TY - JOUR
T1 - Crystal structure analyses of four tourmaline specimens from the Cleopatra's Mines (Egypt) and Jabal Zalm (Saudi Arabia), and the role of Al in the tourmaline group
AU - Bosi, Ferdinando
AU - Balic Zunic, Tonci
AU - Surour, Adel Abdala
PY - 2010/4
Y1 - 2010/4
N2 - Fe-rich "oxydravite" and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nu-bian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and an ordering of Fe3+ at Y. The disordering can be explained by the substitution mechanisms 2 YMg+ZAl+:WOH = 2YAl+ ZMg+WO2- and 2YFe2++ ZFe3++WOH = 2YFe3++ ZFe2++WO2-, which are consistent with reducing the mismatch in dimen-sions between YO6 and ZO 6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO6 octahedra and the smaller Si6O18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit be-tween YO6, ZO6, and TO4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of ZAl combined with the occurrence of B at T in Al-rich tourmaline. ZAl has a greater effect than YAl as long as Al does not fully occupy the Z site.
AB - Fe-rich "oxydravite" and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nu-bian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and an ordering of Fe3+ at Y. The disordering can be explained by the substitution mechanisms 2 YMg+ZAl+:WOH = 2YAl+ ZMg+WO2- and 2YFe2++ ZFe3++WOH = 2YFe3++ ZFe2++WO2-, which are consistent with reducing the mismatch in dimen-sions between YO6 and ZO 6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO6 octahedra and the smaller Si6O18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit be-tween YO6, ZO6, and TO4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of ZAl combined with the occurrence of B at T in Al-rich tourmaline. ZAl has a greater effect than YAl as long as Al does not fully occupy the Z site.
U2 - 10.2138/am.2010.3357
DO - 10.2138/am.2010.3357
M3 - Journal article
SN - 0003-004X
VL - 95
SP - 510
EP - 518
JO - American Mineralogist
JF - American Mineralogist
IS - 4
ER -