Coherent Intergrowths Research Articles

Structural intergrowth of merlinoite/phillipsite and its temperature-dependent dehydration behaviour: a single-crystal X-ray study

AbstractSupposed 'merlinoite' crystals from Monte Somma, Vesuvius (Italy) and Fosso Attici, north of Rome (Italy) represent highly twinned coherent intergrowths between merlinoite and phillipsite on a submicroscopic level. The MER (Immm, a ≈ 14.1, b ≈ 14.2, c ≈ 9.9 Å) and PHI (P 21/m, a ≈ 9.9, b ≈ 14.3, c ≈ 8.7 Å, β = 124.8°) frameworks of similar composition are assembled from identical tetrahedral units, though with a different connectivity. Coherent intergrowth and twinning of the two frameworks lead to P42/mnm pseudosymmetry, which is diagnostic of the intergrowth. Under ambient conditions merlinoite has Immm symmetry or I4/mmm if twinned. a low-symmetry model of space group P121/m1 (a ≈ 14.2, b ≈ 14.2, c ≈ 10 Å, β = 90°) allows structure refinement and quantification of the two frameworks.Upon in situ dehydration to 250°C the evolution of the unit-cell volume of the Monte Somma merlinoite/phillipsite intergrowth displays an intermediate trend between previously studied pure merlinoite from the Khibiny massif (Russia) and Ba-rich phillipsite.The Monte Somma crystal studied by temperature-dependent single-crystal X-ray diffraction methods also contained a subordinate chabazite inclusion with no coherent structural relationship to the merlinoite/phillipsite framework. Thus, the modification of the chabazite framework on dehydration could also be studied.

On the symmetry peculiarities of Bi2WO6single crystals

Bi2WO6single crystals (a=5.452(1), b=5.433(1), c=16.435(1) Å) were studied by X-ray diffraction (MoKα radiation, diffractometer Xcalibur S, CCD-detector) and electron diffraction techniques. Bi2WO6is an archetypal x=1 member of the Aurivillius family of layered perovskites of general formula Bi2O2Ax+1BxO3x-1. Its high piezoelectric performance and nonlinear optical properties have attracted considerable attention. In addition, these crystals offer high ionic conductivity due to the fast oxygen ion transport. In recent years, this compound has been the subject of intense research in the context of catalytic applications. In this work, the Bi2WO6single crystals were grown from solution in melt of Na2WO4–NaF. There were reflections with indexes 0kl, k=2n+1, in the diffraction pattern, contradicting the sp.gr. P21ab. The structure was solved by direct methods and refined in the sp.gr. R1 (R=3.60%, Rw=3.52%). The group P21ab was found to describe the arrangement of heavy atoms Bi and W only (R=17.5%, Rw=18.68%). The structure can be described by three local groups of symmetry – each atomic layer has inherent symmetry: W atoms and O atoms in equatorial vertices of WO6-octahedra have R11b sp.gr., Bi atoms – Bm11, the rest of O atoms – B11b. The oxygen atoms between two Bi sheets can be also described by B11m sp.gr. Preliminary electron diffraction investigation of the Bi2WO6crystals indicated a presence of small amount of a minority phase B1a1 together with the main P21ab phase. The presence of B1a1 phase can be probably explained by Na content in the crystal originating from the flux. Bi2WO6single crystals were studied earlier [1]. TEM showed coherent intergrowths of two distinct modulated variants having different symmetry. This result couldn't be explained by impurity presence because of investigation of pure crystals grown from melt. The work was done with the partial support of the grant for Leading Scientific Schools NSh-1130.2014.5 and RFBR (proj.14-02-00531a).

ChemInform Abstract: Some Turning Points in the Chemical Electron Microscopic Study of Heterogeneous Catalysts

AbstractReview: 91 refs.

Some Turning Points in the Chemical Electron Microscopic Study of Heterogeneous Catalysts

AbstractFrom its initial appearance as a low‐resolution commercial instrument in the early 1960s to its present ultra‐high‐resolution (aberration‐corrected) state, the electron microscope has proved invaluable, often uniquely so, in the elucidation of structural aspects of solid catalysts. When one of us (JMT) used it for the study of surface reactivities of layered solids (graphite and molybdenite), topographical features at a resolution of 3.4 Å could be readily distinguished by using the so‐called gold‐decoration technique, which enabled the catalytic channelling of graphite surfaces (by nanoparticles of various metals) to be charted. This technique also enabled vacancy concentrations in individual (i.e., graphene) layers to be quantitatively determined with unequalled precision. Progressive improvements in electron, optical and other instrumental features became so significant that, by the early 1980s, high‐resolution images of zeolites, intercalates and metal‐carbide‐tipped multiwall carbon nanotubes could be routinely retrieved. High‐resolution studies of zeolites (natural and synthetic) proved particularly illuminating, as they uncovered the ubiquity of coherent (regular and irregular) intergrowths, especially amongst the important pentasil catalysts (ZSM‐5 and ZSM‐11, that is, MFI and MEL structural types).By the mid‐1980s, commercial conventional transmission electron microscopes operating at 200 keV yielded atomically‐resolved structures of nanoparticles of supported noble‐metal catalysts, such as Pt and Au, in which it was found that microtwinning and other unexpected structural irregularities were common. Parallel advances in scanning transmission electron microscopy, pioneered by Crewe and co‐workers, and the use of high‐angle annular dark‐field scanning transmission electron microscopy introduced by Howie and Treacy, added further power to chemical electron microscopy, especially when spectroscopic information like electron energy‐loss and X‐ray emission signals could be routinely recorded.Our own development (PAM and JMT) since the early 2000s of electron tomography (ET) provided deeper insights into the nature of the growing families of nanoporous solids, especially those designated as single‐site heterogeneous catalysts (SSHCs). In these catalysts, the active site may be an integral part of the micro‐ or mesoporous framework (as in MFI and the so‐called MAPOs), or, alternatively be a multinuclear‐single site bimetallic nanocluster like Ru10Pt2 anchored at the pore walls of mesoporous solids. With the dramatic advances in instrumentation, ET now yields quantitatively, features such as fractal dimension in addition to surface areas, pore volumes and visual representation of tortuosity. With the added procedure of “compressed sensing”, far‐reaching possibilities, particularly in ET, now exist for the future study of nanostructured catalysts that have, hitherto, been so vulnerable to electron beam damage as to render them inaccessible to electron microscopic investigations. Another major recent advance, that flows from the work of Vincent and Midgley in the 1990s, is the so‐called precession electron diffraction (PED) procedure, where, by judicious recording (e.g. involving rocking the electron beam in a hollow cone), diffracted intensities can be used for structure determinations because of the elimination of multiple electron (i.e., dynamical) scattering. Solids of catalytic interest that have been structurally determined by PED include Ba6Mn5O15 and MCM‐68. There are real prospects that the PED approach may be able to solve the structures of notoriously beam‐sensitive metal–organic frameworks (MOFs), the catalytic potential of which is continually increasing.Although two other major advances in chemical electron microscopy—the 4 D microscopy pioneered by Zewail, and the in situ electron microscopy of Gai and others—have not been experimentally explored by us, we also summarize their power and future potential in this Essay.

Trace element heterogeneity in molybdenite fingerprints stages of mineralization

Variations in molybdenite trace element chemistry represent a tool for discriminating discrete events in young magmatic-hydrothermal systems and constrain the role of granites in older greenstone belt-hosted gold systems. We show that besides Re and W (typical lattice-bound elements), molybdenite also concentrates chalcophile elements with chalcogenide affinity such as Bi, Pb, and Te (CEs). Elements from the latter group form nano- to micron-scale inclusions which also attract Au and Ag incorporation. High concentration of all elements is attributable to lattice-defects and coherent intergrowths between molybdenite and CE-minerals rather than polytypism. If both groups are used, trace element patterns are useful for interpreting superimposed ore-forming processes. We test the validity of this hypothesis by carrying out Laser-Ablation Inductively-Coupled Mass Spectroscopy spot analysis and element mapping on high-Re molybdenite from the Tertiary Au deposit at Hilltop (NV, USA) and the Archean Boddington Cu–Au deposit (Western Australia). Whereas W and CEs are affected by both deformation and interaction with subsequent fluids, Re is only affected by the latter. An epithermal overprint at Hilltop, recorded in a grain with a CE-rich halo surrounding a core with Re–W oscillatory zoning, upgrades Re content and is also traceable by measurable Au. At Boddington, granite-derived fluids dilute Re in precursor molybdenite, but both granite and orogenic deformation assist CE-mineral coarsening and Au release.

Oxidative dissolution of 4C- and NC-pyrrhotite: Intrinsic reactivity differences, pH dependence, and the effect of anisotropy

The crystallographic diversity of pyrrhotite (Fe1−xS), one of the most common iron sulfide minerals, offers insights into how mineral–fluid interactions are controlled by crystal structures. We have conducted oxidative dissolution experiments on monoclinic 4C-pyrrhotite and ‘hexagonal’ NC-pyrrhotite in aqueous H2O2/H2SO4 and FeCl3/HCl media at pH between 1.8 and 2.9 using polished surfaces of single crystals. Quantification and detailed characterization of the reaction interfaces has been accomplished by confocal 3D topometry and transmission electron microscopy (TEM) in conjunction with focused ion beam (FIB) preparation. Crystallographically coherent intergrowths of 4C- and NC-pyrrhotite in a single sample allowed unambiguous identification of strong intrinsic reactivity differences between the two closely related phases. On {110} faces in the H2O2 medium at 35°C and pH below 2.70, NC-pyrrhotite (N∼4.85) reacts about 50–80% faster than 4C-pyrrhotite. Above pH 2.70, the behavior inverts and 4C-pyrrhotite dissolves faster, while overall reaction rates drop drastically by up to two orders of magnitude. Because the two pyrrhotite phases show only marginally different Fe/S ratios but substantial differences in structural complexity with regards to vacancy ordering, we attribute the reactivity differences to structurally controlled processes at the mineral–water interface. The transition at pH 2.70 is close to the reported isoelectric point of pyrrhotite. We attribute the pH dependent changes in reaction rates and behaviors to protonation/deprotonation of surface sulfhydryl groups and related changes in speciation and bonding mode of reactive oxygen species at the mineral interface. At pH <2.70, we find elemental sulfur as a frequent reaction product in H2O2 and FeCl3 media, indicating incomplete sulfur oxidation. Above pH 2.70, elemental sulfur was not found in H2O2 experiments (no data for FeCl3). Our results show that the effects of crystal anisotropy are strong and directional preference of dissolution changes at the pH 2.70 transition point as well, leading to complex sub-μm-scale textural development at the reaction interfaces throughout the pH range studied. High resolution TEM imaging of cross sections through reacted mineral surfaces show crystalline pyrrhotite up to the reaction interface and the absence of significant non-equilibrium layers or S-enriched (poly)sulfides.

Planar defects in the icosahedral phase in quasicrystal-forming AlCuFe alloys

Electron microscopy has revealed that, upon the formation of a stable icosahedral (i) AlCuFe phase, there arise a large number of planar defects. The revealed defects are nanometer-sized coherent intergrowths of the P1-pentagonal approximant. It has been found that the formation of these defects is the result of a nonequilibrium intermediate transformation. The intergrowths are located in the planes with the fivefold symmetry axis and give rise to an elastic-strain state of the i-phase. Analysis of the diffraction contrast has revealed the presence of phason and phonon atomic displacements due to the mismatch between the quasiperiodic (i-phase) and periodic (P1-phase) lattices. The formed structure exhibits a higher electrical resistance as compared to the i-phase and has been considered as a model state of the imperfect icosahedral phase with preferred phonon displacements.

Local structure of perovskite-based “Pb 2Fe 2O 5”

The reciprocal lattice and defect structure of the crystallographic shear (CS) structures in perovskite-based “Pb 2Fe 2O 5” are investigated using transmission electron microscopy. The reciprocal space for all observed 1/2[110]( h0 l) p CS structures can be described with the same average pseudocubic unit cell with a ≈ a p ( a p – the parameter of the perovskite subcell), and modulation vector q ≈ α a ∗ + γ c ∗ with α/ γ ≈ h/ l and superspace group X2/ m( α0 γ) with a centering vector [1/2,1/2,1/2,1/2]. The defect structure is characterized by numerous mirror twins and coherent intergrowths of 1/2[110]( h0 l) p CS structures with different ( h0 l) p. There are no single CS planes in the perovskite matrix and there is no variation of the perovskite block thickness between the successive CS planes. The crystallographic similarity and the structural and chemical differences between the CS structures in “Pb 2Fe 2O 5” and those in ReO 3 and/or TiO 2-type oxides are discussed.

Hydrothermal alteration experiments of enstatite: Implications for aqueous alteration of carbonaceous chondrites

Abstract— Enstatite is one of the major constituent minerals in carbonaceous chondrites. Hydrothermal alteration experiments (26 in total) of enstatite were carried out at pH 0, 6, 7, 12, 13, and 14, at temperatures of 100, 200, and 300 °C, and for run durations of 24, 72, 168, and 336 h in order to provide constraints on the aqueous‐alteration conditions of the meteorites. The recovered samples were studied in detail by using powder X‐ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).Under acidic and mildly acidic conditions (pH 0, 6), no significant alteration occurred, whereas under neutral to alkaline conditions (pH 7–14), serpentine and saponite formed in various proportions by replacing enstatite. At 300 °C for 168 h, serpentine formed under neutral to moderately alkaline conditions (pH 7, 12), and serpentine and saponite formed as unit cell‐scale coherent intergrowths under highly alkaline conditions (pH 13, 14). The amounts of phyllosilicates have a tendency to increase with increasing pH, temperature, and run duration. There is also a tendency for saponite to form at higher pH and temperature and under longer run‐durations than serpentine.The results indicate that alteration of enstatite is strongly dependent on the experimental conditions, especially pH. They suggest that CM chondrites experienced aqueous alteration under neutral to alkaline conditions, whereas CV and CI chondrites experienced aqueous alteration under more alkaline conditions. The results also suggest that aqueous alteration in CI chondrites occurred at higher temperatures than in CM chondrites, and aqueous alteration in CV chondrites occurred at even higher temperatures than in CI chondrites.

HRTEM study of cation-deficient perovskite-related AnBn− δO 3 n ( n⩾4 δ) microphases in the Ba 5Nb 4O 15–BaTiO 3 system

The occurrence of coherent intergrowths of cation-deficient perovskites in the Ba 5Nb 4O 15–BaTiO 3 system has been examined by high-resolution transmission electron microscopy and selected area electron diffraction. Because of their structural similarity, the simple members Ba 5Nb 4O 15 ( n=5) and Ba 6TiNb 4O 18 ( n=6) form coherent intergrowths—noted 5 P 6 1—by the juxtaposition along the c-axis of P perovskite-like blocks n=5 and one perovskite-like block n=6, with P=1, 2 and 3. More generally, the ability to form intergrowths in the hexagonal perovskite systems is discussed considering the structural characteristics of the simple members. Examples taken from various systems show that the formation of such intergrowths is highly dependent on the size of the A cation present in simple members.

HRTEM observations of structural and chemical modulations in cosalite and its relationship to the lillianite homologues

AbstractThe results of a HRTEM and electron diffraction investigation of a Cu- and Ag-rich cosalite, Cu1.2Ag0.9Pb6.7Bi8.2S20, from Ivigtut, Greenland, are presented. Electron diffraction patterns down [001] show pairs of satellite reflections at Gp±~1/8[140]* andGp±~1/8[40]* in addition to the strong Bragg reflections of the underlying average structure. The superlattice corresponds to a compositional and displacive modulationG±qprimwhereqprimis the primary modulation vector 1/8[140]*. The superlattice is attributed to coupled Ag/Pb/Bi and Cu/□ ordering and associated structural relaxation. The HRTEM images show a complex moire texture due to the structural modulations. Cosalite from Ivigtut, Greenland, also forms coherent intergrowths with members of the lillianite–gustavite series (Pb3Bi2S6–PbAgBi3S6). A model for the intergrowth between the structures is presented based on sharing both S and Pb atoms in the PbS-like slabs in both structures along [010]cosalite.

Cation-Deficient Perovskite-Related (Ba,La)nTin−δO3n (n≥4δ) Microphases in the La4Ti3O12–BaTiO3 System: An HRTEM Approach

The large nonstoichiometric domain, observed in the La4Ti3O12-rich part of the La4Ti3O12–BaTiO3 system, has been shown to correspond to a continuous series of cation-deficient perovskite-related (Ba,La)nTin−δO3n (n≥4δ) microphases. The crystal structures of these microphases have been analyzed by electron diffraction and high-resolution electron microscopy. They can be described as coherent intergrowths of P-perovskite-like blocks La4Ti3O12 and Q-perovskite-like blocks BaLa4Ti4O15, respectively, constitutive of the simple basic terms n=4 and n=5 of the previously identified (Ba,La)nTin−1O3n series. The corresponding BaQLa4(P+Q)Ti3P+4QO12P+15Q microphases can thus be denoted by 4P5Q in a compact form. In fact, only the 4P51 intergrowth sequences have been observed. Eight of these are clearly shown by direct imaging. No intergrowth terms were observed for compositions ranging from BaLa4Ti4O15 (n=5) to Ba2La4Ti5O18 (n=6). The ability of the system to develop coherent intergrowths has been discussed in the light of information provided by an accurate analysis of crystal structures of the basic members of the series: La4Ti3O12 (n=4), BaLa4Ti4O15 (n=5), and Ba2La4Ti5O18 (n=6).

Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka

In the alkali feldspars of the amphibolite- and granulite-facies rocks of Sri Lanka, a late-stage, final exsolution event is observed which produced film lamellae and fine-scale spindles. These were investigated by optical, microprobe, single-crystal, transmission electron microscopy and atomic resolution microscopy techniques. The lamellae and spindles exsolved below the coherent solvus at temperatures as low as 300 to 350° C. Precession photographs and ARM micrographs show that the intergrowth is perfectly coherent. In sections ‖ (010) the rhombic section of the Pericline twins corresponds to analbite or high albite. The albite lamellae and spindles nucleated and grew at low temperatures in a metastable disordered structural state within a tweed-orthoclase matrix and became periodically twinned analbite or high albite, which subsequently developed only a slight increase in Al, Si order. The relationship between twin periodicity and lamellar width, predicted for coherent intergrowths by Willaime and Gandais (1972), is obeyed. In Or-rich grains, in which coherent exsolution is the only exsolution event, the film lamellae tend to be restricted to the rim, the fine-scale spindles to the centre of the grains. The films nucleated heterogeneously at grain boundaries and grew towards the grain centres. Fine-scale spindles probably nucleated homogeneously in the interior part of grains. Heterogeneous nucleation and coherent growth are not mutually exclusive.

Three novel Zintl phases in the magnesium-lanthanum-antimony system based on intergrowth of LaSb and Mg3Sb2 slabs

Reactions of the elements in welded Ta containers at 875-1050[degrees]C produce the black brittle compounds Mg[sub 4.5]La[sub 4]Sb[sub 7] (5-4-7), Mg[sub 4.6]La[sub 3]Sb[sub 6] (5-3-6), and Mg[sub 1.7]La[sub 4.9]Sb[sub 6] (2-5-6). All three consists of coherent intergrowths of Mg[sub 5]Sb[sub 4] (in 5-4-7 and 5-3-6) or Mg[sub 2]Sb[sub 2] (2-5-6) slabs derived from Mg[sub 3]Sb[sub 2] (3-0-2) (anti-[alpha]-La[sub 2]O[sub 3]) with La[sub n+]Sb[sub n] slabs from LaSb (NaCl-type) for n = 3, 2, and 4, respectively, in which a layer of (111) La replaces octahedral (001) Mg at the interface. Crystal data for 5-4-7, 5-3-6, 2-5-6, 3-0-2, respectively, are as follows. Space group, Z: P[bar 3]m1, 1; R[bar 3]m, 3; R[bar 3]m1, 1. a = 4.6206(5), 4.615(1), 4.616(1), 4.559(1) [angstrom]; c = 26.069(7), 66.91(2), 67.67(4), 7.243(2) [angstrom]. No. of independent data, variables: 338, 29; 272, 26; 144, 20; 107, 10. R/R[sub w] (%): 3.0, 3.4; 2.4, 2.8; 2.2, 2.6; 1.5, 2.4. The refined deficiencies at Mg sites, and at the La interface in 2-5-6 as well, result from the above interfacial substitution and are in amounts necessary to afford valence (Zintl) compounds for all three. The 5-4-7, 2-5-6, and 3-0-2 compounds are diamagnetic and show temperature-independent resistivities of the ordermore » of 50, 85, and 530 [mu][Omega]-cm, respectively. Semi-metal behaviors are consistent with this property of bulk LaSb, the intergrowth structures, and Zintl phase concepts. Possibilities for the formation of analogous intergrowth compounds in other systems are suggested.« less

Layer stacking microstructures in a biotite single crystal. A combined hrem-aem study

Abstract A detailed investigation of the as-grown layer-stacking microstructures and the attached microcomposition has been carried out by combined HREM-AEM in a Chinese biotite single crystal. 1M, 2M1, faulted 3T and semi-random IMr n(120) coexist as coherent intergrowths. If no strictly periodic complex polytype is found, we evidence oscillatory stacking modes as quasi-periodic 1D-modulated structures at 1M-2M1 and 1M-3T binary junctions. The transition from order (1M,2M1) to disorder is progressive. Careful study of isolated stacking faults in 1M, 2M1 and 3T matrices shows that they do not follow the least stacking fault energy requirements. Rather they are embryos of alternate polytypes. Microcompositional data show differences between 1M, 2M1 and 1 Mr n(120) structures. Our investigations suggest a significant coupling between the layer-stacking microstructure and the composition. Both of them seem to be fingerprints of the environmental conditions of growth of the biotite mica.

Comparison of the crystal structures of γ-Bi 2MoO 6 and Bi 2WO 6

The crystal structure of γ-Bi 2MoO 6, koechlinite, is compared with the recently re-refined structure of Bi 2WO 6, russellite. Appropriate resetting of the atomic coordinates of γ-Bi 2MoO 6 showed the structures to be isomorphous. Symmetry decomposition of these structures shows the major displacive modes to be virtually identical. The magnitude and/or sign of some of the minor displacive modes, however, are not in agreement. The advantages of X-ray diffraction data over neutron powder data for the refinement of these minor modes are discussed. Transmission electron microscope evidence is presented for the existence of a minority B1 a1 component coherently intergrown with the majority P2 1 ab structure in γ-Bi 2MoO 6, as also observed in Bi 2WO 6. To obtain evidence of the recently reported structural phase transition in Bi 2WO 6 at ∼15°C, diffraction patterns and images of this material were recorded at low temperature. The coherent intergrowths were still found to occur at low temperature.

An electron microscope and X-ray diffraction study of some synthetic (Mg,Mn) 3BO 5 oxyborates

The phase region surrounding the composition (Mg,Mn) 3BO 5, the nominal formula of the phases ludwigite, orthopinakiolite, takéuchiite, and pinakiolite, has been investigated by powder X-ray diffraction and transmission electron microscopy. It has been found that in samples prepared at 1000°C or higher the phases lie in the sequence ludwigite, takéuchiite, and orthopinakiolite as the composition increases in oxygen and Mn 3+ content. All compounds appear to be nonstoichiometric and the composition (Mg,Mn) 3BO 5 has no special significance at 1000°C. The commonest defect microstructures observed were wider or narrower than normal twin plane spacings and coherent intergrowths of one structure type in another. Pinakiolite only formed at temperatures up to approximately 900°C. An alternative twinned form of pinakiolite, utilizing (100) twin planes, not previously observed before in synthetic material, has been characterized. It is suggested that takéuchiite is correctly regarded as a 1:1 intergrowth of ludwigite and orthopinakiolite and that pinakiolite and ludwigite are high and low temperature polymorphs with similar compositions.

Feldspar microtextures and multistage thermal history of syenites from the Coldwell Complex, Ontario

Optical and TEM (transmission electron microscopy) observations of perthites from augite syenites in the Coldwell Complex (Ontario) reveal a complex set of microtextures that outline a multistage thermal history. Regular microtextures (linear or braid texture, straincontrolled, coherent intergrowths) show a progressive evolution from the margin of the intrusion inwards with lamellar spacings in the range 40–100 nm. The textures evolve in a manner similar to those for the Klokken intrusion and reflect differences in cooling rates and bulk composition. Superimposed upon the regular microtexture are 10 μm scale compositional fluctuations which we call “ripples”. The boundary relationships and bulk composition of ripples, which are themselves Ab-rich and Or-rich linear coherent cryptoperthites, suggest that they formed by coarsening during a phase of high-temperature (∼530°C) fluid-feldspar interaction. This was followed by a return to coherent exsolution in which fluid was not involved. Coarse, irregular, patch microperthite cross-cuts all other microtextures. These final “deuteric” intergrowths are believed to result from a further low-temperature (< 380° C) fluid-feldspar interaction and are associated with subgrain formation and the presence of micropores. The outermost syenite sample, against a gabbro ring structure, has distinctive, modified microtextures, indicating that the gabbro is, at least in part, a later intrusion. Our findings show that TEM work on alkali feldspar microtextures can identify discrete thermal events in the cooling history of igneous plutons and illustrates the potential of such microtextures for establishing the relative ages of intrusive rocks.

Nonstoichiometry in oxides

Nonstoichiometry phenomena have been extensively studied in oxide systems. It is proposed that some models developed for oxides could be usefully extended for understanding nonstoichiometry in other systems. Many aspects of nonstoichiometry are closely connected with order-disorder phenomena. The broad field of nonstoichiometric compounds extends from variable composition phases with randomly distributed point defects (entropy controlled systems) to sets of discrete intermediate sompounds, each corresponding with a fully ordered state resulting from assimilation or annihilation of defects (enthalpy controlled systems). Between these two extreme cases, one finds intermediate situations such as clusters with a short range order, shear structures with possibly swinging shear planes, coherent intergrowths and infinitly adaptive structures in which any composition, even irrational, corresponds with an unique ordered structure.

Phyllosilicates in the Mokoia CV carbonaceous chondrite: Evidence for aqueous alteration in an oxidizing environment

Most CV chondrites contain little if any phyllosilicate mineralization. A petrographic and transmission electron microscopy study of the Mokoia CV carbonaceous chondrite shows that the matrix, chondrules, aggregates, and inclusions all contain considerable amounts of phyllosilicates. Those in the matrix are mostly Fe-bearing saponite, while those in chondrules, aggregates, and inclusions are both Febearing saponite and Na-rich phlogopite. Serpentine occurs as minor coherent intergrowths within the phlogopite. The saponite resulted from aqueous alteration of Fe-rich olivine, and the phlogopite resulted from alteration of Ca-rich clinopyroxene, anorthite, spinel, and Ca-Al-Si-rich glass. Iron-rich olivines in the Mokoia matrix contain extremely thin, planar zones parallel to (100), with diffuse streaks along a ∗ in their diffraction patterns. Such planar zones are not known from terrestrial olivines. Saponite forms preferentially along these zones, with its (001) planes roughly parallel to olivine (100). The planar zones probably consist of precipitates of Fe oxide or Fe hydroxide that were produced during oxidation of olivine while it cooled from a high temperature. By formation of the Fe-rich precipitates, olivine became Fe deficient and thus was structurally disturbed. During subsequent hydration, saponite formed by preferentially replacing these regions in olivine. Simultaneously, part of the Fe was mobilized and precipitated as magnetite along the olivine peripheries. The mineralogy and occurrence of phyllosilicates in Mokoia differ from those in the CI and CM chondrites. The differences suggest that aqueous alteration of the three meteorite groups probably occurred under a variety of conditions.