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The crystal structure of intermediate clinopyroxenes along the join diopside-enstatite (CaMgSi_2O_6-Mg_2Si_2O_6)
No full textThe structures of five clinopyroxenes on the diopside–enstatite join have been refined by Rietveld analysis of powder-diffraction patterns obtained with synchrotron radiation. The reliability of the present Rietveld refinements was checked against previous single-crystal data on the sample with composition Di80En20, and it showed reasonable agreement. A difference in the y/b M2 coordinate is related to the lower resolution of the powder-diffraction data of this work, not sufficient to detect the splitting at the M2 site. The Rietveld analysis allowed us to detect the strain modulations on (010) prior to exsolution. The small size of primitive antiphase domains, less than the length-scale of X-ray diffraction, prevents P21/c refinement in samples with composition Di59En41 and Di52En42CaTs2. The refined clinopyroxenes encompass the transition from P21/c to C2/c structures. In the P21/c structure, the A and B chains of tetrahedra undergo a slight straightening with increasing Ca content, and changes similar to those observed at high temperature. The results are in general agreement with large changes in the configuration of the chains at the transition, as observed during the P21/c – C2/c transition at high pressure and high temperature
A high-pressure cubic-to-tetragonal phase-transition in melanophlogite, a SiO2 clathrate phase
No full textThe crystal-structure and the high-pressure behavior of a natural cubic melanophlogite (Sp. Gr. Pm3n at room-conditions), a type-I SiO2 clathrate containing only methane as guest gas, was investigated by X-ray single-crystal diffraction at room-conditions and in situ high-pressure synchrotron powder diffraction with a diamond anvil cell and methanol:ethanol:water = 16:3:1 (MEW) mixture and silicon-oil (Si-oil) as pressure transmitting media up to about 6 GPa.
A cubic-to-tetragonal phase-transition was observed at P 1.14 GPa, in both MEW and Si-oil runs. The tetragonal polymorph is stable up to the highest pressure in the run with MEW, but it switches back to cubic symmetry at P > 3.12 GPa in Si-oil. The different elastic behavior is ascribable to the non-hydrostatic conditions in Si-oil medium at P > 1 GPa.
The elastic behavior of melanophlogite was described fitting the P–V data with a Birch–Murnaghan Equation-of-State, showing similar bulk modulus values for the low-pressure cubic and high-pressure tetragonal phases [i.e. K0,L-cub = 23.4(2.4), K0,tetr = 22.2(9)].
The spontaneous strain for the cubic-to-tetragonal transition is almost completely governed by a compression along the c axis, with negligible contribution along a. An inverse behavior is found with respect to the tetragonal-to-cubic high-temperature phase-transition previously described for Mt. Hamilton melanophlogite. A Landau fit of the spontaneous strain (εss), related to the order parameter (Q) of the transition as εss ∝ Q, showed a first-order character of the P-induced phase-transition and a Pc = 1.2(3) GPa. Symmetry breaking spontaneous strain prevails respect to the volume strain, which is however present and significant
the high temperature P21/c-C2/c phase transitino in iron-free Ca-rich P21/c ciclopyrxenes
No full textan in situ, high-temperature, powder diffraction investigation was performed for iron-free clinopyroxenes with compositions Ca0.40Mg1.60Si2O6, Ca0.52Mg1.46Al0.05Si1.98O6, Ca0.59Mg1.41Si2O6 and Ca0.70Mg1.30Si2O6, up to 850 °C using synchrotron radiation (ESRF, Grenoble). In samples with compositions Ca0.52Mg1.46Al0.05Si1.98O6 and Ca0.59Mg1.41Si2O6, evidence of for the P21/c-C2/c displacive phase transition was seen in changes in lattice parameters at T ap 550 and 300 °C respectively. Landau modelling of the phase transition behaviour for the sample with composition Ca0.52Mg1.46Al0.05Si1.98O6 shows a tricritical behaviour [T c =547(16)]. Comparison with the transition behaviour in other samples with lower Ca contents along the join diopside–enstatite indicates that a decrease in T c , and a switch from first-order to tricritical behavior occurs with increasing Ca content. The change in the transition behaviour was related to an interaction with the antiphase domains at the nanoscale
The high-temperature P2(1)/c-C2/c phase transition in Fe-free pyroxene (Ca0.15Mg1.85Si2O6) : structural and thermodynamic behavior
No full textA high-temperature in situ single-crystal X-ray diffraction study was performed from room T to 1150 °Con two crystals of Fe-free P2(1)/c clinopyroxenes of composition Ca0.15Mg1.85Si2O6 [cell parameters at room T: a = 9.651(2) Angstrom, b = 8.846(2) Angstrom, c = 5.202(1) Angstrom, beta = 108.38(2)degrees, V = 421.4 (2) Angstrom(3)] synthesized by isothermal annealing for 624 h at T = 1370 °C, P = 1 atm. A First order P2(1)/c-C2/c phase transition was found slightly below 1000 degreesC [T-c = 926(39) °C]. The transition was revealed by discontinuous changes in intensities and cell parameters. Prolonged heating at high temperature induced a non-reversible increase in the transition temperature up to more than 1150 °C, without apparent changes in the order of the phase transition. Coupling with strain due to incipient exsolution in a formerly almost defect-free sample is suggested to be responsible for increase in T-c. TEM observations of a sample from the same starting material after further annealing for 72 h at T = 1050 °C, P = 1 atm are consistent with the proposed incipient exsolution model. Annealing was found to induce the formation of a mottled texture oriented parallel to (101).
Results from structure refinement of data collected below the transition at T = 25, 500, 650, 800, and 1000 °C showed only minor changes in the chain configurations, which tire highly differentiated up to 1000 °C, confirming the strong First-order character of the transition
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
Variations 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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