D. J. Barber

On short range ordering in the perovskite lead magnesium niobate

The structural ordering characteristics of lead magnesium niobate (PMN) and solid solutions (PMN-PT) of PMN with lead titanate have been investigated using transmission electron microscopy (TEM). It is proposed that short range, non-stoichiometric 1∶1 ordering of the Mg and Nb cations on an F-centred superlattice generates space charges which dominate the kinetics of the ordering process and inhibit the development of long-range order. Furthermore, it is demonstrated that by introducing off-valent La3+ ions on to the A-site sublattice, the local charges can be at least partially-compensated and an increase in the extent of structural ordering is consequently observed.

Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites

Abstract TEM, HRTEM, HVEM and SEM methods, coupled with energy dispersive X-ray analysis, have been used to study the microstructure and the phases comprising the matrix of carbonaceous chondrites Murchison, Cold Bokkeveld, Nawapali and Cochabamba. A wide variety of phyllosilicate morphologies occurs in each. Very small crystals and clasts of olivine, pyroxene and other unhydrated minerals are mixed intimately with the phyllosilicates. Intergrowths of carbonates and Sulfides within the phyllosilicates also occur, as well as a ubiquitous spongey material which is difficult to characterize, but contains elementary phyllosilicate units and embryo crystals. The identifiable large crystalline phyllosilicates are principally Fe-rich serpentine-group minerals and intermediate more Mg-rich chrysotilelike group members, with characteristic ~ 7.0–7.4 A basal layer spacings. Complex interlayered and intergrown hydrous minerals also occur associated with the spongey material, and other poorly crystalline silicates and finely divided Fe-Ni sulphides. Fe/Si and Mg/Si ratios vary on a sub-micron scale, and the morphologies of the larger phyllosilicate crystals correlate broadly with these variations. Small crystals of sodium chloride and potassium chloride have been identified, occluded within a predominantly organic mass. The matrix minerals have a multistage history of formation in which the effects of aqueous alteration are dominant. Few, if any, of matrix minerals can be unmodified nebular condensates, although some clasts and inclusions have escaped alteration and predate the alteration process.

Thin foils of non-metals made for electron microscopy by sputter-etching

Established techniques of making thin foils of non-metallic materials are reviewed briefly and their inadequacies outlined. The preparation of foils by sputter-etching is described and the effects on the sample of ion current density, and angle of ion incidence are discussed. Results on specimens of ceramics, glasses and minerals are presented. From these it is concluded that sputter-etching is a valuable and nearuniversal thinning technique for non-metals.

Prediction of ion-bombarded surface topographies using Frank's kinematic theory of crystal dissolution

The two kinematic theorems of crystal dissolution by Frank are applied to surfaces undergoing ion-bombardment in order to determine the topographies evolved in any given time. The predicted shapes show good agreement with experimental observations.

The microstructure of Semarkona and Bishunpur

Abstract The ordinary chondrites, Semarkona and Bishunpur, appear to have experienced in situ hydrous alteration. Here we report the results of the first detailed TEM examination of them, which supports this conclusion. In Semarkona, all but the most magnesian mafic minerals have been destroyed, and primary sulphides, most likely troilite, have been partially altered to phyllosilicates and Ni-rich pyrrhotite. In Bishunpur, alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix. During hydrous alteration of both meteorites, the temperature probably did not exceed 260°C. The equilibrium composition of the gas present during the alteration of Semarkona was not solar but was dominated by H2O. The formation of smectite does not require the presence of liquid water. However, evidence for the redistribution of elements, such as Ca, Zn, and Si, over tens of microns or more implies that there was at least some transient grain-boundary fluid present. The alteration in Semarkona and Bishunpur is far from complete. Thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the CM and CI carbonaceous chondrites. Superficially, the mineralogy, mineral chemistry, and bulk chemistry of Semarkona matrix-rims resemble those of some chondritic interplanetary dust particles (CIDPs). The ordinary chondrite parent bodies may, therefore, be a source of some CIDPs.

The Semarkona meteorite: First recorded occurrence of smectite in an ordinary chondrite, and its implications

Abstract Semarkona is an unshocked unequilibrated ordinary chondrite. Much of the interchondrule matrix and the mesostases of some chondrules and clasts have been hydrothermally altered. Analyses of altered materials and opaque matrix are presented. Ca and Fe have been redistributed from primary, anhydrous phases into calcite and Na, Fe smectite, respectively. In Semarkona the process possibly requires open system behavior inconsistent with the conventional view of “metamorphism” of the ordinary chondrites. The low petrologic type previously assigned to Semarkona is the result of retrograde metamorphism, under aqueous conditions, of a higher temperature mineral assemblage. Semarkona, and possibly Bishunpur, should be assigned to petrologic type 2.

A TEM study of ordering in the perovskite, Pb(Sc1/2Ta1/2)O3

Ordering behaviour and the factors which influence the ordering have been studied in both single crystals and hot-pressed ceramics of Pb(Sc1/2Ta1/2)O3 using TEM techniques, including low-temperature microscopy. The characteristics of the structurally-ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases. The superlattice reflections are interpreted in terms of Pb2+ cation displacements. The non-interaction of ordered domains and ferroelectric domains is explained within this model.

TEM studies of Pb(Mg1/3Nb2/3)O3-PbTiO3 ferroelectric relaxors

Abstract TEM was used to observe nanoscale microstructural changes which occur over a range of PMN-PT compositions. Increasing PT additions to the PMN-PT solid solution modify ferroelectric and structurally ordered domains. The relevance of nanostructural changes to the corresponding dielectric properties are briefly discussed.

Ferroelectric domain configurations in a modified-PZT ceramic

Transmission electron microscopy and electron diffraction have been used to study ferroelectric domain configurations in a modified-PZT ferroelectric ceramic which has rhombohedral symmetry. Wedge-shaped domains with head-to-tail dipole arrangements and fine lamellar domains are most commonly observed. Switching between these domain types and various other domain configurations is interpreted, and the role of crystal imperfections in pinning the domain boundaries is discussed.

Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites

Optical microscopy, electron probe microanalysis, scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in Bishunpur, Krymka, Semarkona, Chainpur (all LL3), Tieschitz and Sharps (both H3). The size-distribution of grains in matrix in Bishunpur obeys a power law. Rims around chondrules tend to be finer grained than matrix and may be layered. All rims and matrices studied contain clasts of forsteritic olivine and low-Ca pyroxene. In Bishunpur the clasts may be cemented by amorphous “glue”, rich in normative albite, and partly altered to smectite. Rims in Krymka, Sharps, Chainpur and Tieschitz have a densely packed olivine groundmass with a grain-size of about 0.1 μm. In Krymka, groundmass olivine is Fo20–30 and forms interlocking dendrites. Various Fe oxides are present in rims and matrices. Grain-size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules. From analyses, matrices/rims in the LL-group apparently are enriched in a feldspathic component. Tieschitz rims are slightly enriched, but here the feldspathic component is present as white matrix. Sharps rims are not enriched in a feldspathic component. Opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules, partly induced by contraction of protopyroxene on inversion to clinopyroxene. Fragmented silica-rich chondrule mesostases reacted with Fe oxides and Na to form the groundmass of fayalitic olivine and feldspathic “glue”. A low-temperature, nebular or pre-solar, component is limited to 3 vol.% of each meteorite, so equilibrium condensate was not the carrier of volatiles such as Tl and Bi.