilicates: examination by cathodoluminescence and transmission electron microscopy .

Summary The origin, whether primary (magmatic) or secondary (Ca-metasomatic) after elpidite, of the Ca-rich zirconosilicates armstrongite and gittinsite in the subsolvus granite of the Strange Lake peralkaline complex (Quebec-Labrador border, Canada) has been a well debated topic. These zirconosilicates, along with zircon and amphibole, account for most of the Zr in the associated Zr-, REE-, Y-, Nb-, and Be-rich ore deposit. In this study, cathodoluminescence (CL) examination and transmission electron microscopy (TEM) have been used to shed light on the relationships among and origin of these zirconosilicates. Optical and CL observations indicate that elpidite undergoes epitaxial replacement by armstrongite and, subsequently, by gittinsite. This gittinsite is generally associated with quartz and variable amounts of hematite. TEM results suggest the bulk replacement of elpidite by poorly crystalline armstrongite, followed by gittinsite and quartz, or zircon and quartz. The Ca-rich compositions of elpidite and its structural similarity with armstrongite imply the existence of a solid solution with armstrongite. Moreover, the enrichment of gittinsite in Mn and Fe (and in some cases Y), and armstrongite and zircon in Y, as determined by analytical electron microscopy (AEM), correlates with the presence of these elements in the Ca-metasomatic fluid. These structural and compositional relationships indicate that the primary, magmatic elpidite is replaced by armstrongite and, subsequently, by gittinsite and quartz during Ca metasomatism of the complex.

---

Résumé L'origine, primaire (magmatique) ou secondaire après l'elpidite (metasomatose du Ca), des zirconosilicates riches en calcium, soit l'armstrongite et la gittinsite, dans le complexe granitique sub-solvus de Strange Lake (Québec-Labrador, Canada) a fait l'objet de controverses. Ces zirconosilicates, ainsi que le zircon et l'amphibole, contiennent la majeur partie du zirconium présent dans ce complexe qui comprend un gisement important de Zr, Terres-Rares, Y, Nb et Be. Dans cette étude, un examen des relations texturales entres les zirconosilicates par cathodoluminescence (CL) et par microscope électronique en transmission (MET) a été effectué afin de clarifier leur origine. Les observations en optique naturelle et en CL montrent que l'armstrongite et, subséquemment, la gittinsite, se sont formés par remplacement épitaxique de l'elpidite. La gittinsite est généralement accompagnée par du quartz et par des quantités variables d'hématite. Les résultats de la MET suggèrent un remplacement de l'elpidite par de l'armstrongite pauvrement cristallisé, suivi, soit par de 1a gittinsite et du quartz, soit par du zircon et du quartz. La présence d'elpidite riche en Ca et sa structure cristalline similaire a celle de l'armstrongite indique q'il existe une solution solide depuis l'elpidite vers l'armstrongite. De plus, un enrichissement en Mn et en Fe de la gittinsite (parfois aussi en Y), et en Y de l'armstrongite et du zircon, déterminé par analyse au microscope électronique (AEM), est corrélé avec la présence de ces éléments dans un fluide responsable de la métasomatose du Ca. Les relations de structure et de composition entre l'elpidite primaire, magmatique, et l'armstrongite, la gittinsite et le quartz, indiquent que l'elpidite a été remplacée par ces derniers durant un épisode de métasomatose du Ca.

     

The structure of grain boundaries in granite-origin ultramylonite studied by high-resolution electron microscopy

The structure of grain boundaries in a granite-origin ultramylonite, composed mainly of fine-grained feldspar and quartz, was studied by high-resolution electron microscopy (HREM). At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in adjacent grains meet at the interface with no other appreciable phases. In these boundaries, some of the straight segments correspond to a low-index plane of one of the connected grains. Boundaries containing voids, with a spheroidal shape elongated along the boundaries, were observed only between quartz grains. It is suggested that these boundaries were formed by healing of microcracks. The structural width of major boundaries, deduced from lattice-fringe imaging, is less than about 0.5 nm. Received: 15 September 1998 / Revised, accepted: 8 April 1999     

Effect of early nucleation conditions on synthetic muscovite polytypism as seen by high resolution transmission electron microscopy

Three isobaric (P _{H 2O}=1 kb) and isothermal (T _N=355, 502 and 630° C) nucleation experiments of OH-muscovite have been carefully studied by powder X-ray diffraction and high resolution transmission electron microscopy. The intimate stacking sequences of layers within individual spiral-free crystallites are determined by lattice (or structure)-imaging techniques. At T _N=355° C, disordered stacking involving local 1M and 2M ₁ order, largely dominates. A few nuclei with 1M structure coexist with disordered ones. At T _N=502° C, most of micas are of more or less heavily faulted 1M structure but coexist with heavily faulted 2M ₁ nuclei. At T _N=630° C, almost all the population is of 2M ₁ structure with rare stacking faults often located in the middle of the sequence. The nature of intrinsic and extrinsic stacking faults within 1M and 2M ₁ matrices is described. Decreasing temperature and/or increasing supersaturation are found to promote disorder in muscovite. The role of the style of packing of the two early first condensed layers (and related specific distortions of the single layer structure) on the selection of the basic structure developed through further 2D-nucleation, as suggested by Takeda and Ross (1975), is shown to be counterbalanced by environmental parameters like T and supersaturation. These high resolution transmission electron microscopy (HRTEM) observations on screw dislocation-free crystallites give experimental support to the basic assumptions of a recent faulted matrix model which has been proposed to explain the origin of complex polytypes of mica according to a subsequent spiral growth mechanism.     

Models of rock varnish formation constrained by high resolution transmission electron microscopy

Nanometre-scale transmission electron microscope imagery of manganiferous rock varnishes from Death Valley, California, Peru, Antarctica, and Hawaii confirms prior infrared mineralogy studies. The building blocks of rock varnish are clay minerals that are cemented to the rock by oxyhydroxides of manganese and iron. Rock varnish is layered on the scale of nanometres, with the basic structure defined by the subparallel alignment of detrital clay minerals. Although only a few examples of bacteria were found, possible cell-wall encrustations are ubiquitous and aligned with the clay minerals. Mn–Fe oxides appear to be mobilized from bacterial casts and then reprecipitate on clay minerals that weather into monolayers. These observations have implications for varnish dating and palaeoenvironmental techniques: K–Ar and uranium-series dating of rock-varnish oxides can only yield minimum ages; however, this small spatial scale of mobilization would not alter the signal from micron-scale microchemical laminations.     

Analytical transmission electron microscopy of oscillatory zoned grandite garnets

Oscillatory zoned grandite garnets from skarns of the Oslo rift region (southern Norway) have been studied using analytical transmission electron microscopy (ATEM). Our results show lamellae with a width varying from about 10 nm to more than 500 nm in both periodic and non-periodic sequences, which represent oscillatory zoning at a very fine scale. The compositions of adjacent lamellae alternate bimodally from X_and=0.55-0.75 to X_and=0.95-1.0. High resolution (HRTEM) images show that the compositional interfaces are sharp and coherent. Oscillatory zoning at the nm-scale puts constrains on the origin of oscillatory zoning in grandites and underlines the dynamic nature of the crystal growth process. The influence of growth rate is discussed against the background of local growth dynamics and external forcing at different length scales. This study emphasises the importance of adequate spatial resolution in analysing chemical variations in grandite garnets and shows that commonly applied methods (e.g. optical microscopy, backscattered electron imaging and electron or ion microprobe) are not sufficient to detect 'real' zonation patterns. Finally, proposed hypotheses on the origin of birefringence of grandite garnets with intermediate compositions are reviewed and discussed on the basis of the observed microstructure.     

An investigation by transmission electron microscopy of planar deformation features in naturally shocked quartz

We have investigated by Transmission Electron Microscopy (TEM) the planar deformation features(PDF) in quartz from various sites where shock metamorphism has been detected optically: impact sites; the Vredefort complex (South Africa) and the K/T boundary at Raton Basin (Colorado). PDFs are straight narrow bands of glass or microtwins or huge densities of dislocations. Such contrasting microstructures must reflect different shock scenarios. In the Vredefort complex the post-shock thermal history has strongly overprinted the shock-induced defects. In the Raton Basin samples very tiny bubbles have precipitated on the dislocations, strongly suggesting a lengthy annealing at moderate pressure and temperature. This new information should be taken into account in any model of the origin of the K/T transition. In any case, observation of PDFs by optical microscopy alone is not sufficient to unambiguously conclude on their origin.     

A comparison of electron staining agents for the transmission electron microscopy examination of coal

A systematic comparison was made using several different stains on ultrathin sections of coal. Two stains (uranyl acetate+lead citrate and potassium permanganate) stood out by enhancing contrast and bringing out additional detail during the electron microscopy of coal.     

Further studies on torbanites and coorongite using transmission electron microscopy and C-isotope analysis

Transmission electron microscopy of torbanites from various localities in eastern Australia has revealed micro-organisms of possible methanogenic affinity. Botryococcus remains in the torbanites displayed close resemblance to the rubbery B. braunii accumulations deposited in recent bodies of freshwater origin. Compaction and collapse of structure seem to be the major visible alteration to the algal colonies. C-isotope compositions of the torbanites favour methanogenic activity. Comparison with recent botroyococcal accumulations support diagenetic alteration at a very early stage of deposition.     

Analysis of natural and beneficiated ferruginous bauxites by both X-ray diffraction and X-ray fluorescence

The quantitative determination of the main compounds Al (OH)₃ (gibbsite) as Al₂O₃ and hydrated Fe-oxides as Fe₂O₃ in natural and beneficiated ferruginous bauxites (low-grade bauxites) by X-ray diffraction (XRD) is presented employing the external standard method. The calibration parameters for the analysis by XRD were ascertained by using ten synthetic mixtures of the pure oxides Al₂O₃ and Fe₂O₃ which were calcined at 1200°C. The concentration ranges for Fe₂O₃ were 10–70 wt.% and those for alumina 30–90 wt.%. The standard deviation for the calibrations of the two oxides was 3.33 wt.%. The results obtained by XRD for both oxides were compared with those attained by X-ray fluorescence. The precision of the determinations by XRD was ascertained by analysing a natural sample consecutively 5 times. The accuracy was verified by the analysis of two international standard reference materials with low silica contents.