THE ROEDDERITE-CHAYESITE SERIES FROM SPANISH LAMPROITES - CRYSTAL-CHEMICAL CHARACTERIZATION

Members of the roedderite-chayesite series in lamproites from Cancarix (SE Spain), crystallized from late magmatic residua under low P-H2O, high temperature (similar to 1100 degrees C), oxidizing conditions. They exhibit the following main chemical variations: 0.14 less than or equal to Na less than or equal to 0.62 atoms per formula unit (apfu); 0.80 less than or equal to K less than or equal to 1.00 apfu; 2.97 less than or equal to Mg less than or equal to 4.33 apfu; 0.00 less than or equal to Fe2+ less than or equal to 1.19 apfu; 0.42 less than or equal to Fe3+ less than or equal to 0.87 apfu; they are hexagonal (10.120 less than or equal to a less than or equal to 10.135 Angstrom, 14.305 less than or equal to c less than or equal to 14.326 Angstrom), P6/mcc. The characteristic chemical substitution is: Fe3+ + square reversible arrow Fe2+ + (K+Na)(+). Six crystal structures have been refined to 0.020 g R(obs) less than or equal to 0.026. They have the osumilite/milarite-type structure, with Si entering the double tetrahedral T1 six-membered rings, and Mg and Fe entering both the ring-linking T2 tetrahedra, and the A octahedra. The 12-coordinated C site, located between two double rings of T1 tetrahedra, is occupied mainly by K and subordinately by Na. Furthermore, Na occupies the partially empty nine-fold coordinated B site which occurs both in the ideal (z = 0) and in a split-atom position along the c direction.