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

Abstract

The crystal chemistry and crystal structure of gem-quality elbaite from the Pederneira mine, São José da Safira, Minas Gerais, Brazil, were investigated by electron microprobe analysis in wavelength dispersive mode, unpolarized Fourier transform infrared spectroscopy, singlecrystal neutron and X-ray diffraction. The average chemical formula of the sample resulted X (Na 0.73 Ca 0.06 h 0.21 )and the chemical analysis showed an almost homogeneous distribution of the principal elements over the crystal, with significant amounts of Fe (wt% FeO * 3.1) and Mn (wt% MnO * 1.3) which substitute Y Al. The infrared spectrum over the diagnostic range of fundamental hydroxyl stretching vibrations is characterized by three strong absorption bands at 3,591 (m 1 ), 3,562 (m 2 ), and 3,491 (m 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 ''donor'' and O5 as ''acceptor.'' The O3-H bond distance corrected for ''riding motion'' is 0.9834 Å , and HÁÁÁO5 = 2.238(7) Å , O3ÁÁÁO5 = 3.152(4) Å and O3-HÁÁÁO5 = 157.7(5)°. The reasons of the positional disorder at the O1 and O2 sites, observed in both neutron and X-ray refinements, are discussed.

First accurate location of two proton sites in tourmaline: A single-crystal neutron diffraction study of oxy-dravite

“…The orientations of the O3ÀH3 vector in oxydravite of this study, in flour-buergerite of Tippe and Hamilton (1971) and in fluor-elbaite of Gatta et al (2012) are mutually consistent, indicating the occurrence of a small angle between the O3ÀH3 vector and [0001] direction:~3.4º in oxy-dravite (at 10 K),~1.3º in fluor-buergerite (unknown temperature) and~5.4º in fluor-elbaite (at 298 K). Such experimental findings are consistent with the pleochroic scheme for the OH-stretching absorption bands measured on the holotype sample of oxy-dravite by FTIR spectroscopy.…”

“…Grice and Ercit, 1993;Hawthorne, 1996;Gatta et al, 2012): i.e. two proton sites occur in the tourmaline structure, labelled here as H1 and H3 (Tables 2a and 2b), bonded to O1 and O3, respectively.…”

“…Gonzales-Carreño et al, 1988;Grice and Ercit, 1993;Hawthorne, 1996;Bosi et al, 2005;Skogby et al, 2012). However, only two singlecrystal neutron diffraction studies have been devoted so far to the crystallographic location of H in tourmaline: on fluor-buergerite reported by Tippe and Hamilton (1971) and on fluor-elbaite reported by Gatta et al (2012). These two studies could reveal only the position of the H bonded to O3, but not that which was bonded to O1.…”

“…with O3 as 'donor' and O5 as 'acceptor') were: O3ÀH3 = 0.94(5) Å , H3_O5 = 2.46(5) Å , O3_O5 = 3.334 Å and O3ÀH3_O5 = 155(2)º in fluor-buergerite and O3ÀH3 = 0.964(7) Å (and 0.9834 Å corrected for ''riding motion''), H3_O5 = 2.238(7) Å , O3_O5 = 3.152(4) Å and O3ÀH3_O5 = 157.7(5)º in fluor-elbaite. A careful inspection of the difference-Fourier map of the nuclear density, coupled with unusually high displacement parameters of the O1, O2 and O3 sites at 298 K, led Gatta et al (2012) to the FIG. 3.…”

Neutron diffraction in gemology: Single-crystal diffraction study of brazilianite, NaAl3(PO4)2(OH)4

“…In this light, the aim of the present study is a reinvestigation of the crystal structure and crystal chemistry of a natural brazilianite (from Telírio) at ambient temperature by means of single-crystal X-ray and neutron diffraction and electron microprobe analysis in wavelength-dispersive mode, to provide: (1) the reliable location of the proton sites and the real topological configuration of the OH-groups, for a full description of the atomic relationship via the H-bonds; (2) the anisotropic displacement parameters of all the atomic sites, H sites included. This experiment follows a series of crystal structure investigations we have recently performed on gemstone minerals containing light elements (H, Li, Be, and B) by single-crystal neutron diffraction (Gatta et al 2010(Gatta et al , 2012a(Gatta et al , 2012b, a method offering definite advantages in these cases (Rinaldi et al 2009). …”