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

Fe-rich "oxydravite" and dravite from the Late Proterozoic ophiolitic melange of the Arabo-Nubian 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 Mossbauer 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 Fe(3+) at Y. The disordering can be explained by the substitution mechanisms 2(Y)Mg+(Z)Al+(W)OH = 2(Y)Al+(Z)Mg+(W)O(2-) and 2(Y)Fe(2+)+(Z)Fe(3+)+(W)OH = 2(Y)Fe(3+)+(Z)Fe(2+)+(W)O(2-), which are consistent with reducing the mismatch in dimensions between YO(6) 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 YO(6) 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 MgO(6), octahedra and the smaller Si(6)O(18) 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 between YO(6), ZO(6), and TO(4) 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 (Z)Al combined with the occurrence of B at Tin Al-rich tourmaline. (Z)Al has a greater effect than (Y)Al as long as Al does not fully occupy the Z site.