The crystal structure of rapidcreekite, Ca 2 (SO 4 )(CO 3 )(H 2 O) 4 , a 15.517(2), b 19.226(3), c 6.1646(8) Aa, V 1839.0(4), Aa 3 , Pcnb, Z = 8, has been solved by direct methods and refined to an R index of 4.4% based on 943 observed (5sigma ) reflections measured with MoKalpha X-radiation. There are two unique Ca sites, both of which are coordinated by six oxygen anions and two (H 2 O) groups in a square antiprism arrangement. There is one unique S site coordinated by a tetrahedron of oxygen anions, and one C site coordinated by a triangle of oxygen anions. (Caphi 8 ) polyhedra share edges to form chains along [001] that are cross-linked in the [010] direction into a sheet by (CO 3 ) triangles and (SO 4 ) tetrahedra; the resulting sheet is the structural unit of rapidcreekite. Inter-unit linkage is provided by an array of H-bonds involving the (H 2 O) groups of the sheet; this weak interstitial bonding accounts for the perfect {100} cleavage of rapidcreekite. The structure of rapidcreekite is related to the structure of gypsum by twinning of the gypsum structure along alternate rows of (SO 4 ) groups coupled with incorporation of (CO 3 ) groups along each twin plane.

First Page Preview

First page PDF preview
You do not currently have access to this article.