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

Abstract

Fe-rich “oxydravite” and dravite from the Late Proterozoic ophiolitic mélange 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 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 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+ +WO 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 T in Al-rich tourmaline. Z Al has a greater effect than Y Al as long as Al does not fully occupy the Z site.