A single-crystal neutron and X-ray diffraction study of elbaite

作者：Gatta G Diego^*; Danisi Rosa M; Adamo Ilaria; Meven Martin; Diella Valeria

来源：Physics and Chemistry of Minerals, 2012, 39(7): 577-588.

DOI：10.1007/s00269-012-0513-0

• Elbaite
• Tourmaline
• Pederneira deposit
• Single-crystal X-ray and neutron diffraction
• Microprobe analysis
• Infrared spectroscopy
• Hydrogen bonding

The crystal chemistry and crystal structure of gem-quality elbaite from the Pederneira mine, So Jos, da Safira, Minas Gerais, Brazil, were investigated by electron microprobe analysis in wavelength dispersive mode, un-polarized Fourier transform infrared spectroscopy, single-crystal neutron and X-ray diffraction. The average chemical formula of the sample resulted (X) (Na(0.73)Ca(0.06)a-(0.21)) (Y) (Al1.24Li1.02Fe0.41Mn0.18Mg0.07) (Z) Al-6 (T) Si6O18 (B) (BO3)(3) (V) (OH)(3) (W) (OH0.46F0.54) and the chemical analysis showed an almost homogeneous distribution of the principal elements over the crystal, with significant amounts of Fe (wt% FeO similar to 3.1) and Mn (wt% MnO similar to 1.3) which substitute Al-Y. The infrared spectrum over the diagnostic range of fundamental hydroxyl stretching vibrations is characterized by three strong absorption bands at 3,591 (nu(1)), 3,562 (nu(2)), and 3,491 (nu(3)) cm(-1), respectively. The neutron structure refinement showed: (a) the presence of boron at the B site only and (b) the absence (at a significant level) of aluminum at the T site, which appears to be fully occupied by silicon. Only one proton site was successfully located. The H bond configuration is described with O3 as %26quot;donor%26quot; and O5 as %26quot;acceptor.%26quot; The O3-H bond distance corrected for %26quot;riding motion%26quot; is 0.9834 a%26quot;%26lt;%26lt;, and H center dot center dot center dot O5 = 2.238(7) a%26quot;%26lt;%26lt;, O3 center dot center dot center dot O5 = 3.152(4) a%26quot;%26lt;%26lt; and O3-H center dot center dot center dot O5 = 157.7(5)A degrees. The reasons of the positional disorder at the O1 and O2 sites, observed in both neutron and X-ray refinements, are discussed.

• 出版日期2012-7