Cells of magnetotactic bacteria are used as model systems for studying the magnetic properties of ferrimagnetic nanocrystals. Each individual bacterial strain produces magnetosomes (membrane-bounded magnetic crystals) that have distinct sizes, shapes, crystallographic orientations and spatial arrangements, thereby providing nanoparticle systems whose unique magnetic properties are unmatched by synthetic chemically-produced crystals. Here, we use off-axis electron holography in the transmission electron microscope to study the magnetic properties of isolated and closely-spaced bullet-shaped magnetite (Fe3O4) magnetosomes biomineralized by the following magnetotactic bacterial strains: the cultured Desulfovibrio magneticus RS-1 and the uncultured strains LO-1 and HSMV-1. These bacteria biomineralize magnetite crystals whose crystallographic axes of elongation are parallel to <100> (RS-1 and LO-1) or <110> (HSMV-1). We show that the individual magnetosome crystals are single magnetic domains and measure their projected in-plane magnetization distributions and magnetic dipole moments. We use analytical modelling to assess the interplay between shape anisotropy and the magnetically preferred <111> magneto-crystalline easy axis of magnetite.

High resolution imaging by transmission electron microscopy has revealed a mechanism for the weathering of intermediate microcline in a humid, temperate climate. Dissolution of the feldspar begins at the boundary of twinned and untwinned domains and produces circular holes which enlarge to form negative crystals. Amorphous, ring-shaped structures develop, about 25 Å in diameter, within the larger holes. These rings, in turn, crystallize to an arcuate phase having a 10-Å basal spacing and then to crinkled sheets of illite or dehydrated montmorillonite. The 10-Å layer silicate shows an irregular stacking sequence, including 10-,-20-, and 30-Å sequences. Included plagioclase crystals show a similar mechanism of weathering and, moreover, are more intensely weathered.

Orthopyroxene (En85) weathers initially by vacancy diffusion, and through this process hydration occurs and a sequence of biopyriboles develops, culminating in a talc-like layer silicate whose structure joins coherently to the orthopyroxene structure. Oxidation of Fe2+ to Fe3+ colors the altering pyroxene yellow. The ‘talc’ does not remain in structural coherence with the pyroxene after it has exceeded a few tens of nanometers in size; it is replaced by a mixture of talc and smectite. In some areas the mixture has an epitactic relation to the pyroxene, but commonly it fills faceted solution holes without crystallographic relation to the parent structure. Continued weathering extends the yellow zone at the periphery of the orthopyroxene, and the alteration product increases in smectite and decreases in 'talc’ During this stage of the reaction, MgO and SiO2 are released to form colorless true talc around the altering pyroxene. Eventually, the yellow alteration may become a smectite pseudomorph after orthopyroxene or it may be changed entirely to a mixture of vein talc and iron oxides. The complete conversion of orthopyroxene to talc plus oxides thus takes place through three sequential mineral reactions without the development of a noncrystalline phase.

The smectite to illite reaction was studied by transmission and analytical electron microscopy (TEM/AEM) in argillaceous sediments from depths of 1750, 2450, and 5500 m in a Gulf Coast well. Smectite was texturally characterized as having wavy 10- to 13-Å layers with a high density of edge-dislocations, and illite, as having relatively defect-free straight 10-Å layers. The structures of smectite and illite were not continuous parallel to (001) at smectite-illite interfaces. AEM data showed that the smectite and illite were chemically distinct although smectite had a more variable composition. Illite formation appeared to have initiated with the growth of small packets of illite layers within subparallel layers of smectite matrix. With increasing depth, ubiquitous thin packets of illite layers increased in size until they coalesced.

A model for the transition requires that the structure of smectite was largely disrupted at the illite-smectite interface and reconstituted as illite, with concomitant changes in the chemistry of octahedral and tetrahedral sites. At least partial Na-K exchange of smectite preceded illite formation. Transport of reactants (K, Al) and products (Na, Si, Fe, Mg, H20) through the surrounding smectite matrix may have taken place along dislocations.

The smectite-to-illite conversion process for the studied samples does not necessarily appear to have required mixed-layer illite/smectite as an intermediate phase, and TEM and AEM data from unexpanded samples were found to be incompatible with the existence of mixed-layer illite/smectite in specimens whose XRD patterns indicated its presence.

The rate of dissolution of akaganéite in HCl increased with time over the bulk of the reaction leading to a sigmoid dissolution vs. time curve. The bulk of the dissolution of lepidocrocite could be described by the cube root law. Transmission electron microscopy examination of partly dissolved crystals of akaganéite showed that acid attack proceeded mainly along the [001] direction. Initially, the tapered ends of the crystals became squared, and as dissolution continued the lengths of the crystals decreased steadily. At the same time, the crystals were gradually hollowed out. Acid attack was most pronounced at the edges of the crystals of lepidocrocite and appeared to involve a disruption of the hydrogen bonds that link the sheets of octahedra making up the structure. Defects also acted as sites for preferential acid attack. Dissolution of multi-domainic crystals involved preferential attack along the domain boundaries, as well as at the edges of the crystals. Single-domain crystals were well developed, but appeared to contain internal imperfections, which promoted the formation of holes on the otherwise unreactive (010) faces.

Imogolite was synthesized at 25°C by aging partially neutralized solutions containing monomeric silicic acid and polymeric hydroxy-aluminum ions for 7 years. Solutions having an initial Si/Al molar ratio of about 0.5 and pHs of 4.0–4.5 produced the largest yields of imogolite, followed by those having an initial Si/Al ratio of about 1, although imogolite was not the principal product. Electron microscopic examination showed a small amount of imogolite fibers embedded in a noncrystalline gel-like substance. Traces of imogolite were detected in solutions having an initial Si/Al ratio of about 2, but no imogolite was found by electron microscopy in products from solutions having an initial Si/Al ratio of about 4. Only gibbsite formed from solutions having initial Si/Al ratios of <0.27. The diameter of the tubular structural unit of the imogolite produced in these experiments was 23 ± 2Å, close to that of natural imogolite.

The thermal decomposition of a fibrous talc was studied by transmission electron microscopy (TEM) and selected-area electron diffraction (SAD). Small changes in the lengths of a and b unit-cell parameters were noticeable at 500°C, but the talc laths did not change morphologically until 800°C. At this temperature striations began to appear in the TEM image, and the talc SAD reflections began to develop faint satellite streaks. At 900°C the striations appeared to be crystallites, and reflections of orthorhombic enstatite were noted. Both TEM and SAD evidence showed that the enstatite crystallites were formed in three orientations corresponding to the three pseudohexagonal a axes of the talc. Thus, triple superposition of the electron diffraction pattern at the three equivalent angles explains the high symmetry star-shaped pattern. At 1000° to 1100°C the enstatite crystallites were shorter and thicker, and the streaks in the SAD pattern were nearly absent. Above 1200°C only one orientation of the enstatite crystallites was found. Noncrystalline regions were also detected at 900°C and became progressively scarce at 1000° and 1100°C. They were not detected at 1200°C. At 1300°C cristobalite was detected in some SAD patterns. The crystallographic axes and unit-cell parameters of the talc and enstatite were also topotactically related as follows: at (5.3 Å)//ce (5.2 Å); bt (9.16 Å)//be (8.8 Å); d(001)t (18.84 Å)//ae (18.2 Å). These results are compatible with an inhomogeneous decomposition mechanism as proposed by earlier workers.

Iddingsite rimming olivine in a basanite from the Limberg, Germany, is composed of saponite and goethite. Transmission electron microscopy of ion-thinned, oriented crystals suggests a two-stage alteration process. At first, the olivine breaks into a mosaic of 50-Å diameter {110} bounded needle-shaped domains which change to a metastable hexagonal phase having a = 3.1 Å and c = 4.6 Å, probably of close-packed, metal-oxygen octahedra. This reaction opens solution channels in the olivine which are detectable from about 20-Å diameter and are parallel to the olivine y-axis. Laths of smectite, one or two layers thick, 20 Å wide, and as much as 100 Å long parallel to their y-axis nucleate from the metastable phase and begin to fill in the solution channels. The laths orient with smectite (001) parallel to olivine (100). As the channels widen, prismatic {110} goethite crystals form directly from the metastable hexagonal phase. This first stage thus provides heterogeneous nuclei of smectite and goethite, formed epitactically and perhaps topotactically from a metastable intermediary.

In a second stage,these nuclei enlarge by deposition from solution as water migrates readily through the solution channels. A reduction in total volume allows smectite veins to form, misoriented with respect to the olivine. The reaction conserves iron, requires the addition of aluminum and water, and releases magnesium and silicon. Electron microprobe analyses of the iddingsite indicate that the smectite is saponite.

The structural damage produced by dry grinding and acid leaching of chrysotile was studied by transmission and scanning electron microscopy, infrared spectroscopy, X-ray powder diffraction, and thermogravimetric analysis. Severe dry grinding converted the chrysotile fibers into fragments having strong potential basic reaction sites. These sites were immediately neutralized by molecules present in the atmosphere (e.g., H2O, CO2). Acid leaching transformed the chrysotile fibers into very porous, non-crystalline silica, which was easily fractured into short fragments. The damage produced in the chrysotile structure by grinding or leaching was assessed by monitoring the intensity of various infrared absorption bands.

Transmission electron microscopic (TEM) examination has shown that multi-domainic crystals of synthetic goethite consist of almost parallel intergrowths, each of which is slightly misoriented with respect to its neighbors. These intergrowths emanate from a central nucleus within the crystal. They can nucleate along both the x and y crystal axes, but subsequent growth is mainly in the z direction.

The formation of multi-domainic goethites from ferrihydrite was favored by high pH (≥ 13) and, at lower pHs, by the addition of NaNO3 to the system. Decreasing the temperature of synthesis from 70° to 20°C also enhanced domain formation. The nucléation of domains was confined to the initial stage of goethite formation. Domains probably formed when crystal growth was very rapid or when adsorbed species blocked the appropriate sites on the nucleus material.

Transmission electron microscope (TEM) images of mixed-layer illite/smectite (I/S) from Gulf Coast shales obtained earlier by the authors have been reexamined by comparing them with the calculated images of G. D. Guthrie and D. R. Veblen. Ordered two-layer periodicity was not detected in the 1750- and 2450-m depth samples, for which X-ray powder diffraction (XRD) showed 20% and 40% illite randomly interstratified in I/S, respectively. Two-layer periodicities that occur in images of the 5500-m depth sample were inferred to reflect ordered I/S. XRD data for the same sample imply the presence of 80% illite in RI-ordered I/S. The two-layer periodicities were observed in slightly overfocused images, consistent with the image calculations of Guthrie and Veblen, with strong dark fringes inferred to correspond to smectite interlayers. Two-layer periodicities were observed only in small domains of a few of the images, consistent with the requirement of special orientation of layers, which varies continuously over a wide range. The lack of more frequent observations of ordered periodicities in TEM images may reflect the lack of the special observation conditions and chemical heterogeneity of illite and smectite layers. Ordered mixed-layering may exist in those specimens for which XRD indicates such ordering, in contrast to the previous interpretation of the authors.

Transmission electron microscopy has been used to characterize coexisting pyrophyllite and muscovite in low-grade metamorphosed pelites from Witwatersrand and northeastern Pennsylvania. The Witwatersrand sample consisted largely of porphyroblasts of interlayered muscovite and pyrophyllite in a fine-grained matrix of the same phases. In both textures, muscovite and pyrophyllite occurred as interlayered packets (with apparently coherent interfaces) from about 300 Å to a few micrometers in thickness, with no mixed layering. Their compositions were determined with a scanning transmission electron microscope to be

$$(\Box{_{{\rm{0}}{\rm{.11}}}}{\rm{ }}{{\rm{K}}_{{\rm{1}}{\rm{.72}}}}{\rm{Na}_{{\rm{0}}{\rm{.17}}})(A}{{\rm{l}}_{{\rm{3}}{\rm{.91}}}}{\rm{ F}}{{\rm{e}}_{{\rm{0}}{\rm{.03}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.05}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.01}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{6}}{\rm{.11}}}}{\rm{ Al}}{{\rm{l}}_{{\rm{1}}{\rm{.89}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}$$

$$(\Box_{{\rm{1}}{\rm{.90}}}{\rm{N}}{{\rm{a}}_{{\rm{0.06}}}}{{\rm{K}}_{{\rm{0}}{\rm{.04}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.94}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.01}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.05}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{7}}{\rm{.94}}}}{\rm{A}}{{\rm{l}}_{{\rm{0}}{\rm{.06}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}},$$

The pyrophyllite and muscovite in the Pennsylvania shale likewise occurred only as coexisting coherent to sub-parallel packets as thin as 200 Å, with compositions of

$$(\Box_{{\rm{1}}{\rm{.89}}}{\rm{N}}{{\rm{a}}_{{\rm{0.04}}}}{\rm{C}}{{\rm{a}}_{{\rm{0}}{\rm{.02}}}}{{\rm{K}}_{{\rm{0}}{\rm{.05}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.93}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.04}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.02}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.01}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{7}}{\rm{.92}}}}{\rm{A}}{{\rm{l}}_{{\rm{0}}{\rm{.08}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}$$

$$({\rm{N}}{{\rm{a}}_{{\rm{0}}{\rm{.04}}}}{\rm{C}}{{\rm{a}}_{{\rm{0}}{\rm{.02}}}}{{\rm{K}}_{{\rm{2}}{\rm{.03}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.54}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.24}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.16}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.06}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{6}}{\rm{.09}}}}{\rm{A}}{{\rm{l}}_{{\rm{1}}{\rm{.91}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}.$$

The compositions of these coexisting pyrophyllite and muscovite define a solvus with steep limbs and extremely limited solid solution. Illite is a white mica, intermediate in composition between pyrophyllite and muscovite, formed at much lower temperatures than muscovite. These relations show that illite is metastable relative to pyrophyllite + muscovite in all of its diagenetic and low-grade metamorphic occurrences. This further implies that illite precursor phases, such as smectite, are also metastable. The prograde reactions involving smectite, illite, and muscovite are therefore inferred to represent Ostwald-step-rule-like advances through a series of metastable phases toward the equilibrium states attained in the greenschist facies. “Illite crystallinity” can therefore be a measure of reaction progress, for which temperature is only one of several determining factors.

The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Alternating mica-like and smectite-like layers of rectorite give rise to periodically varying contrast in 10-Å lattice fringes, yielding a periodicity of 20 Å in a transmission electron microscopic study. Expansion of rectorite using dodecylamine hydrochloride yields a three-layer repeat of thickness 32–35 Å, consisting of a basic 20-Å unit, identical to that in images of collapsed, dehydrated rectorite, and a 12-15-Å thick, intercalated organic layer.

Thin packets of layers derived by grinding the sample are only 20 Å thick, or multiples thereof. Serrated edges of rectorite grains likewise have steps 20 Å in height, implying that mechanical cleavage occurs readily along the weakly bonded, smectite-like interlayers. The “fundamental” 20-Å unit is proposed to be compositionally centered on an interlayer bounded by two identical T-O-T layers, each of which has compositionally different tetrahedral sheets. Such structural considerations suggest that resultant 20-Å units (“rectorite units”) are unique in structure and chemistry relative to true illite. These results further imply that grinding and other treatment of coherent crystals of clay minerals may produce individual unit layers. Moreover, when coupled with size-separation, such treatment may yield X-ray powder diffraction data that reflect reconstituted layer sequences.

In alkaline media and at 70°C dilute suspensions of ferrihydrite transformed to goethite between pH 11.2 and 14 and to a mixture of goethite and hematite above and below this pH range. Increasing the temperature of the transformation or the concentration of the suspension reduced the pH range in which goethite alone formed. The morphology of goethite was chiefly a function of the pH of the system. Acicular crystals formed at all pHs and exclusively above pH 12.2. Epitaxial twinned crystals predominated at pHs below 11, and twins free from hematite formed at higher pHs. Increasing the suspension concentration, ionic strength, or temperature extended the pH range over which twinned crystals formed. Electron micrographs showed that twins formed mainly during the initial stage of the transformation, whereas acicular crystals formed over a longer period. Thus, the twins appeared to nucleate in the ferrihydrite; nucleation of acicular particles took place in solution.