Population Of Cation Sites Research Articles

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An omphacite from the Nybø eclogite pod, Norway, has a chemical composition which plots within the gap, often regarded as a miscibility gap, which is evident in all previous compilations of natural jadeite-rich acmite-poor omphacite compositions. Its cation site populations determined by crystal structure refinement accord well with its chemical composition analysed by electron microprobe. Its disordered space group C2/c symmetry extends the known compositional range of C2/c pyroxene. The postulated miscibility gap is regarded as real and wide at low temperatures, but it appears to narrow with increasing temperature or ultimately with increasing acmite proportion. The adjacent primitive symmetry field appears to initially widen with increasing temperature or increasing acmite proportion, at the expense of the diminishing miscibility gap, before finally closing with further increase.

Atomic arrangements around the O3 site in Al- and Cr-rich oxy-tourmalines: a combined EMP, SREF, FTIR and Raman study

A study of natural oxy-tourmalines belonging to the system oxy-dravite–chromo-alumino-povondraite–oxy-chromium-dravite from the Sludyanka crystalline complex (Southern Baikal region, Russia) was carried out to explore the characteristic vibrational bands in the principal (OH)-stretching frequency and their relations to the O3 anion site of the tourmaline structure. Relevant information was obtained using electron microprobe analysis (EMPA), structural refinement (SREF), infrared (IR) and Raman single-crystal spectroscopy. The studied oxy-tourmalines are characterized by the substitution Al ↔ Cr, which is accompanied by redistribution of Mg over the Y and Z sites. The occurrence of strong correlations between relative peak area intensities for two IR bands at 3,565 and 3,519 cm−1 and cation site populations derived from SREF and EMP data allowed assignment of the band at 3,565 cm−1 to the cluster [ Y Mg Z Al Z (Al,Mg)]–O3 and the band at 3,519 cm−1 to the cluster [ Y Cr Z (Cr,Al) Z (Cr,Al,Mg))]–O3. It appears that the combination of polarized IR and Raman spectra collected with the electric vector E⊥c and E//c may provide a useful characterization of the local (OH) environments around the O3 site of the tourmaline structure.

Characterization of lithium sites in dehydrated LiCaNaKLSX by [formula omitted]Li MAS NMR spectroscopy

The Li cation sites of dehydrated LiCaNaKLSX (low silica X) were studied by 7 Li magic-angle spinning nuclear magnetic resonance spectroscopy. The number of crystallographically distinct Li cation sites for dehydrated LiCaNaKLSX is at least three, attributed to the SI ′, SII, and SIII sites. The cation site populations were determined by the line intensities of the central transition. When the Ca exchange level is increased from 0% to 8.5% with a constant Li content, the number of SI ′ and SII sites occupied by Li decreased slightly and that of SIII sites increased. The increased Li population in SIII sites is associated with the location of Ca cations at the SI and SII sites. In addition, the increase in the adsorption capacity of nitrogen caused by the presence of Ca can be explained by the increase in the Li occupancy at SIII sites.

Cation location and migration in lanthanum-exchanged zeolite NaY studied by X-ray powder diffraction and MAS NMR spectroscopy

The population of cation sites in a lanthanum-exchanged zeolite NaY (LaNaY-73) in the course of calcination and at conditions similar to catalysis (T= 623 K, continuous flow of m-xylene and nitrogen over calcined zeolite LaNaY) has been studied by ex situ and in situ X-ray powder diffraction. Ex situ magic-angle spinning (MAS) NMR spectroscopy at room temperature complemented the diffraction results. During calcination of LaNaY, a cation migration process (lanthanum from supercage to sodalite cage, sodium from sodalite cage to supercage), after pretreatment at 363 K, was observed. In the calcined sample all lanthanum cations are located at position SI′ in the sodalite cage while 10 of the 16 sodium cations per unit cell occupy position SII in the supercage. A sodium migration from the sodalite cages to position SII in the supercages was observed by ex situ temperature-dependent experiments on samples calcined between 298 and 623 K. X-Ray powder diffraction, as well as multinuclear solid-state NMR spectroscopy, has shown that adsorption of m-xylene induces sodium migration from the sodalite cages to position SII in the supercage, whereas the positions of lanthanum are not influenced. Furthermore, m-xylene could be located by in situ X-ray powder diffraction at conditions similar to catalysis at the preferred adsorption site in front of the six-ring window of the supercage, as determined by ex situ diffraction experiments at low temperature. The positions and populations of the cations derived at 623 K agree with the results obtained from ex situ diffraction experiments carried out at this temperature.

Iron site populations from Mössbauer spectroscopy in Ti-bearing garnets from Mt Vulture (Italy)

The present work concerns the determination of the cation site populations and crystal chemical formulae of some Ti-rich silicate garnets. The samples come from Mt. Vulture, a volcanic complex located on the east side of the Lucania Apennines (south Italy). Mossbauer spectroscopy (MS) has been used in order to obtain the iron site occupancies. Two different models for interpreting the Fe2+ peak positions in the Mossbauer spectra are compared, and some methodological aspects are discussed. The derived iron distributions have then been combined with the data obtained by electron probe microanalysis (EPMA) and single-crystal X-ray diffraction analysis (XRD) to obtain consistent crystal chemical formulae for these compounds.

Influence of the temperature on the cation distribution in dehydrated and hydrated SrY zeolites

The structures of SrY zeolite were determined in the dehydrated and in the hydrated form by X-ray powder diffractometry. The influence of the temperature on the variation of the cation location was investigated in the range of 323 K to 723 K for the hydrated forms and between 723 K and 1123 K in the dehydrated forms. For the dehydrated structures a Boltzmann-like cation distribution was observed above 823 K. With increasing temperature the cations become more and more equally distributed over the different cation sites. Below 823 K a linear variation of the cation site populations is observed, which is explained in terms of domain disorder. In the hydrated samples the cation site distribution depends on the residual water inside the zeolite cavities, which is a function of temperature.

Influence of weakly coordinated cations and basic sites upon the reaction of H 2S and CO 2 on zeolites

Batch formation of COS and H 2O by heterogeneous reaction of H 2S and CO 2 on A- and X-type zeolites with various degrees of alkaline earth cation-exchange, on faujasite-type zeolites with various Si Al ratios, and on alkaline cation-exchanged X-type zeolites, has been studied. Results obtained at different reaction times illustrate the dual function of the zeolites as selective sorbents causing a shift in reaction equilibrium and as catalysts. Correlations between the catalytic activity and population of cation sites with low oxygen coordination have been found. Furthermore, the catalytic activity could be correlated with the efficiency of dissociative H 2S adsorption (literature data) and the basicity of zeolites. These correlations are regarded as being in accordance with the concept of basic catalysis on zeolites.

The crystal structure of vesuvianite from Nakatatsu mine: Reinvestigation of the cation site-populations and of the hydroxyl groups.

Refinement of the crystal structure of a vesuvianite [Ca18.6 (Mn0.3Fe0.7) (OH)2 (Mg2.1Fe0.7Ti0.5Al4.7) Al4.2 (OH, O)8 Si18.2 O68; a = 15.561(8) c = 11.777(7) A; space group P4⁄nnc] from Nakatatsu mine, Fukui Prefecture, central Japan, was carried out by a least-squares method with X-ray single-crystal data. The final R index for 1901 observed independent reflections is 0.030. From site population refinement and observed interatomic distances of cation sites, the C-sites (square antiprism) and B-sites (tetragonal pyramid) are half occupied by Ca2+ and Fe2+ (+Mn2+), respectively, the A-sites (octahedron) are almost completely occupied by Al, and the AlFe-sites (octahedron) are occupied by Mn2+, Fe2+, Mg2+, Fe3+, Ti4+ and Al3+. From bond-valence calculations, the proton donor atoms are both O(10) and O(11). The cation site-populations and the hydroxyl groups of the structure models reported by ealier studies have been compared and reviewed.Based on the structure analysis, the chemical formula of vesuvianite belonging to the space group P4⁄nnc should be written as Ca19 (Mn, Fe2+) (F, OH)2 (Mg, Fe2+, Mn, Al, Fe3+, Ti)8Al4 (OH, F, O)8 (SiO4)10(Si2O7)4.

Crystal-chemistry of a unique jadeite-rich acmite-poor omphacite from the Nybø eclogite pod, Sørpollen, Nordfjord, Norway

An omphacite from the Nybø eclogite pod, Norway, has a chemical composition which plots within the gap, often regarded as a miscibility gap, which is evident in all previous compilations of natural jadeite-rich acmite-poor omphacite compositions. Its cation site populations determined by crystal structure refinement accord well with its chemical composition analysed by electron microprobe. Its disordered space group C2/c symmetry extends the known compositional range of C2/c pyroxene. The postulated miscibility gap is regarded as real and wide at low temperatures, but it appears to narrow with increasing temperature or ultimately with increasing acmite proportion. The adjacent primitive symmetry field appears to initially widen with increasing temperature or increasing acmite proportion, at the expense of the diminishing miscibility gap, before finally closing with further increase.