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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
High-pressure behavior of space group P2/n omphacite
No full textA single-crystal X-ray diffraction (XRD) study, using a diamond-anvil cell at high pressure and room temperature, was performed on a crystal from a natural space group P2/n omphacite sample with composition very close to Jd 55Di 45 and with a high degree of order in cation distribution. Unit-cell pa-rameters were determined at 13 different pressures up to about 7.5 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) fitted to the P-V data yielded V 0 = 421.43(4) Å 3, K T0 = 122(1) GPa, and K' = 5.1(3). The K T0 value for this sample lies between the data obtained for the two end-members jadeite and diopside, and describes a slight positive curvature trend. During the same experiment, intensity data were collected and crystal structures were refined at 5 pressures up to 7.3 GPa. Both M1 and M2 polyhedra volumes showed a slight but significant change in slope at about 4 GPa. This behavior can likely be explained in terms of tilt angle variation of TA and TB tetrahedral, which also showed a change in slope with pressure, rather than in terms of bond length compression anomaly
Ortho-pyroxene From the Serra De Mage Meteorite - Structure Refinement and Estimation of C2/c Pyroxene Contributions To Apparent Pbca Diffraction Violations
No full textTransmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Mage cumulate eucrite shows augite lamellae thicker than 500 Angstrom, irregularly spaced within the orthopyroxene matrix and parallel to (100), with a(Opx)* = a(Aug)*, [101](Opx)*, = c(Aug)*, and b(Opx)* = b(Aug)*. Streaking along a* owing to stacking disorder was observed in selected-area electron diffraction (SAD) patterns. Slabs of clinopyroxene were observed, but no Guinier-Preston zones were detected. Single-crystal X-ray diffraction (XRD) data were collected and structure refinements completed in space group Pbca, converging to R(obs) = 2.81 and 5.17% for the two samples. Because of the presence of some Okl forbidden reflections with k = 2n + 1 and l = 4n + 2, structure refinement was attempted in space group P2(1)ca, as suggested by previous studies, and in space groups P12(1)/c1 and P112(1)/a, as suggested by the Laue symmetry of the reflections, which is 2/m rather than mmm. However, the high residual indices R and the anomalous values of many displacement parameters and bond distances did not support the lower-symmetry space groups. The violations of the extinction conditions of the Pbca space group and the monoclinic symmetry of the reflections are due to overlap of reflections from the C2/c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%
Thermal history of Acapulco and ALHA81261 acapulcoites constrained by Fe2+-Mg ordering in orthopyroxene
No full textThe quenched Fe2+-Mg ordering states of four orthopyroxene crystals from the Acapulco meteorite and two from the Antarctic ALHA81261 acapulcoite were determined from X-ray single-crystal diffraction and electron-microprobe analysis. The closure temperatures of cation ordering were 49028 and 46543 degrees C for Acapulco and ALHA81261, respectively. Kinetic analysis of the Fe2+-Mg intracrystalline reaction, on the basis of Mueller's theory [1,2], was carried out using the method developed by Ganguly [3] to measure the cooling rates of the host rock, assuming both asymptotic and exponential models. The determined ordering states yield cooling rates, near their closure temperatures, of approximately 0.17 and 0.05 degrees C per day for Acapulco and ALHA81261 respectively, slightly slower than that measured for the FRO90011 lodranite [4]. A break-up of the A-L parent body, with subsequent radiative heat loss of small ejected fragments, may explain such rapid cooling. The closure temperatures obtained for the two acapulcoites were then compared with those calculated, using the same method, for other achondrites and their cooling rates with those measured for the FRO90011 lodranite [4] and the Estherville mesosiderite [5]. The sequence of decreasing closure temperatures from the FRO90011 lodranite to the Landes IAB iron meteorite reflects decreasing cooling rates. Silicates enclosed in iron meteorites, and thus involved in core formation processes, deeper in the interior of a parent asteroid, seem to have cooled more slowly than meteorites that were affected by partial melting events
Iron Distribution In Ortho-pyroxene - A Comparison of Mossbauer-spectroscopy and X-ray Refinement Results
No full textThe Fe-Mg distribution over the MI and M2 octahedral sites in five natural orthopyroxenes has been determined by single-crystal X-ray diffraction (XRD) and Mossbauer spectroscopy (MS). The results from the two methods are in good agreement for two Fe/Mg intermediate composition samples, provided that the MS data are thickness-corrected. For two Fe-poor and one Al-rich sample, the results diverge ; XRD gives a more ordered Fe-Mg distribution than MS, with up to 0.05 more Fe atoms per formula unit in the M2 position. The thickness effect on MS for ordered orthopyroxene is shown to cause considerable distortion of the spectra, resulting in an underestimate of the degree of ordering. For normally thick absorbers (5 mg Fe/cm2), MS data will allocate approximately 2% too much of total Fe to the M1 position. However, the Fe distribution obtained from MS data can be corrected by extrapolation to zero thickness according to the formula Fe(M1)/Fe(tot)corr = [Fe(M1)/Fe(tot)meas - 0.004085t]/(1 - 0.003031t), where t is the Fe > of the absorber in mg Fe/cm2
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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