THE CRYSTAL-STRUCTURE OF JORGENSENITE

The crystal structure of jorgensenite, Na-2(Sr,Ba)(14)Na2Al12F64(OH)(4 ), a 16.059(7), b 10.974(4), c 7.277(3) Angstrom, beta 101.70(3)degree s. monoclinic, C2/m, Z = 1, has been solved and refined to an R index of 7.4% using 1632 observed reflections recorded with MoK alpha X-rays . The structure is very similar to that of jarlite, ,Na,square)(14)Al- 12(Mg,square)(2)F-64(OH,H2O)(4). Sheets of corner-sharing AlF6 and NaF 6 octahedra are intercalated with slabs of edge-and face-sharing Sr ph i(9), (Sr,Ba)phi(9) and Sr phi(10) polyhedra. The following site-occup ancies were obtained: X(1) = X(3) = Sr, X(2) = 0.40 Sr + 0.60 Ba, Al(1 ) = Al(7) = Al, Na(1) = Na(2) = Na. The principal differences from the jarlite structure are (1) the occupancy of X(2) by Ba rather than Na las in jarlite) and (2) the occupancy of Na(2) by Na rather than Mg la s for the Mg site in jarlite). Natural compositions show considerable solid-solution between end-members.