1,721,018 research outputs found
Micro-FTIR and EPMA Characterisation of Charoite from Murun Massif (Russia)
No full textCombined micro-Fourier transform infrared (micro-FTIR) and electron probe microanalyses (EPMA) were performed on a single crystal of charoite from Murun Massif (Russia) in order to get a deeper insight into the vibrational features of crystals with complex structure and chemistry. The micro-FTIR study of a single crystal of charoite was collected in the 6000–400 cm−1 at room temperature and after heating at 100°C. The structural complexity of this mineral is reflected by its infrared spectrum. The analysis revealed a prominent absorption in the OH stretching region as a consequence of band overlapping due to a combination of H2O and OH stretching vibrations. Several overtones of the O-H and Si-O stretching vibration bands were observed at about 4440 and 4080 cm−1 such as absorption possibly due to the organic matter at about 3000–2800 cm−1. No significant change due to the loss of adsorbed water was observed in the spectrum obtained after heating. The occurrence of well-resolved water bending vibration bands at about 1595 and 1667 cm−1 accounts for more than one structural water molecule as expected by charoite-90 polytype structure model from literature. The chemical composition of the studied crystal is close to the literature one
Structural anomalies in tobelite-2M2 explained by high resolution and analytical electron microscopy
No full textA transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the single- crystal structure refinements
Time evolution of Fe-oxydation/deprotonation process in fe-rich phlogopite under isothermal conditions
No full textCrystal chemistry of trioctahedral micas-2M 1 from Bunyaruguru kamafugite (southwest Uganda)
No full textThe crystal chemistry of 2M1 micas from Bunyaruguru kamafugite (southwest Uganda) was studied by electron probe microanalysis, single-crystal X-ray diffraction, Mössbauer and Fourier transform infrared spectroscopy. Chemical analyses showed that the studied crystals are Ti-rich, F-poor phlogopites with an annitic component, Fetot/(Fetot + Mg), ranging from 0.15 to 0.22. Unit-cell parameters from single-crystal X-ray data are in the range: 5.3252(1) ≤ a ≤ 5.3307(1), 9.2231(3) ≤ b ≤ 9.2315(3), 20.1550(6) ≤ c ≤ 20.1964(8) Å, and 94.994(2) ≤ β ≤ 95.131(2)°. Anisotropic structure refinements, in the space group C2/c, converged to 2.77 ≤ R1 ≤ 3.52% and 2.91 ≤ wR2 ≤ 4.02%. Mössbauer spectroscopy showed that the studied sample has: VIFe2+ = 60(1)%, VIFe3+ = 24(1)%, and IVFe3+ = 16(1)%. FTIR investigations pointed to the occurrence of Fe3+-oxy substitutions and ruled out the presence of vacancy mechanisms. The overall crystal-chemical features are consistent with the following substitutions: tetraferriphlogopite [IVFe3+ ↔ IVAl]; Ti-oxy [VIM2+ + 2 (OH)− ↔ VITi4+ + 2 (O2−) + H2↑] and Al, Fe3+, Cr-oxy [VIM2+ + (OH) − ↔ VIM3+ + O2− + 1⁄2 (H2)↑]; Al, Fe3+-Tschermak [VIM2+ + IVSi4+ ↔ VIM3+ + IVAl]; kinoshitalite [XIIK + IVSi4+ ↔ XIIBa2+ + IVAl] and [XIIK+ + IVAl3+ ↔ IVSi4+ + XII□]. The estimation of the OH− content for Ugandan mica-2M1 was obtained, for the first time, from the linear regression equation c = 0.20(2) × OH− (gpfu) + 19.93(2) derived from literature data of 2M1-samples with known OH− content. The orientation of the O-H vector with respect to c* was found in the range from 2.0 to 6.9°
Tobelite and NH4+-rich muscovite single crystals from Ordovician Armorican sandstones (Brittany, France): Structure and crystal chemistry
No full textThe crystal structures of tobelite and NH4+-rich muscovite from the sedimentary rocks of the Armorican sandstones (Brittany, France) have been solved for the first time by single crystal X-ray diffraction. The structural study was integrated by electron probe microanalyses, X-ray photoelectron and micro-Fourier transform infrared spectroscopy. The crystals belong to the 2M2 polytype with the following unit-cell parameters: a = 9.024(1), b = 5.2055(6), c = 20.825(3) Å and β = 99.995(8) for tobelite and a = 9.027(1), b = 5.1999(5), c = 20.616(3) Å and β = 100.113(8)° for NH4+-rich muscovite. Structure refinements in the space group C2/c converged at R1 = 8.01%, wR2 = 8.84% and R1 = 5.59%, wR2 = 5.63% for tobelite and NH4+-rich muscovite, respectively.
X-ray photoelectron spectroscopy revealed nitrogen environments associated either to inorganic (B.E. 401.31 eV) and to organic (B.E. 398.67 eV) compounds. Infrared spectra showed, in the OH- stretching region (3700-3575 cm-1), two prominent bands, centered at ~ 3629 and ~ 3646 cm-1, and two shoulders at ~ 3664 and ~ 3615 cm-1 which were assigned to Al3+Al3+-OH- arrangements having OH- groups affected by different local configurations. In addition, a series of overlapping bands from about 3500 to 2700 cm-1 characteristic of the NH4+-stretching vibrations, a main band at ~ 1430 and a shoulder at ~ 1460 cm-1 which were associated to the NH4+ bending vibration (ν4) were also present.
The ammonium concentration was semi-quantitatively estimated in both crystals from the absorbance of the OH--stretching and NH4+-bending vibrations in the infrared spectra. Additional estimate was obtained for the NH4+-rich muscovite by considering the normalized peak area between K2p3/2 and N1s in the X-ray photoelectron spectrum. The obtained values were also in agreement with those derived from the interlayer spacing in the simulated X-ray powder diffraction spectra.
The results of this integrated approach converged to (K0.18Na0.01NH4+0.62)Σ=0.81
(Al1.98Fe2+0.02)Σ= 2.00(Si3.19Al0.81)Σ= 4.00O10.00OH2.00 for tobelite and to (K0.46Na0.03Ba0.01NH4+0.36)Σ=0.86
(Al1.98Mg0.01Fe2+0.01V3+0.01)Σ=2.01(Si3.13Al0.87)Σ=4.00O10.00F0.08OH1.92 for NH4+-rich muscovite
3T-phlogopite from Kasenyi (SW Uganda) kamafugite: crystal chemistry and structure
No full text3T trioctahedral micas are rarer than it is thought. This is likely due to the occurrence of apparent polytypism, so that 1M polytype twins result in a diffraction pattern simulating a 3T periodicity [1, 2]. Most of the 3T trioctahedral micas found in nature to date belong to muscovite-polylithionite-annite system [3, 4, 5, 6]. X-ray diffraction studies on these micas have often reported partial tetrahedral ordering and/or different patterns of octahedral ordering [3, 6].
In the present work, a 3 T trioctahedral mica from Kasenyi (south west Uganda) kamafugite was studied via Electron Probe Microanalysis (EPMA) and Single Crystal X-ray Diffraction (SCXRD).
Main EPMA data gave: SiO2 = 38.7(2), Al2O3 = 13.08(9), MgO = 20.4(2), TiO2 = 4.8(1), FeOtot = 5.51(9), Cr2O3 = 0.90(7), K2O = 9.64(5), Na2O = 0.29(1), BaO = 0.15(5) and F = 0.13(5) wt%. The analysed crystal may be classified as a Ti-rich, F-poor mica with a composition intermediate between the annite and phlogopite end members.
Anisotropic single crystal X-ray refinement was performed in the <i>P</i>3<sub>1</sub>12 space group and converged to R1 = 4.34 and wR2 = 3.33 %. Unit cell parameters were: a = b = 5.3235(3) and c = 30.188(2) Å. Mean bond length distances of M1, M2 and M3 follow the pattern M1 = M2 < M3, suggesting partial octahedral cation ordering. Conversely, mean bond lengths of T1 and T2 point to a disordered cation distribution over tetrahedral sites.
Finally, the overall crystal chemical features indicates the occurrence in the studied sample of the following substitution mechanisms: tetraferriphlogopite; Ti-oxy and Al, Fe3+, Cr-oxy; Al, Fe3+-Tschermak; kinoshitalite and XIIK+ + IVAl3+ « IVSi4+ + XII. Such substitutions are the same as those found in 1M-2M1 coexisting micas from the same rock sample [7]
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