Influence of the X-site composition on tourmaline's crystal structure
- Autor(en)
- Eleanor J. Berryman, Bernd Wunder, Andreas Ertl, Monika Koch-Müller, Dieter Rhede, Katharina Sarah Scheidl, Gerald Giester, Wilhelm Heinrich
- Abstrakt
The crystal structures of synthetic K-dravite [XKYMgZ3AlT6Si6O18(BO3)V3(OH)W3(OH)], dravite [XNaYMgZ3AlT6Si6O18(BO3)V3(OH)W3(OH)], oxy-uvite [XCaYMgZ3AlT6Si6O18(BO3)V3(OH)W3O], and magnesio-foitite [XY(Mg2Al)ZAlT6Si6O18(BO3)V3(OH)W3(OH)] are investigated by polarized Raman spectroscopy, single-crystal structure refinement (SREF), and powder X-ray diffraction. The use of compositionally simple tourmalines characterized by electron microprobe anal. facilitates the detn. of site occupancy in the SREF and band assignment in the Raman spectra. The synthesized K-dravite, oxy-uvite, and magnesio-foitite have significant Mg-Al disorder between their octahedral sites indicated by their resp. av. <Y-O> and <Z-O> bond lengths. The Y- and Z-site compns. of oxy-uvite (YMg1.52Al1.48(10) and ZAl4.90Mg1.10(15)) and magnesio-foitite (YAl1.62Mg1.38(18) and ZAl4.92Mg1.08(24)) are refined from the electron densities at each site. The Mg-Al ratio of the Y and Z sites is also detd. from the relative integrated peak intensities of the Raman bands in the O-H stretching vibrational range (3250-3850 cm-1), producing values in good agreement with the SREF data. The unit cell vol. of tourmaline increases from magnesio-foitite (1558.4(3) Å3) to dravite (1569.5(4)-1571.7(3) Å3) to oxy-uvite (1572.4(2) Å3) to K-dravite (1588.1(2) Å3), mainly due to lengthening of the crystallog. c-axis. The increase in the size of the X-site coordination polyhedron from dravite (Na) to K-dravite (K) is accommodated locally in the crystal structure, resulting in the shortening of the neighboring O1-H1 bond. In oxy-uvite, Ca2+ is locally assocd. with a deprotonated W (O1) site, whereas vacant X sites are neighbored by protonated W (O1) sites. Increasing the size of the X-site-occupying ion does not detectably affect bonding between the other sites; however, the higher charge of Ca and the deprotonated W (O1) site in oxy-uvite are correlated to changes in the lattice vibration Raman spectrum (100-1200 cm-1), particularly for bands assigned to the T6O18 ring. The Raman spectrum of magnesio-foitite shows significant deviations from those of K-dravite, dravite, and oxy-uvite in both the lattice and O-H stretching vibrational ranges (100-1200 and 3250-3850 cm-1, resp.). The vacant X site is correlated with long- and short-range changes in the crystal structure, i.e., deformation of the T6O18 ring and lengthening of the O1-H1 and O3-H3 bonds. However, X-site vacancies in K-dravite, dravite, and oxy-uvite result only in the lengthening of the neighboring O1-H1 bond and do not result in identifiable changes in the lattice-bonding environment.
- Organisation(en)
- Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Naturhistorisches Museum Wien (NHM), Technische Universität Berlin
- Journal
- Physics and Chemistry of Minerals
- Band
- 43
- Seiten
- 83-102
- Anzahl der Seiten
- 20
- ISSN
- 0342-1791
- DOI
- https://doi.org/10.1007/s00269-015-0776-3
- Publikationsdatum
- 02-2016
- Peer-reviewed
- Ja
- ÖFOS 2012
- 104026 Spektroskopie, 105116 Mineralogie, 105113 Kristallographie
- Schlagwörter
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/62bc752d-f560-461e-8073-d2e094087e6a