The roedderite-chayesite series from Spanish lamproites: chemical and crystallographic relations

Members of the roedderite-chayesite series in lamproites from Cancarix (SE Spain), crystallized from late magmatic residua under low PH20, high temperature (~ l l00~ oxidizing conditions. They exhibit the following main chemical variations: 0.14 ~< Na ~< 0.62 atoms per formula unit (apfu); 0.80 ~<K ~< 1.00 apfu; 2.97 ~< Mg ~< 4.33 a.pfu; 0.00 ~< Fe 2+ ~< 1.19 apfu; 0.42 ~< Fe 3+ ~< 0.87 apfu; they arehexagonal (10.120 ~< a ~< 10.135 A, 14.305 ~< c ~< 14.326 A), P6/mcc. The characteristic chemicalsubstitution is: Fe 3+ + [] ~ Fe 2+ + (K+Na) § Six crystal structures have been refined to 0.020 ~<Robs ~< 0.026. They have the osumilite/milarite-type structure, with Si entering the double tetrahedral TIsix-membered rings, and Mg and Fe entering both the ring-linking 72 tetrahedra, and the A octahedra.The 12-coordinated C site, located between two double rings of T1 tetrahedra, is occupied mainly by Kand subordinately by Na. Furthermore, Na occupies the partially empty nine-fold coordinate