Band 90, Heft 8-9

We studied the sizes and shapes of magnetite nanocrystals produced by several types of uncultured magnetotactic bacteria to understand whether their size distributions can be used for identifying the biogenic origin of magnetite crystals in geological samples. The two-dimensional projections of the crystals were measured on transmission electron microscope (TEM) images, and features of crystalsize and shape-factor distributions (CSD and SFD, respectively) were analyzed. In agreement with previous results, most magnetite CSD curves are asymmetric and negatively skewed; however, one magnetotactic strain produced particles that have a normal size distribution. A statistical analysis of CSDs and SFDs (both from this and previous studies) reveals similarities among magnetite from magnetotactic strains from various locations. In particular, crystals in a cultured marine strain (MC-2) were indistinguishable from magnetite from a freshwater strain. We tested whether CSDs of distinct magnetosome types can be recovered from the shape and size data of all particles combined in samples that contain several types of magnetosomes; such samples can be used as models for rocks that contain magnetite nanocrystals of unknown and, presumably, various origins. If the SFDs of the distinct magnetosome types occurring in the same sample differ, the CSDs of individual magnetosome types can be retrieved from bulk data. In such cases the characteristic shape of the size distribution can be used for identifying magnetite as originating from magnetotactic bacteria

Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about fluid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks, including meta-granite. The rocks were affected by two sets of folding (F1 and F2) and ductile shearing associated with F1 during ca. 1.6.1.8 Ga tectonic activity in this belt. Quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism; a prograde M1 event that culminated at 480 ± 40 °C, 6.4 ± 0.4 kbar, and an M2 event that caused retrogression of the M1 assemblages. Field and petrographic observations in combination with data on chemical composition of tourmaline indicate three stages of tourmaline growth. The earliest stage is represented by pre-tectonic (pre-metamorphic) greenish-yellow tourmaline cores in the tourmalinite from the Surda area (Association A). The textural relations demonstrate that in the second stage, greenish-yellow cores of oscillatory zoned tourmaline in the metagranite (albite + quartz + biotite + chlorite ± muscovite ± apatite ± magnetite, Association C), very fine tourmaline grains in muscovite schist (muscovite ± quartz, Association B) and greenish yellow cores of tourmaline in biotite-muscovite schist (biotite + muscovite + magnetite + chlorite + apatite+ quartz, Association D) crystallized during the F 1 -M 1 events. Post-tectonic, greenish-blue tourmaline, belonging to the third stage, replaces the earlier tourmaline grains along their margins and cracks. Compositionally, tourmalines of all four associations fall in the alkali group. Most of the data fall in the dravite field with a few analyses straddling the boundary between schorl and dravite. An abrupt compositional change was noted across the different color zones in the tourmaline grains. This change is explained by one or more of the coupled substitutions, ◻Al(NaR) -1 , CaR(NaAl) -1 , and Mg(Fe) -1 , where R = Mg + Fe 2+ + Mn. Integrating the field, petrographic, and phase-compositional data, it has been demonstrated that tourmaline growth in the second and third stages was induced by infiltration driven, B metasomatism during prograde metamorphism of the rocks. Deep-seated magma was the source of the B-bearing hydrothermal fluid

The crystal structure and microstructure of bornite have been of great interest to mineralogists since the pioneering studies of Morimoto and his colleagues (Morimoto 1964; Koto and Morimoto 1975). Because many questions about bornite remain, we are re-examining its structure using HRTEM. Highresolution transmission electron microscope (HRTEM) images taken along the <101> axis of bornite and processed by CRISP show variations of contrast between superstructure images and sublattice (1α) images. This contrast variation in the processed images is assumed to arise from vacancy ordering. Atom coordinates in the 4a-I and 6a-I superstructures are determined with the assumptions that superstructures possess the same symmetry as the sublattice and metal atoms occupy tetrahedral sites. Final structural models of 4a-I and 6a-I superstructures are calculated using simultaneous equations involving structure factors derived from Fourier transforms (FT) of HRTEM images, and the results are confirmed by image and diffraction simulations. With these models, the non-systematic extinctions are explained by the statistical vacancy ordering patterns. However, due to various approximations, the structures proposed are only possible models, but not unique solutions. So far, no low-temperature structure of bornite (2α4α2α) has been observed, but 1α, 2α, and 4α-I or 6α-I superstructure domains are always found to coexist in bornite near room temperature. We have also found that electron beam damage can induce the 1α2α1α superstructure

Based on magnetic structure and TEM studies of bornite we propose a new 2α Fe/Cu ordered superstructure model with symmetry F4̅3m and formula Cu 8 Fe 4 S8. First principles calculations (LAPW implemented in WIEN97 code) of two Fe/Cu ordering schemes indicate that, at the ground state, the Fe atoms should fill the tetrahedral sites of sulfur atoms in the anti-fluorite cube, and the vacancies are associated with Cu atoms in the zincblende cube of the superstructure. The calculated magnetic moment of iron in the Fe/Cu-ordered structure is about 4 μ β , which was not expected from previous work because bornite has both covalent and metallic bonds that make crystal-field theory inappropriate for explaining the magnetic moment of Fe. High-resolution transmission electron microscopy (HRTEM) image simulations for the newly proposed structure model are much closer to experimental HRTEM images than those for the model proposed by Kanazawa et al. (1978)

Picocyanobacteria were found to play an important role in calcite precipitation in oligotrophic lakes. In this study, investigations on the interface between cyanobacteria and attached biogenic calcite crystals have been performed to gain further insights into the mechanisms of nucleation of these precipitates. Ultramicrotomy, the conventional preparation technique of thin sections for Transmission Electron Microscopy (TEM) investigations, often fails when working on heterogeneous samples containing soft organic material and hard minerals. Thus, in this study the thin sections were prepared using Focused Ion Beam (FIB) milling. This approach is usually applied in material sciences but until recently was not very common in environmental research. Different analytical TEM methods like Electron Spectroscopic Imaging (ESI) and Electron Energy Loss Spectrometry (EELS) were used to test the suitability of FIB-milling for the preparation of organic/inorganic interface specimens. With this approach we were able to analyze both organic and the inorganic phases of the same sample. Elemental maps of the samples were also calculated. By analyzing the structure of the C K-absorption edge, the different bonding forms of the organic carbon cell and the inorganic carbon of the crystal could be clearly distinguished

The magnetic susceptibility and Curie temperatures T c have been investigated for a series of synthetic samples with solid-solution compositions ranging from pure magnetite (Fe 3 O 4 ) to hercynite (FeAl 2 O 4 ). The determined T c can be fitted by a straight line, which also fits the theoretical values for these end-members. With increasing hercynite concentration, susceptibility curves for one heating and cooling cycle become irreversible, indicating changes in the structural state of the samples during annealing. These changes occur in specific temperature ranges for each composition. For a sample of composition Mag 40 Hec 60 , irreversible changes occurring between about 200 and 300°C are likely due to changes in the cation distribution, whereas above 300°C, compositional Fluctuations due to spinodal decomposition are evident. The exsolution mechanism has been investigated using energy- filtered transmission electron microscopy, which has allowed direct imaging of the compositional Fluctuations consistent with the theoretical predictions of spinodal decomposition

Enthalpies of formation of rutherfordine, UO 2 CO 3 , andersonite, Na 2 CaUO 2 (CO 3 ) 3 (H 2 O) 5 , and grimselite, K 3 NaUO 2 (CO 3 ) 3 (H 2 O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, ΔHr-ox, is .99.1 ± 4.2 kJ/mol for the reaction UO 3 (xl, 298 K) + CO 2 (g, 298 K) = UO 2 CO 3 (xl, 298 K). The Δ Hr-ox for andersonite is .710.4 ± 9.1 kJ/mol for the reaction Na2O (xl, 298 K) + CaO (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + 5H 2 O (l, 298 K) = Na 2 CaUO 2 (CO 3 ) 3 (H 2 O) 6 (xl, 298 K). The Δ Hr-ox for grimselite is .989.3 ± 14.0 kJ/mol for the reaction 1.5 K2O (xl, 298 K) + 0.5Na 2 O (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + H 2 O (l, 298 K) = K 3 NaUO 2 (CO 3 ) 3 H 2 O (xl, 298 K). The standard enthalpies of formation from the elements, ΔHfº f are .1716.4 ± 4.2, .5593.6 ± 9.1, and .4431.6 ± 15.3 kJ/mol for rutherfordine, andersonite, and grimselite, respectively. Energetic trends of uranyl carbonate formation from the binary oxides and ternary carbonates are dominated by the acid-base character of the binary oxides. However, even relative to mixtures of UO 2 CO 3 , K 2 CO 3 , and Na 2 CO 3 or CaCO 3 , andersonite and grimselite are energetically stable by 111.7 ± 10.2 and 139.6 ± 16.1 kJ/mol, respectively, suggesting additional favorable interactions arising from hydration and/or changes in cation environments. These enthalpy values are discussed in comparison with earlier estimates

The structural changes upon cation substitution in natural AB 2 O 6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb 2 O 6 - FeNb 2 O 6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Q m ~ 0.7. A discontinuity at this Q m value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modifications of the polyhedra by folding along the common edge

Compression mechanisms of A 2+ B 4+ O 3 ilmenites with compositions MgSiO 3 (stable at high pressures), MgGeO 3 (stable at moderate pressures), and MgTiO 3 (stable at ambient pressure) were investigated at high pressure by single-crystal structure analysis, using both synchrotron radiation and an MoKα rotating-anode X-ray generator. The distortions of AO 6 (A:Mg) and BO 6 (B: Si,Ge,Ti) octahedra under pressure were parameterized by bond length, shared-face area, site-volume ratio, and A 2+ -B 4+ interatomic distance across the shared edges and shared face. The AO 6 octahedral volume is much more compressive than the BO 6 octahedral volume. Of the three samples, both the AO 6 and BO 6 octahedra are most rigid in MgSiO 3 . The A 2+ -B 4+ interatomic distance becomes more shortened with increasing pressure than do the A 2+ -A 2+ and B 4+ -B 4+ distances. The compression of Mg-Si is more remarkable than that of Mg-Ge and Mg-Ti. The A-B interatomic distance along c is more compressed with increasing pressure than A-A and B-B along a. The short A-B distance across the shared face becomes more shortened than the A-A and B-B distances across the shared edge. The cation position moves in the direction of c with pressure and tends to approach the center of the AO 6 and BO 3 octahedra with increasing pressure. The regularity of the octahedra is enhanced at higher pressure. Both quadratic elongation and bond angle variance verify the reduction of the deformation of AO 6 and BO 6 octahedra with pressure

We investigated dehydration processes in uranyl phosphate minerals of the meta-autunite group that consist of uranyl phosphate sheets and interlayer cations, and water molecules. Meta-autunite [Ca(UO 2 PO 4 ) 2 ⋅6H 2 O], metasaléeite [Mg(UO 2 PO 4 ) 2 ⋅8H 2 O], and metatorbernite [Cu(UO 2 PO 4 )2⋅8H 2 O] were selected for our study. The change in basal spacing between two adjacent uranyl phosphate sheets with temperature was examined by temperature-controlled X-ray diffraction (XRD) analysis from room temperature (RT) to 300 °C to determine structurally distinct, dehydrated phases. Thermogravimetric and differential thermal analyses (TG/DTA) were also performed under conditions similar to those used for the XRD analysis to clarify the hydration states of the dehydrated phases. Retention of the structure of the uranyl phosphate sheets under a high vacuum, equivalent to 300 °C, was confirmed by transmission electron microscopy. Meta-autunite, metasaléeite, and metatorbernite decreased their basal spacings by losing water molecules. Comparison of the TG/DTA and XRD results indicates that the changes in basal spacings of the dehydrated phases with temperature are as follows: 8.32 Å (6 H2O per unit formula) at RT, 7.31 (? H 2 O) and 6.68 Å (? H 2 O) at 75 °C, 6.34 Å (2 H 2 O) at 120°C, and 5.81 Å (1 H 2 O) at 300°C for meta-autunite; 8.29 (8 H 2 O) and 7.73 Å (? H 2 O) at RT, 6.62 Å (probably, 2 H 2 O) at 40 °C, 6.54 Å (2 H 2 O) at 160 °C, and 5.52 Å (1 H 2 O) at 300 °C for metasaléeite; and 8.61 Å (8 H 2 O) at RT, 8.07 Å (4 H 2 O) at 100 °C, 6.58 Å (2 H 2 O) at 200 °C, and 5.60 Å (1 H 2 O) at 300 °C for metatorbernite. The dehydration processes revealed by XRD and TG/DTA under similar experimental conditions are slightly different from those obtained by previous studies. Our results clearly demonstrate the presence of previously unknown dehydrated phases of the meta-autunite group minerals with basal spacings less than 6Å that may have distinct thermodynamic properties

A coherent mechanism for the transformation of olivine to its spinel structure polymorph has been investigated in Mg 2 GeO 4 olivine at conditions ranging from 6 GPa to 18 GPa at 300 to 1250°C using a multi-anvil apparatus. This transformation mechanism, which produces plates of spinel lying parallel to the (100) planes in olivine, becomes increasingly important above 10 GPa at temperatures between 300 and 400°C. The primary effect of temperature above 400°C is to increase the length of the lamellae. The primary effect of pressure is to increase the number of lamellae that have nucleated. The distribution of transformation is very inhomogeneous. Lamellae first appear in patches associated with grain edges and corners. As the degree of transformation increases, patches become more extensive and more heavily populated until the majority, if not all grains, contain large numbers of lamellae that span large portions of their host grains. The mechanism that produces the lamellae requires three factors, high pressure overstep of the equilibrium boundary, some level of differential stress (either microscopic or macroscopic), and high enough temperature to facilitate rearrangement of the cations. It is most consistent with a martensitic-like transformation mechanism. The results of this study suggest that olivine may exhibit similar behavior in cold, rapidly subducting lithospheric slabs. If conditions are too cold for olivine to transform by reconstructive nucleation and growth, it could instead progressively transform by the martensitic-like mechanism as conditions deviate further and further from the equilibrium phase boundary

The structure of CaTiOGeO 4 (CTGO) has been refined using single crystal X-ray diffraction data. CTGO is isostructural with titanite, CaTiOSiO 4 . The displacive P2 1 /a-A2/a phase transition analogous to titanite has been studied by in situ heating X-ray powder diffraction and transmission electron microscopy. The transition is accompanied by the disappearance of superstructure reß ections, k + l = 2n + 1, which are replaced by diffuse scattering for T > T c . The diffuse scattering is seen as streaks along b* in high-temperature TEM SAD. Lattice parameters as a function of temperature and composition were determined by X-ray powder diffraction between room temperature and a maximum of 1123 K. Strain analysis of CTGO indicates a transition temperature of T c = 588 ± 4 K and the additional occurrence of an isosymmetric anomaly at T i = 800 ± 25 K. There is complete solid-solution along the join CaTiO(GexSi 1-x )O 4 . The lattice parameters across the solid solution vary continuously and the structural phase transitions were identified based on the determination of spontaneous strain associated with the transitions. The e11 and e13 components dominate the strain tensor. All compositions across the solid solution exhibit close to tricritical behavior and nearly constant scalar strain

The powder IR spectra of synthetic hydrogrossular, Ca 3 Al 2 (O 4 H 4 ) 3 , were recorded at temperatures between 298 and 10 K and the polarized single-crystal Raman spectra at 298 and 4 K. The results were interpreted using factor group analysis which predicts the number and symmetry of the different IR- and Raman-active modes. A first attempt was made to assign the observed bands in the IR and Raman spectra to various atomic or polyhedral motions and to make a first-order lattice dynamic analysis. The mode assignments for grossular, Ca 3 Al 2 (SiO 4 ) 3 , and hydrogrossular were compared. The measured spectra are considerably different in the high-wavenumber region, where O-H stretching modes occur, between 298 and 10/4 K. In the Raman spectra two different symmetry O-H bands are observed at 3648 cm -1 (A 1g + E g ) and 3653 cm -1 (F 2g ) at room temperature, while at 4 K several O-H bands are present. At room temperature an IR-active O-H band located around 3662 cm -1 narrows and shifts to higher wavenumbers and also develops structure below about 80 K. Concomitantly, several weak intensity O-H bands located around 3600 cm -1 begin to appear and they become sharper and increase in intensity with further decreases in temperature down to 10 K. The spectra indicate that the vibrational behavior of individual OH groups and their collective interactions measurably affect the lattice dynamic (i.e., thermodynamic) behavior of hydrogrossular. The (O 4 H 4 ) group in garnet is best described as a “chemical component” that can substitute for SiO 4 tetrahedra and not as a “structural or polyhedral unit”. The line widths of the IR and Raman O-H bands at low and room temperature were interpreted qualitatively based on the time scales associated with the Raman and IR experiment, which are different in magnitude. The results of this study demonstrate the need for making spectroscopic measurements at low temperatures when studying the vibrational behavior of O-H modes in minerals

Natural Mn-oxide nanoparticles and grain coatings are ubiquitous in the environment and profoundly impact the water quality and quality of sediments through their ability to degrade and sequester contaminants. These oxides, which are believed to form dominantly via oxidation of Mn 2+ by marine and freshwater bacteria, have extremely high sorptive capacities for heavy metals. We have used XANES, EXAFS, and synchrotron (SR)-XRD techniques to study biogenic Mn oxides produced by spores of the marine Bacillus sp. strain SG-1 in seawater as a function of reaction time under in-situ conditions. An EXAFS model was developed to fully account for the structure and features in the data, providing realistic structural information. The first observed biogenic solid-phase Mn-oxide product is a layered phyllomanganate with hexagonal sheet symmetry and an Mn-oxidation state similar to that in δ-MnO 2 , between 3.7 and 4.0. XRD and SEM-EDS data show the biooxides to have a phyllomanganate 10 Å basal plane spacing and an interlayer containing Ca. With time, a phyllomanganate oxide with pseudo-orthogonal sheet symmetry appears. Fits to these EXAFS spectra suggest the octahedral layers have relatively few Mn octahedral site vacancies in the lattice and the layers bend to accommodate Jahn-Teller distortions creating the change in symmetry. A reaction mechanism is proposed to account for the observed products. The phyllomanganate oxides observed in this study may be the same as the most abundant Mn-oxide phases suspended in the oxic and sub-oxic zones of the oceanic water column that are of global importance in trace metal and nutrient cycling

Hydration of the <1 μm size fraction of SWy-1 source clay (low-charge montmorillonite) was studied by modeling of X-ray diffraction (XRD) patterns recorded under controlled relative humidity (RH) conditions on Li-, Na-, K-, Mg-, Ca-, and Sr-saturated specimens. The quantitative description of smectite hydration, based on the relative proportions of different layer types derived from the fitting of experimental XRD patterns, was consistent with previous reports of smectite hydration. However, the coexistence of smectite layer types exhibiting contrasting hydration states was systematically observed, and heterogeneity rather than homogeneity seems to be the rule for smectite hydration. This heterogeneity can be characterized qualitatively using the standard deviation of the departure from rationality of the 00l reflection series (ξ), which is systematically larger than 0.4 Å when the prevailing layer type accounts for ~70% or less of the total layers (~25% of XRD patterns examined). In addition, hydration heterogeneities are not distributed randomly within smectite crystallites, and models describing these complex structures involve two distinct contributions, each containing different layer types that are interstratifed randomly. As a result, the different layer types are partially segregated in the sample. However, these two contributions do not imply the actual presence of two populations of particles in the sample. XRD profile modeling also has allowed the refinement of structural parameters, such as the location of interlayer species and the layer thickness corresponding to the different layer types, for all interlayer cations and RH values. From the observed dependence of the latter parameter on the cation ionic potential (v/r; v = cation valency and r = ionic radius) and on RH, the following equations were derived: Layer thickness (1W) = 12.556 + 0.3525 × (v/r . 0.241) × (v × RH . 0.979) Layer thickness (2W) = 15.592 + 0.6472 × (v/r . 0.839) × (v × RH . 1.412) which allow the quantification of the increase of layer thickness with increasing RH for both 1W (one water) and 2W (two water) layers. In addition, for 2W layers, interlayer H 2 O molecules are probably distributed as a unique plane on each side of the central interlayer cation. This plane of H 2 O molecules is located at ~1.20 Å from the central interlayer cation along the c* axis

This paper reports the results of hydrothermal synthesis in the system Na 2 O-MgO-FeO-Fe 2 O 3 -SiO 2 - H 2 O. Four samples of stoichiometric magnesioriebeckite composition, ideally ◻Na 2 Mg 3 Fe 2 3+ Si 8 O 22 (OH) 2 , were run at 700-800 °C, 0.4 GPa, and redox conditions varying from NNO (Nickel.Nickel Oxide) to NNO + 2.3 log f O₂ . Powder XRD and SEM-EDX show a high (>85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa 2 Mg 4 Fe 3+ Si 8 O 22 (OH) 2 ]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reß ect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mössbauer spectra show that in all samples, Fe 3+ is completely ordered at M2, whereas Fe 2+ occurs at the M1, M3, and M4 sites. The Fe 3+ /Fe 2+ ratio is a function of f O₂ : for increasing oxidation conditions, there is significant increase in M2Fe 3+ and decrease in Fe 2+ , notably in M 4 Fe 2+ . Mössbauer spectra also show significant variation in M1 Fe 2+ and M3 Fe 2+ quadrupole splitting as a function of the Fe 3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies <3680 cm -1 , assigned to octahedral M1,3 (Mg, Fe 2+ )-OH-A◻ configurations, and a broad band consisting of four overlapping components related to M1,3 (Mg, Fe 2+ ) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing f O₂ of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the C (Mg,Fe 2+ ) 1 B (Mg,Fe 2+ ) 1 CFe 3+ -1 B Na -1 and the A Na 1 C (Mg,Fe 2+ )1 A ◻ -1 CFe 3+ -1 exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f O₂ during crystallization

series of natural sphalerite samples, characterized by an inhomogeneous (zonal) distribution of Fe, Mn, and Cd atoms substituting for Zn, has been investigated by electron probe microanalysis and X-ray element distribution mapping. The cation distributions are markedly inhomogeneous due to the reciprocal effects of the minor elements on their respective solubilities. In particular, zonal partitioning between Fe and Mn and between both cations and Hg was observed in sphalerite, as well as coupled Cu-In and Cu-Fe substitutions. Distinctly different distribution patterns were identified in Mn-free samples with a low Fe-content, as compared to Mn-bearing samples. In the former case, Fe and Cd distribution patterns are very similar, whereas in the latter case, Cd is distributed homogeneously and Mn and Fe patterns are antithetic. The different oscillatory zoning observed in Mn-free samples is attributed to a fast, self-organized solute (Fe, Cd) deposition, although an alternative external origin cannot be ruled out. On the other hand, in the presence of Mn, zoning may be related to an absorption process at the mineral-fluid interface controlled by a competition between Mn and Fe that may explain the observed limited coupled concentration of these elements in sphalerite. The homogeneous distribution of Cd suggests relatively slow crystal growth

The structure of multi-component silicate melts and glasses (e.g., Ca-Mg and Ca-Na aluminosilicates) can provide insight into the properties of natural silicate melts and has implications for relevant magmatic processes. In spite of its importance, the atomic and molecular structure of most multicomponents (e.g., quaternary) melts and glasses has not been fully described, primarily because of insufficient resolution obtained with conventional spectroscopic and scattering methods; the information obtained by these methods is compromised by severe inhomogeneous peak broadening due to structural complexity. Here we report the first 17 O and 27 Al 3QMAS NMR spectra for quaternary, Ca-Mg and Ca-Na peralkaline aluminosilicate glasses (i.e., M/Al > 1, M is one monovalent or one-half a divalent cation). These data reveal new details into the molecular structure of multi-component aluminosilicate melts, which include the presence of a substantial fraction of V Al in the Ca-Mg aluminosilicate glasses and IV Al-O- IV Al in both glasses at 1 atm. Traditional models of glass structure do not support the presence of such species given these high-silica, peralkaline compositions. These results suggest that Al avoidance is violated in the multi-component peralkaline aluminosilicate glasses, and that the presence of Mg 2+ in the melts increases the extent of disorder in the melts (compared with Ca 2+ and Na + ). These factors lead to an increase in configurational entropy and the activity coefficients of the oxides, and may provide an explanation for the decrease in viscosity of these complex melts

Makarochkinite, [VII] (Ca 1.64 Na 0.25 Mn 0.11 ) [VI] (Fe 2+ 3.56 Fe 3+ 1.46 Ti 0.61 Mg 0.25 Mn 0.02 Nb 0.038 Ta 0.007 ) [IV] (Si 4.48 Be 0.91 Al 0.54 Fe 0.07 O 18 )O 2 (end-member Ca 2 Fe 4 2+ Fe 3+ TiSi 4 BeAlO 20 ) from electron-microprobe data, Mössbauer spectroscopy, and single-crystal structure refinement, occurs in a granitic pegmatite near Lake Ishkul', Il'men Mountains (Southern Urals), Il'men State Reserve, Chelyabinsk Oblast', Russia. Associated minerals include danalite, phenakite, titanite, potassian calcic amphibole (ferro-edenite and hastingsite), biotite, ilmenite, magnetite, ferrocolumbite, fergusonite-(Y), and samarskite-(Y). Makarochkinite forms equant masses 5.50 mm across, black in hand specimen; luster vitreous; it is opaque except in slivers <1 μm thick. It is brittle; Moh's hardness 5.5.6, and has no discernable cleavage; fracture is uneven. Twinning is absent. The measured density is 3.93(1) g/cm 3 ; calculated density 3.933 g/cm 3 . It is optically biaxial, α = 1.835 ± 0.014 and γ = 1.865 ± 0.015 from reß ectance data, pleochroism marked: X = greenish brown, Y = yellowish brown, Z = reddish brown; Y ≤ X < Z. Single-crystal X-ray diffraction gives triclinic symmetry, space group P1̅ . , Z = 2, wR(F 2 ) (all) = 0.096 for 6497 reß ections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) Å, α = 105.707(8), β = 96.227(6), γ = 124.861(6)°, V = 735.7(3) Å 3 . The eight strongest lines in the powder pattern [d-spacing (Å),(I),(hkl)] are 7.997(57)(100), 4.779(29)(011), 3.120(32)(012), 2.924(69)(01̅3), 2.676(77)(2̅03), 2.530(100)(2̅1̅3), 2.410(28)(22̅3), 2.075(39)(41̅1). Of the aenigmatite-sapphirine-surinamite group minerals, makarochkinite is compositionally closest to rhönite and høgtuvaite. It is distinguished from rhönite by Fe 2+ > Mg and by the presence of 0.91.0.98 Be per 20 cations; Be occupies the two most polymerized T sites in roughly equal amounts. Distinction from høgtuvaite is based on occupancy of the M7 site, which is dominated by Fe 3+ in høgtuvaite and by Ti in makarochkinite

Lava tubes are often coated with spectacular lava stalactites that are thought to form by a process of lava remelting. Here, we present results from lava stalactites collected inside a master lava tube that fed the 1792−1793 Etna flank eruption, which show features rather different from their Hawaiian or Icelandic counterparts. We analyzed three types of stalactites recognized at Mt. Etna on the basis of their morphology, and compared their features with those of the lava flow hosting the tube. Three-dimensional morphologic analyses by SEM, petrographic observations, and mineral and glass composition measured by SEM-EDS, allowed us to infer processes and conditions of stalactite formation. Our results indicate that in all the analyzed stalactites, the nature, abundance and composition of phenocrysts is similar to that of the host lava flow. This finding suggests a common mechanical origin for different types of stalactites, caused by drainage of the tube and dripping of fluid lava from the roof. However, the composition of interstitial glass is significantly different from that of the glassy groundmass measured in historical volcanic rocks of Mt. Etna and suggests that, once stalactites solidi field, they were affected by a process of partial melting. Partial melting involved between 12 and 25% of the bulk rock, causing the wide compositional variation and enrichment in K2O measured in our samples

Crystal size distributions (CSDs) provide information about the nucleation and growth conditions of crystals. In the present study, we investigated CSDs of garnets in metapelites in the Bushveld contact aureole to examine the mechanisms of garnet nucleation and growth and to deduce controlling factors of CSDs. The CSD shapes were evaluated in terms of the Law of Proportionate Effects crystal growth and of the thermally accelerated, diffusion-controlled nucleation and growth. We adopted both the nonparametric and parametric stereological methods to estimate three-dimensional crystal size distributions from two-dimensional measurements of crystal diameters. The studied garnets showed various CSD shapes: asymptotic, lognormal, and near-symmetric distributions. These varieties of CSDs are generally controlled by heating rate, metamorphic temperature, and duration of thermal effects due to the intrusion of the igneous body, the Rustenburg Layered Suites. The chemical composition of the host sedimentary rocks is an additional important factor in controlling CSD shapes: it affects garnet growth as a supply source of nutrients for the mineral. At the contact between the Rustenburg igneous rocks and the aureole, where a rapid increase of temperature and a migmatitic extraction of siliceous melts occurred, the CSD is asymptotic showing constant- or accelerating-nucleation rates. In contrast, at some distance from the igneous contact where a relatively slower rise of temperature occurred and lognormal to near symmetric CSDs were mainly generated, suggesting decaying-rate nucleation dominated. However, even in such thermal conditions, differences in chemical compositions of host rocks affected growth mechanisms of garnet porphyroblasts, which resulted in relatively symmetric and pseudo-lognormal CSDs of garnets in biotite- and quartz-rich layers, respectively, in a pelitic sample

Hydrated alumino-phosphate-sulfate (APS) minerals of the woodhouseite and crandallite series are found as microcrystalline aggregates replacing volcanic glass and apatite and filling pores in sandstones of the fluvial Lower Cretaceous Chaswood Formation, southeastern Canada. Mineral grains from heavy mineral separates and rock samples were analyzed with an electron microprobe. As the crandallite component of the grains increases, REE and F content increases. The APS minerals formed during early diagenesis as a result of the flux of meteoric water below the water table, beneath regional intraformational unconformities at which oxisols had developed. Sulfate was derived from the oxidation of very early diagenetic pyrite, and phosphate from dissolution of detrital apatite. The APS minerals are commonly associated with detrital dolomite, which may have played a role in increasing pH in the circulating meteoric water. Zoning of APS grains to lower S concentrations outward suggests that SO 4 2- concentration was a determining factor in APS mineralogy and mineral composition

A chemical and paragenetic study has been performed on Sc silicates and Sc-bearing beryl occurring in Hercynian NYF-miarolitic pink granite and granophyric leucogranite at Baveno and Cuasso al Monte, Western Southern-Alps, Italy. In the Baveno and Cuasso al Monte plutons, detailed field work allowed the discovery of a significant number of crystals of bazzite, thortveitite, scandiobabingtonite, cascandite, and jervisite representative of all the known morphological and color varieties of these minerals in the two localities. Other studied samples belong to the historic collections of the Natural History Museum of Milan. Except for beryl, which crystallizes relatively early in aplitic granophyre, all the other Sc minerals crystallize as late-stage phases in cavities associated with fluorite. Chemical analyses reveal moderate Sc enrichment at the rim of beryl crystals. Bazzite displays a relatively large chemical variation, from .primitive. compositions enriched in Fe 2 O 3 and Al 2 O 3 , to .highly evolved. compositions with values of Sc 2 O 3 up to 17.54 wt%. Scandiobabingtonite shows a perfect inverse correlation between Fe 3+ and Sc concentrations, suggesting complete solid solution between babingtonite and its Sc analogues. A wide variety of compositions have been determined for cascandite, significantly extending the compositional range of this mineral. In particular, the MnO content ranges from 0.37 to 4.87 wt%. The jervisite crystals analyzed in this work have rather homogeneous compositions much closer to the end-member if compared with the holotype analysis reported in literature. Thortveitite shows a wide range of compositions with variation in Sc 2 O 3 , Y 2 O 3 , HREE, and Fe 2 O 3 . Significant Fluctuations of the Sc/Yb ratios are in agreement with similar complex variations in the ratios between REE (e.g., Y/Dy) reported in the literature for crystals of gadolinite-group minerals from the same localities. Two different genetic models are discussed to explain the precipitation of Sc silicates as late stage phases in cavities. (1) during the latest stages of magma crystallization, HFSE and Sc were extracted from the silicate liquid and partitioned into fluids due to the complexing effect of F. Indeed, in view of the NYF geochemistry of the granite and the significant abundance of the associated F-bearing minerals, Fluorides (but not other complexing agents such as carbonates and phosphates) played the major role in concentrating HFSE and Sc. In cavities, such elements resulted in a series of rare accessory phases when F was extracted from Fluids because of the precipitation of zinnwaldite and Fluorite. (2) HFSE, Sc, and Y+REE were mainly incorporated by gadolinite-(Y) and siderophyllite crystallizing from residual magma. Many of the accessory phases crystallized in cavities because of the aggressive effect of subcritical hydrous, F-rich Fluids on the previously formed gadolinite-(Y) (liberating REE, Y, B, Be, Fe, Ca), siderophyllite (liberating Fe, Ti, and possibly Nb-Ta, Sc, etc.), and feldspars (liberating Ca, P, Cs, Ba). This second model is consistent with the widespread hydrothermal alteration of the Baveno and Cuasso al Monte granites and granophyres

Typical liquidus temperatures can be over 1000°C greater than the glass transition temperatures for high-pressure aluminosilicate melts so the effect of temperature must be determined if glass data is to be used to approximate the structural speciation present in geologic melts. This study has investigated the effect of fictive temperature (T f , taken as the temperature where the melt structure is the same as that of the glass) on the percentage of [5] Al and [6] Al species in two high-pressure (10 GPa) Na-aluminosilicate glasses (Na 3 AlSi 7 O 17 and NaAlSi 3 O 8 ) where one glass of each composition was quenched from the high-pressure melt while the other was annealed near the glass transition temperature. The 27 Al MAS NMR spectra of the Na 3 AlSi 7 O 17 samples show that the higher T f (quenched) glass contains more high-coordinated Al than the lower T f (annealed, 475°C) glass. However, the 27 Al spectra of the NaAlSi 3 O 8 samples show the opposite temperature dependency, which in addition to the lack of NBO in this glass, may suggest differing mechanisms for the generation of high-coordinated Al

A natural high-pressure phase of titanium oxide with α-PbO 2 -structure has been found in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China. High-resolution transmission electron microscope observations have revealed an orthorhombic lattice, corresponding to α-PbO 2 - type TiO 2 with cell parameters a = 0.461 nm, b = 0.540 nm, c = 0.497 nm and space group Pbcn. It occurs as nanometer-thick (<2 nm) lamellae between multiple twinned rutile crystals, providing additional evidence of very high-pressure, metamorphism at 7 GPa, 900°C. This implies subduction of continental material to a depth of more than 200 kilometers. α-PbO 2 -type TiO 2 could be an extremely useful index mineral for ultrahigh-pressure

Morphology of kaolinite crystals with high-crystallinity (Keokuk, Iowa) has been investigated by electron back-scattered diffraction (EBSD) and high-resolution scanning electron microscopy (SEM). EBSD patterns from the specimen generally correspond to those expected from ordered kaolinite. Elongated hexagonal crystals always show the a-axis parallel to the elongated direction. The sidefacets of these crystals are in parallel to the c-axis, which is inclined by about 15° from the normal to the basal plane. The Miller indices of the side-facets are exactly ±(110), ±(11̅0), and ±(010). A facet indexed as ±(13̅0) is developed in some crystals. These morphological characteristics must be reflected in the ordered stacking sequences (the position of the octahedral vacancy site and the direction of the interlayer shift) of the Keokuk kaolinite. Inversely, the crystallinity of individual kaolinite grains may be evaluated from their morphology in a SEM. The feasibility to discriminate the enantiomers in kaolinite using EBSD is also described