Crystal chemistry of Cu-bearing tourmalines

The crystal structures of two elbaites from the Paraiba deposit with copper contents of 3.51 and 1.61 wt% CuO [a = 15.881(1), 15.840(3); c = 7.112(1), 7.1028(9), respectively], as well as a synthetic Cu-bearing olenite with a CuO content of 8.39 wt% [a = 15.840(4), c = 7.091(1)] have been refined to R-indices of 2.2, 3.1, and 4.1% using X-ray single-crystal diffraction. On the basis of original and published data (for six structures) the crystal-chemical relationships of copper-bearing tourmalines has been analyzed. It is shown that copper cations and cations of other 3d elements (Mn, Zn, Fe) occupy almost exclusively the Y site. Such an ordered distribution results in a change of the size of the Y-octahedra. There is an inverse correlation between the content of 3d elements (mainly Cu2+ cations) and Al3+ cations in the Y-octahedron and also between the value of < Y-O > and the content of Al3+ cations in the Y site. The direct correlation between values of < Y-O >-distance in the structures of Cu-bearing elbaites and the parameters (a, c) of the unit cell have been found. Distortions of polyhedra in the structures of Cu-bearing elbaites are similar, and significantly higher than those present in the structure of the synthetic Cu-bearing olenite.