Ion substitutions and structural adjustment in Cr-bearing tourmalines

The geological settings of Cr-bearing tourmaline occurrences are reviewed and the crystal chemistry of Cr-bearing tourmalines is analysed on the basis of new X-ray single-crystal structure refinements (R 0.019-0.025) of three crystals from the Ural deposits (Cr (sub 2) O (sub 3) content approximately 6 wt.%) and one from Umba Valley, Tanzania ( approximately 1 wt.% Cr (sub 2) O (sub 3) ) and published data (13 structures) on tourmalines from different deposits with Cr content up to 36 oxide wt.%. The boundary between the ordered and the disordered distribution of Cr between Y and Z octahedral sites is determined near 1.03 + or - 0.27 Cr apfu and relationships between Cr content and structural parameters (unit-cell parameters, octahedral bond-lengths, charge of Y and Z sites and polyhedral deformation) are established. An increasing Cr content in tourmaline causes an increase of both lattice parameters, of <Z-O> and <X-O> bond-lengths and a regular variation of the charge of the Y and Z octahedra. In general, the structure becomes less deformed by reducing the distortion of the octahedra and of the tetrahedral ring.