The crystal structure of the new mineral dyrnaesite-(La), Na8CeIVREE2(PO4)6

Dyrnaesite-(La), Na-7.89(Ce0.94Ca0.06)(Sigma 1.00)(La1.14Ce0.40Pr0.10Nd0.24Ca0.12)(Sigma 2.00)(PO4)(6) is orthorhombic, Pnma, a = 18.4662(7), b = 16.0106(5), c = 7.0274(2) angstrom, V = 2077.7(1) angstrom(3), Z = 4. The crystal structure is related to the group of Na3REE(XO4)(2) compounds (with X = P, V, As), based on the aphthitalite/glaserite structural type. Dyrnaesite is distinct in having ordered Na vacancies, and a rare-earth element (REE) site occupied preferentially by Ce4+. This also distinguishes it from closely related vitusite-(Ce) [Na3REE (PO4)(2)]. The relation of their unit cells is: a(d) = b(v), b(d) = 3a(v), c(d) = 1/2c(v). The distinct Ce4+ site in dyrnaesite-(La) has smaller coordination with shorter bond lengths than the other REE site in the same structure or the REE sites in vitusite-(Ce). It is adjacent to the predominately vacant Na site, which in its turn has the largest coordination of all Na sites in the structure. REE sites, or Na sites in a [010] row (similar to [100] in vitusite-(Ce)) assume two types of coordinations with and without mirror symmetry and two different configurations of surrounding PO4 tetrahedra. This summarizes the topological difference to vitusite-(Ce) where the corresponding coordinations are similar in the same row and intermediate in character to the two types in dyrnaesite-(La).