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Crystal chemistry of synthetic amphiboles along the richterite - ferro-richterite join
No full textIn this work we relate the synthesis and crystal-chemical study
of amphiboles along the join richterite - ferrorichterite
[nominally Na(NaCa)(Mg5-xFe2+
x)Si8O22(OH)2], at 700°C, 1
kbar, under redox conditions imposed by a MW solid buffer.
XRPD shows essentially single-phase run products at both endmember
compositions, with minor (10-15%) pyroxene and
quartz for intermediate compositions. Interestingly, there is a
significant evolution of the amphibole morphology across the
solid-solution series: Mg end-member richterite is extremely
acicular (length up to 15 μm, diameter up to 1 μm) whereas for
increasing Fe in the system the amphibole becomes increasingly
stubby, with a length:diameter ratio≈ 3:1 for the Fe -rich endmember.
Notably, the refined cell parameters show linear
variations across the series, suggesting a complete Mg-Fe2+
solid-solution along the join. The OH-stretching FTIR spectra
show complex patterns for intermediate compositions with up to
eight bands which can be assigned to the various configurations
involving Mg/Fe2+ at M(1,3), locally associated with both full
and empty A-sites. Combination of Mössbauer and FTIR
spectroscopy however shows that the composition of the
amphibole is much more complex than expected just from
XRPD data: Fe2+ is in fact disordered among all M-sites, and,
despite the strongly reducing conditions during syntheses, there
is significant Fe3+ in the amphibole; it is fully ordered at M(2).
For increasing Fe in the system, the amount of Fetot in the
amphibole is systematically lower than expected as is Fe2+ at the
M(1-3) octahedra, while M(2)Fe3+ increases toward more ironrich
compositions (Fig. 1). These results definitively stress the
need for a proper characterization of the experimental run
products because these may depart from the expected
stoichiometry in a significant way. Direct EMP or single-crystal X-ray refinement is highly
desirable, however spectroscopic methods may provide a
valuable alternative, particularly when the Fe3+ vs. Fe2+
composition is needed
Single-crystal polarized ftir spectroscopy and neutron diffraction refinement of cancrinite
No full textWe relate here a combined single-crystal polarized-light FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites from Cameroun and Canada, respectively. Electron microprobe analyses show both samples to be almost end-member carbonate-cancrinites [ideal chemical formula Na6Ca2(Si6Al6O24)(CO3)2•2H2O].
The structural refinements show that the extra-framework content in the large 12-membered rings channels is represented by one independent Na-site (Na2) and two independent, and statistically distributed, CO3 groups. The geometry of the CO3 groups appears to be almost regular, with C1-OC1 ~ 1.298(7) Å and C2-OC2 ~ 1.300(5) Å, in agreement with the previous studies [1,2,3]. The atoms of the carbonate-groups are not perfectly coplanar, being z(C1) ≠ z(OC1) and z(C2) ≠ z(OC2). The H2O molecules and a further Na-site (Na1) lie in the cancrinite-cage; the H2O oxygen site (OW) lies off from the triad axis, giving rise to a statistical configuration with three equivalent and mutually exclusive water molecules, as already suggested by [1,2]. The high-quality neutron data show that the water molecule is almost symmetric, with very similar Ow-H1 and Ow-H2 bond-distances, and is slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with two possible bonds for H1 (i.e. OW-H1•••O2 and OW-H1•••O4) and two for H2 (i.e. OW-H2•••O3 and OW-H2•••O2). The Ow•••O donor-acceptor distances are all > 2.7 Å.
The polarised-light FTIR spectra show two main absorptions, at 3602 and 3531 cm-1, respectively. The former is polarised for E c, while the latter is polarized for E c. On the basis of the neutron diffraction data, the 3602 cm-1 band is assigned to the anti-symmetric stretching mode (ν3), while the 3531 cm-1 band is assigned to the symmetric stretching mode (ν1) of the same water molecule, in agreement with the presence of a single bending mode at 1630 cm-1. One additional weak component at 4108 cm-1 could possibly indicate the presence of low amounts of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1300-1500 cm-1 range are strongly polarized for E c, and are assigned to the vibrations of the CO3 group.
References. [1] H.D. Grundy, I. Hassan, Can. Mineral., 20, 239-251, 1982; [2] P. Ballirano, A. Maras, Eur. J. Mineral., 16, 135-141, 2004; [3] I. Hassan, S.M. Antao, J.B. Parise, Am. Mineral., 91, 1117-1124, 2006
Oxidation or cation re-arrangement? Distinct behavior of riebeckite at high temperature
Full text linkIn this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mossbauer spectroscopy and powder X-ray difraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400-450 degrees C), then it undergoes a rapid (within similar to 50 degrees C) dehydrogenation, forming oxo-riebeckite, which is stable up to similar to 900 degrees C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (E-a) for the riebeckite to oxo-riebeckite transition is 166 +/- 6 kJ/mol.Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750-800 degrees C); however, in the 550 < T < 700 degrees C range, it undergoes a significant rearrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700-800 degrees C range has the same chemical formula as riebeckite but has a disordered and non-standard cation distribution at the octahedra, i.e., (M(1))(Fe3+Fe2+)(M(2))(Fe3+Fe2+)Fe-M(3)(2+); we call this phase "R-C(3+) disordered riebeckite". For T >= 800 degrees C, it decomposes to aegirine + fayalite + cristobalite + H2O.External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures
Mössbauer research at University Roma Tre
No full textThe principal goal of the Rome group is studying crystalline materials, especially minerals: MS information (Fe oxidation state, Fe3+/Fetot ratios, Fe ions ordination number, Fe atoms aggregation state) is used together with stmchlral and chemical data collected
on the same single-phase material to retrieve the topochemical
model and to explain the physical properties.
Outstanding materials shldied so far are: nahlral and synthetic spinels, natural borosilicates (tourmaline and
axinite), natural amphiboles (especially fibres), natural and synthetic zeolites, but also obsidian, pumice, day and other heterogeneous materials
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
Atomistic insight into lithospheric conductivity revealed by phonon-electron excitations in hydrous iron-bearing silicates
No full textAmphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system. Amphiboles are hydrous silicates occurring in many rock types in the continental crust and subduction zones. Here, in situ Raman spectroscopy of grunerite reveals temperature-activated electron-phonon excitations that provide an atomistic insight into the role of amphiboles in the anisotropic lithospheric conductivity
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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