THE CRYSTAL STRUCTURE OF STURMANITE

Abstract

Sturmanite, ideally Ca6Fe 3+ 2(SO4)2.5[B(OH)4](OH)12•25H2O, is trigonal, P31c, Z = 2, with a 11.188(9), c 21.91(7) A, V 2375.1(5) A 3 . The crystal structure has been refined to an R index of 0.079. As in the case of ettringite, two coaxial elements parallel to [001] can be considered as the most distinctive features of sturmanite. The main one is a column of polyhedra formed by Fe-octahedra and Ca-polyhedra. The Fe(OH)6 octahedra are located along the three-fold axes that pass through the origin of the cell. They are linked to three VIII Ca-polyhedra, located around the three-fold axes, in between the Fe(OH)6 octahedra. The second structural element parallel to [001] contains [SO4] and B(OH)4 tetrahedra; it is located along three-fold axes at (⅓, ⅔, z) and (⅔, ⅓, z). Both structural modules are linked to each other by a complex system of hydrogen bonds. On the basis of a complete determination of the structure, the chemical composition of sturmanite is close to Ca6[Fe 3+ 0.6Al0.2Mn 4+ 0.2]2 [SO4]2.7[B(OH)3]0.3 [B(OH)4](OH)12•24H2O.