The crystal structure of betpakdalite, and a new chemical formula:: {Mg(H2O)6}Ca2(H2O)13[Mo6+8As5+2Fe3+3O36(OH)](H2O)4

The crystal structure of betpakdalite, monoclinic, C2/m, a 19.531(2), b 11.061(1), c 15.257(2) Angstrom, beta 131.57(1)degrees V 2466.6(5) Angstrom(3), Z = 2, has been refined to an R index of 2.5% for 3077 observed (5 sigma) reflections measured on a single crystal with MoK alpha X-radiation. A crystal fragment was analyzed with an electron microprobe. Compared with previous work, the chemical analysis showed the absence of K and the presence of Mg. Moreover, the site assigned to K in previous work is far more stereochemically compatible with occupancy by Mg. The chemical formula of betpakdalite is redefined as {Mg (H2O)(6)} Ca-2 (H2O)(13) [Mo-8(6+) As-2(5+) Fe-3(3+) O-36 (OH)] (H2O)(4).