1,720,977 research outputs found
Photochemical benzylation of 1,4-dicyanonaphthalene. X-ray crystal structure of 6,11-dicyano-5,11-methano-5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene
No full textFluoro-tremolite, IMA 2016-018
No full textFluoro-tremolite
Ca2Mg5Si8O22F2
Limecrest-Southdown quarry, Sparta, New Jersey, USA
Amphibole supergroup
Monoclinic: C2/m; structure determined
a = 9.846(2), b = 18.050(3), c = 5.277(1) Å, β = 104.80(2)°
3.381(57), 3.276(38), 3.126(67), 2.940(43), 2.706(100), 2.592(34), 2.531(59), 2.337(36)
Type material is deposited in the collections of the Franklin Mineral Museum, 32 Evans
Street, Franklin, 07416 New Jersey, USA, catalogue number 771
Heterapentalenes. The reaction of 5H-pyrazolo[1',2':2,3][1,2,3]triazolo[4,5-a]phenazin-4-ylium-5-ide andsome of its derivatives with dimethyl acetylenedicarboxylate
No full textDeprotonation of Fe-dominant amphiboles: Single-crystal HT-FTIR spectroscopic studies of synthetic potassic-ferro-richterite
No full textHigh-temperature Fourier transform infrared (HT-FTIR) spectroscopy was used to characterize the deprotonation process of synthetic potassic-ferro-richterite of composition A(K0.90Na0.07)B(Ca0.54Na1.46) c F e 4.22 2 + F e 0.78 3 + T S i 8 O 22 W O H 1.70 O 0.30 2 - ^
mcleft(
mFe-4.22^2 +
mFe-0.78^3 +
ight)^
mT
mS
mi-8
mO-22^
mWleft(
mO
mH-1.70
mO-0.30^2 -
ight). Unpolarized single-crystal spectra were collected both in situ and on quenched samples, and heating experiments were conducted in air, at a rate of 10 °C/min. The room-T spectrum measured before annealing shows a main band at 3678 cm-1 and a minor band at 3622 cm-1; these are assigned to local configurations involving Fe2+ at M(1)M(1)M(3) and facing a filled and an empty alkali-site, respectively. At 400 °C, a new band grows at 3656 cm-1; this is the most intense feature in the pattern at 450 °C. At T ≥ 500 °C, all peaks decrease drastically in intensity, and finally disappear at T > 600 °C. The total absorbance measured in situ increases significantly in the 25 < T < 450 °C range, although the spectra collected on quenched samples show no OH loss in the same T range. This feature is consistent with an increase of the absorption coefficient (ε) with T, the reason for which is still unclear. However, this feature has significant implications for the quantitative use of FTIR data in HT experiments. Examination of the relevant OH-stretching bands shows that iron oxidation occurs preferentially at the M(1,3) sites associated with occupied A sites. The deprotonation temperature indicated by FTIR for single-crystals is around 100 °C higher that that obtained by HT-X-ray diffraction (XRD) on single crystal by Oberti et al. (2016), whereas that obtained by HT-XRD on powders is intermediate. This unexpected observation can be explained by considering that: (1) the iron oxidation process, which is coupled to deprotonation and is probed by XRD, occurs preferentially at the crystal surface where it is triggered by the availability of atmospheric oxygen; (2) the proton diffusion, probed by FTIR, is slower that the electron diffusion probed by XRD; thus, the temperature shift may be explained by a much longer escape path for H in single-crystals than in powders. These results suggest that possible effects due to crystals size should be carefully considered in HT experiments done on Fe-rich silicates
The crystal chemistry of gismondines : the non-existence of K-rich gismondines
No full textGismondine samples from 17 localities have been analyzed by electron microprobe and TG analysis and their unit cell dimensions have been determined. The chemical composition varies within narrow limits from the stoichiometric formula Ca4Al8Si8O32. 18H2O. The Si/(Al+Si) ratio varies from 0.514 to 0.542, Ba, Fe and Mg are always absent and K, Na and Sr are low or absent. The chemical and X-ray data demonstrate the non-existence of K-rich gismondines. The K-rich analyses from the literature are explained by the frequent phillipsite-gismondine intergrowths. Unit-cell dimensions turned out to be rather constant, and no correlation with chemical composition has been found. The largest correlation is between two chemical parameters : Si/(Al+Si) and (Na+K)/(Na+K+Ca). The narrow field of chemical variability of this zeolite is attributed to the (Si,Al) ordered distribution.Des échantillons de gismondine de 17 localités différentes ont été examinés à la microsonde électronique et par thermogravimétrie ; les paramètres de la maille ont été déterminés. La composition chimique, à peu près constante, est proche de la composition théorique Ca4Al8Si8O32. 18H2O. Le rapport Si/(Si+Al) oscille entre 0,514 et 0,542 ; Ba, Fe et Mg sont toujours absents, la teneur en K, Na et Sr est faible, ou ces éléments sont absents. Les données chimiques et les diagrammes de poudre montrent l'inexistence des gismondines riches en K. Les gismondines riches en K de la littérature, sont imputées à des intercroissances fréquentes phillipsite-gismondine. Les paramètres de la maille et la composition chimique ne peuvent pas être corrélés : seul, le rapport Si/(Si+Al) peut être relié au rapport (Na+K)/(Na+K+Ca). Dans le groupe des zéolites, la gismondine montre une composition chimique très constante avec une distribution ordonnée (Si, Al).Vezzalini Giovanna, Oberti Roberta. The crystal chemistry of gismondines : the non-existence of K-rich gismondines. In: Bulletin de Minéralogie, volume 107, 6, 1984. pp. 805-812
The crystal structure of peprossiite-(Ce), an anhydrous REE, and Al mica-like borate with square-pyramidal coordination for Al.
No full text
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
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