1,720,957 research outputs found
Disorder and modulation in first aragonite precipitates from Obstanser Eishöle (Austria)
No full textAragonite is thermodynamically metastable at near-surface conditions, and still it is relatively widespread
in marine and terrestrial sediments. In this contribution we propose the detailed chemical and crystallographic
analysis of fresh aragonitic precipitates. Wet samples, collected from a dolomitic cold cave in the East Alpine
range, were directly taken from underground dripping water and from the surface of aragonite speleothems.
Calcium carbonate nano- and microcrystals are always found in association with magnesite and hydromagnesite
and incorporate variable amounts of magnesium and possibly hydroxyl groups.
The typical size of the analyzed precipitates ranges from some tens of nanometers to few microns. Advanced
electron crystallographic tools were therefore necessary for a proper structural characterization. Indeed, in the
last ten years electron diffraction (ED) turned into a robust protocol for phase identification and ab-initio
structure determination. Such evolution was mostly propelled by the development of semi-automatic routines
for 3D data collection (Mugnaioli & Gemmi, 2018). The concept at the basis of 3D ED is the same as for
single-crystal X-ray diffraction, but electrons allow sampling single crystals 10 to 1000 times smaller, despite
the presence of surrounding crystals of other mineralogical phases.
3D ED revealed that first calcium carbonate precipitates have a structure strictly related to conventional
aragonite. Still, diffuse scattering and satellite reflections appear along aragonite {110} and point to a reduction
of symmetry into the monoclinic system (Németh et al., 2018). Following the order-disorder description
of aragonite proposed by Makovicky (2012), such disorder can be associated with the same mechanism
responsible for the twinning in mature aragonite. The frequent (or systematic) inversion of the stacking vector
can be imposed by the incorporation of magnesium in the structure, whose atomic radius and coordination
significantly differ from those of calcium. In turn, the necessity to include magnesium and hydroxyl groups in
the lattice may be the very factor that favors the crystallization of aragonite in respect to calcite, which should
otherwise be the stable mineral phase at near-surface conditions. Such ‘monoclinic-aragonite’ seeds might
therefore represent the key step for the formation of large amount of metastable aragonite sediments
3D electron diffraction in nano-geology: present and perspectives
Full text linkWhen working on advanced research topics in
geosciences, one must often deal with small yields and
cryptocrystalline polyphasic samples. Conventional optical
and X-ray crystallographic tools may not be sufficient for the
proper characterization of these samples. The development of
efficient probes able to investigate the nanoworld becomes
therefore crucial for pushing forward our understanding about
the geochemical and mineralogical processes that regulate
Earth and extraterrestrial environments.
In the last ten years, electron diffraction (ED) evolved
from a qualitative method restricted to few dedicated TEM
users, to a robust protocol for phase identification and abinitio structure determination [1]. Such change has been
mostly propelled by the development of routines for 3D data
collection. This methodology is in principle equivalent to
single-crystal X-ray diffraction, but allows sampling crystals
of few tens on nanometers.
We will show here some examples of recent applications
of ED in geosciences, namely how to achieve an easy and
relatively fast characterization of minor and cryptocrystalline
phases in natural and experimental samples. We were able
identify and characterize modulated phases able to carry
hydrogen at upper-mantle conditions, to follow aragonite
growth from the first nucleation seeds [2] and to identify
mineralogical phases and polytypes in non-equilibrated
extraterrestrial samples and in impact rocks [3]
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
Monoclinc aragonite, the precursor of metastable aragonite formation
No full textAragonite is a wide-spread crystalline form of CaCO3 on the Earth surface. Although calcite is the
thermodynamically stable CaCO3 form at ambient conditions, aragonite precipitates in the ocean and
in some continental settings. It is abundant in the shells of various organisms such as molluscs
and corals as well as in stromatolites, and its abiotic formation is favored in waters of a Mg2+/Ca2+
ratio > 1.5. Despite decades of research it is still unclear why metastable aragonite is so abundant.
Here, we present evidence of a new CaCO3 polymorph, which sheds light on this long-standing
question
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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