THE CRYSTAL STRUCTURE OF RAPIDCREEKITE, Ca2(SO4)(CO3)(H2O)4, AND ITS RELATION TO THE STRUCTURE OF GYPSUM

MARK A. COOPER and FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2

Abstract

The crystal structure of rapidcreekite, Ca2(SO4)(CO3)(H2O)4, a 15.517(2), b 19.226(3), c 6.1646(8) Å, V 1839.0(4) Å3, Pcnb, Z = 8, has been solved by direct methods and refined to an R index of 4.4% based on 943 observed (5 ) reflections measured with MoK X-radiation. There are two unique Ca sites, both of which are coordinated by six oxygen anions and two (H2O) 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. (Ca 8) polyhedra share edges to form chains along [001] that are cross-linked in the [010] direction into a sheet by (CO3) triangles and (SO4) tetrahedra; the resulting sheet is the structural unit of rapidcreekite. Inter-unit linkage is provided by an array of H-bonds involving the (H2O) 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 (SO4) groups coupled with incorporation of (CO3) groups along each twin plane.