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Short-range order and Long-range order in amphiboles: A model for the interpretation of infrared spectra in the principal OH-stretching region
No full textFTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O2- at the anionic O3 site in amphiboles
No full textThis paper reports a single-crystal unpolarized- light FTIR study in the OH-stretching region of a suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied have fairly constant M-site composition, with Al-[6](tot) similar to 0.55 atoms per formula unit (apfia), Ti-[6] similar to 0.45 apfia, and Fe-[6](3+) similar to 0.40 apfu. SIMS and SREF data show all samples to have an 03 anion composition of OH approximate to O2- approximate to 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption consisting of several overlapping bands; three main components can be recognized: similar to 3710, 3686, and 3660 cm(-1), respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites allows five of these components to be assigned to local configurations involving OH-O-2- at the 03 site, thus showing that coupling with an O2- anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of O2- in amphiboles in chemically favorable cases, i.e., in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving Ti-M1(4+) or Fe-M1(3+) Fe-M3(3+) are associated with O2- at both adjacent 03 sites, and that Al-M3 is focally associated with OH, confirming SRO models based on structure refinement results
Raman and FTIR spectroscopy of synthetic amphiboles: II. Divalent (Mg-Co) substitutions at the octahedral sites
No full textAl-Mg disorder in a gem-quality pargasite from Baffin Island, Nanavut, Canada
No full textThe crystal structure of gem-quality pargasite from Soper River, near Kimmirut, Baffin Island, Nunavut, Canada, (K0.24Na0.73)(Sigma0.97) (Ca1.86Na0.14)(Sigma2.00) (Mg4.15Fe0.07Mn0.01Al0.71Ti0.06)(Sigma5.00) (Si6.45Al1.55) O-22 [(OH)(1.25)F0.63O0.122-](Sigma2.00), a 9.8814(6), b 17.967(1), c 5.2927(4) Angstrom, beta 105.263(5)degrees, V 906,5 Angstrom(3), C2/m, Z = 2, has been refined to an R index of 2.9% using 1506 observed intensities measured with MoKalpha X-radiation. The crystal used for collection of the X-ray intensity data was then analyzed with an electron microprobe. The amphibole composition is very close to that of end-member pargasite. There is significant disorder of Al over the M(2) [1.40 Mg + 0.54 Al + 0.06 Fe3+] and M(3) [0.81 Mg + 0.17 Al + 0.02 Fe2+] sites. Such disorder has been observed in Mg-rich pargasite from a high-pressure paragenesis in the western Alps. The infrared spectrum of this amphibole in the principal OH-stretching region shows a complex envelope that can be resolved into six bands by analogy with the spectra of synthetic (OH,F)-bearing pargasite in the literature. The relative intensities of the component bands are in accord with the observed OH:F ratio in this pargasite, and indicate complete short-range disorder of OH and F. The occurrence of Al-Mg disorder over M(2) and M(3) in the Soper River pargasite, from a lower-pressure environment, indicates that this disorder is a compositional feature of pargasite, rather than a result of crystallization or equilibration at high pressure and temperature. The pattern of electron density in the A cavity of the structure may be interpreted in terms of K and Na at the A(m) site and Na at the A(2) site. Moreover, the specific site-populations are conformable with the composition of this amphibole and the most favorable patterns of short-range order
Raman and FTIR spectroscopy of synthetic amphiboles: I. The OH librational bands and the determination of the OH-F content of richterites via Raman spectroscopy
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FTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O<sup>2-</sup> at the anionic O3 site in amphiboles
No full textThis paper reports a single-crystal unpolarized-light FTIR study in the OH-stretching region of a
suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied
have fairly constant M-site composition, with [6]Altot ~0.55 atoms per formula unit (apfu), [6]Ti ~0.45
apfu, and [6]Fe3+ ~0.40 apfu. SIMS and SREF data show all samples to have an O3 anion composition
of OH ≈ O2- ≈ 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption
consisting of several overlapping bands; three main components can be recognized: ~3710, 3686, and
3660 cm-1, respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components
can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites
allows five of these components to be assigned to local configurations involving OH-O2- at the O3 site,
thus showing that coupling with an O2- anion through an A-cation significantly affects band position.
Infrared spectroscopy can detect the presence of O2- in amphiboles in chemically favorable cases, i.e.,
in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving M1Ti4+
or M1Fe3+ M3Fe3+ are associated with O2- at both adjacent O3 sites, and that M3Al is locally associated
with OH, confirming SRO models based on structure refinement results
Characterization of short-range order in potassium-fluor-richterite by infrared spectroscopy in the OH-stretching region
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