1,720,974 research outputs found
Thomsonite: a detailed refinement with cross checking by crystal energy calculations
No full textThe thomsonite [12399-54-1], from Death Valley, California, is orthorhombic, space group Pncn, with a = 13.089, b = 13.047, and c = 13.218 Å. The structure was refined to a final R of 0.035. The ordering scheme for the Si-Al distribution doubles the value of the c parameter, typical of all other fibrous zeolites. The possible Si-Al distribution is discussed for samples with an Si/Al ratio >1. Two cation sites were found. One site, occupied by Ca and Na, is at the center of a regular square antiprism, which is linked to 2 equiv. polyhedrons forming a chain parallel to c. The other site, occupied by Ca and Sr, statistically distributed over 2 equiv. positions near a center of symmetry, and is 6-fold coordinated. Four H2O mols. were found at all the H atoms located. The cation distribution and the H positions were verified by electrostatic energy calcns
A Crystal-chemical Model For Pbca Ortho-pyroxene
No full textThe study of a crystal-chemical model for Pbca orthopyroxene was undertaken using the results of X-ray single-crystal structure refinements and chemical analyses of about 200 samples with different compositions and degrees of order. Multiple linear correlations between compositional and structural variables were searched for using the statistic package SPSS. The coefficients and constant terms of linear equations that allow the prediction of cell parameters and interatomic distances for any orthopyroxene, starting from its crystal-chemical formula, were calculated. From the predicted distances, the geometric refinement program DLS-76 yields the relevant atomic positions, which agree satisfactorily with those measured experimentally. The same results were obtained more quickly using coefficients that directly express the correlation between the atomic fractions at the structural sites and the atomic positions. The calculation of atomic positions makes it possible to study the effects induced on the structure by any variation of chemical composition and cation distribution; in particular it has allowed the prediction of structural properties for fictive end-members. The coefficients for the calculation of cation-O mean bond distances of the regular polyhedra M1 and SiB appear to be nearly correlated to the ionic radii of the cations at these sites, thus justifying the introduction of (M1-O) and (SiB-O) bond distances in the linear equations used for determining or checking M1 and SiB site populations. The prediction of atomic positions can be used for lattice-energy calculations, in order to develop a structure-energy model for orthopyroxene. The coefficients and constant terms of linear equations for the atomic fractions as a function of cell parameters, atomic positions, and mean atomic numbers at M1 and M2 sites were also calculated in order to test the possibility of predicting, for any orthopyroxene, its cation distribution in the absence of chemical analysis
Crystal-chemical Implications of the Mg-2+-Fe-2+ Distribution In Orthopyroxenes
No full textStructure reflnement data of natural and heated orthopyroxenes with different Fe/
(Fe + Mg) ratios were used for a systematic analysis of structural variations as a function of
chemical composition and ordering degree. The mean T-O bond distances do not vary
significantlyw ith increasingir on content and thereforea ny changeo f (TB-O) distancesm ay
provide a measure of Si/Alrv substitution, which occurs only at the TB site. The mean Ml-O
distancei ncreasesli nearly with iron content as a consequencoef a homogeneouse xpansiono f
Ml site.T he mean M2-O distancea lso increasesw ith iron content,b ut the linear plot is the
sum of very different trends of the individual bond lengths; among these, the M2-O3 distancese
xhibit a peculiarb ehaviorc onsistentw ith a 4 + 2 coordination of the M2 polyhedron.
The R3+ cations, which enter the structure with the Si/Altr substitution, appear to order
totally in the Ml site
Coupling between non-convergent ordering and transition temperature in the C2/c <-> P2(1)/c phase transition in pigeonite
No full textA Landau potential with linear-quadratic coupling has been developed to describe interactions between a non-convergent order parameter, Q(OD), for Fe/Mg ordering, and the order parameter, Q(D), for the C2/c-P2(1)/c phase transition in pigeonite. Spontaneous strain relationships, and expressions for the effect of ordering on the transition temperature derived from this expansion, have been tested by single crystal X-ray diffraction methods. Lattice parameters collected from a natural pigeonite crystal with composition En(47)Fs(44)Wo(9), in situ at temperatures up to 1050 degreesC, reveal that increasing Q(OD) could act to suppress Q(D) by a mechanism which includes overlapping and opposing strain fields. In a second experiment, the intensities of superlattice reflections (h + k = 2n + 1) were followed in situ at temperatures up to 500 degreesC. The crystal was heated ex situ successively at 700, 750, 800, and 850 degreesC between repeated in situ measurements in order to produce changes in the degree of cation order. The resulting data sets, giving the temperature dependence of Q(D)(2) for different fixed values of Q(OD), are consistent with the initial Landau model. In particular, they show a strong and linear dependence of transition temperature on Q(OD). The fourth order coefficient of the expansion describing the phase transition is perhaps also renormalized by changes in Q(OD). It is suggested that the influence of Q(OD) on the phase transition could be greater than the influence of the phase transition on the equilibrium variation of Q(OD)
HT P21/c-C2/c phase transition and kinetics of Fe2+-Mg order-disorder of an Fe-poor pigeonite : implications for the cooling history of ureilites
No full textA natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature TTr = 944°C, first order in character. The Fe-Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in a range of temperatures between the closure temperature of the Fe-Mg exchange reaction and the transition temperature. Isothermal disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression of ln kD versus 1/T yielded the following equation: In KD = -3717(±416)/T(K)+1.290(±0.378); (R2 = 0.988). The closure temperature (Tc) calculated using this equation was ~740(±30)°C. Analysis of the kinetic data carried out taking into account the e. s. d.'s of the atomic fractions used to define the Fe-Mg degree of order, performed according to Mueller's model, allowed us to retrieve the disordering rate constants C0Kdis+ for all three temperatures yielding the following Arrhenius relation: In(C0K+dis) = In K0 - Q/(RT) = 20.99(±3.74) -26406 (±4165)/T(K); (R2 = 0.988) An activation energy of 52. 5(±4) kcal/mol for the Fe-Mg exchange process was obtained. The above relation was used to calculate the following Arrhenius relation modified as a function of XFe (in the range of The cooling time constant, η = 6 × 10-1 K-1 year-1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late cooling history of the ureilite parent body after impact excavation
Thermal history of ALH 84001 meteorite by Fe2+-Mg ordering in orthopyroxene
No full textA single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was
studied by X-ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information
about its thermal history. Both sets of data were used to measure the Fe2+-Mg order degree between
the M1 and M2 sites expressed by the distribution coefficient kD. The 529 ± 30 °C closure temperature
(Tc) of the Fe2+-Mg ordering process of ALH 84001 orthopyroxene (Fs28) was calculated using
Stimpfl (2005a, 2005b) lnkD versus 1/T equation obtained for intermediate iron sample. At this Tc, the
orthopyroxene cooling rate, calculated by Ganguly’s (1982) numerical method, was 0.1 °C/day. This
study puts new constraints on the last high-temperature thermal episode recorded by orthopyroxene.
With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and
we interpret the Tc of 529 °C as a lower limit for this impact heating. Our data confirm that
experimentally defined physical conditions for the formation of magnetite from decomposition of
carbonates took place on the Martian surface during event I3
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
CINETICA DEL PROCESSO DI ORDINE-DISORDINE Fe 2-Mg IN UNA PIGEONITE P21/c POVERA IN FERRO
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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