Observation of talc-tremolite interfaces by high resolution electron microscopy

Talc — a sheet silicate and tremolite — a chain silicate are found by electron microscopy and X-ray microanalysis to occur intergrown as bicrystals with sharp, fault-free boundaries in a talc-rich gemstone nephrite from Scortaseo, Puschlav Valley, Swiss Alps. The plane of intergrowth is the (010)-plane for both components. Curving, stepped interfaces are observed more rarely, and these suggest a reaction from tremolite to talc which stopped before completion. The Mg-ions necessary for this reaction may stem from the associated dolomite rock.     

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.     

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.     

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.     

Trace elements,crystal defects and high resolution electron microscopy

Intergrowth defects (intercalation of different structure types) are common in some rock-forming minerals and can be demonstrated to affect trace element distributions, especially when new crystallographic site types are introduced by the intercalated structure. Atoms that do not readily substitute into the host crystal may be incorporated within the intercalated structure. Trace element partitioning patterns, EPR data and mineral dislocation densities suggest that dislocations do not exercise primary control on the partitioning of compatible trace elements, but the effects of dislocations on incompatible elements may be important.Intergrowth defects in geologically important materials include disordered intergrowth structures in Sulfides and the stacking and chain-width disorder that is found in pyroxenes and amphiboles from a range of occurrences. Intergrowth defects may range from a few Ångstroms wide to optically resolvable size. It is important to evaluate their effects when interpreting geological trace element data.     

Direct major- and trace-element analyses of rock varnish by high resolution laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS)

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to determine major and trace element concentrations in rock varnish samples from the Lahontan Range, near Fallon, NV and from a remote wilderness area near the San Juan River, in southeastern Utah. The data indicate that rapid LA-ICPMS analyses provide ample analytical resolution for semi-quantitative compositional determinations of both trace and major elements in the varnish despite the presence of a rock substrate component in most analyses. The overall major element contents of rock varnish from the two localities are grossly similar to rock varnish from other locations analyzed by solution ICPMS, electron microprobe, and energy dispersive scanning electron microscopy (SEM). Differences between microprobe and LA-ICPMS analyses may stem from different sampling scales and different degrees of substrate involvement. It was possible to detect significant variations in trace element contents in the rock varnish samples. The Lahontan Range is situated within a belt of W mineralization, and varnish from that locality contained significantly higher W and Mo contents than varnish from localities outside the W belt. Lead, Tl, Bi, Cd and As contents of varnish-coated pebbles from near the San Juan River in southeastern Utah, varied by an order of magnitude as a function of the position of the sampling site on the pebble. Elevated heavy element contents on the skyward-facing varnish surfaces indicate that heavy elements may be preferentially scavenged at the locations most likely to receive direct inputs of atmospherically-deposited airborne particulates. The source of metal-rich airborne particulates, in this case, is probably any one of several large coal-fired power plants in the Four Corners region, proximal to the San Juan study area. These patterns indicate that rock varnish chemistry is influenced by atmospherically-derived fluxes of both dissolved and particulate constituents, and that rock varnish can be used as a passive environmental indicator for a wide variety of elements, in much the same manner as moss and lichens.     

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.     

High resolution electron microscopy of tourmaline crystals

Summary High resolution electron micrographs have been obtained from crystals of two types of tourmaline, aligned in a goniometer stage so that the electron beam is parallel to thec axis. Images having a resolution of about 3 Å taken out of focus show direct indications of the atomic arrangement; they suggest medium-range ordering of the metal atoms in buergerite and unexpectedh31l symmetry in elbaite-schorl solid solution. The appearance of ordering is tentatively explained in terms of electron-irradiation effects.

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Hochauflösende Elektronenmikroskopie von Turmalinkristallen

Zusammenfassung Hochauflösende Elektronen-Mikrographien wurden von zwei Typen von Turmalinkristallen, die auf einer Goniometereinrichtung so orientiert waren, daß der Elektronenstrahl parallel zurc-Achse verlief, erhalten. Bilder mit einer 3 Å-Auflösung, die entfokusiert aufgenommen wurden, geben direkte Hinweise auf die Atomanordnung; sie deuten auf Ordnung mittlerer Reichweite der Metallatome in Buergerit und zeigen unerwarteterweiseh31l-Symmetrie bei Elbait-Schörl-Mischkristallen. Das Auftreten von Ordnung wird als vermutlicher Elektronenbestrahlungseffekt gedeutet.

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With 3 Figures     

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.     

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.     

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.

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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.

     

Graphitization of Korean anthracites as studied by transmission electron microscopy and X-ray diffraction

The different modes of high-resolution transmission electron microscopy (TEM) were applied to Korean anthracites, semi-graphites and graphites. Whereas X-ray diffraction data yield averaged values, TEM is the only technique able to bring out the heterogeneity of phases which are different in morphology and in microtexture. The thermal behavior of samples was studied using laboratory heat-treatments up to 2800°C. In heat-treated anthracites for example, an increasing degree of graphitization results in phase changes which can be quantified by TEM. The study of a larger sampling appears however necessary to relate crystallographic variations to geological data.     

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.     

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     

Periodicity faults in chain silicates: A new type of planar lattice fault observed with high resolution electron microscopy

The existence of errors in the periodicity of the tetrahedral chains in pyroxenes and pyroxenoids is predicted. With high resolution electron microscopy of rhodonite, (Mn, Ca)₅[Si₅O₁₅], and pyroxmangite, (Mn,Fe,Ca)₇ [Si₇O₂₁], it is demonstrated that these errors are arranged into planar periodicity faults. The chain periodicity faults are compared with other planar faults described for chain silicates: chain multiplicity faults and chain arrangement faults.     

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.     

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.     

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.