emical (EPM data), TG–DSC studies and structural investigations indicate that mosandrite presents a particular chemical composition (low Ca, Na and F contents, high amount of H2O), unit-cell parameters (a 7.398, b 5.595, c 18.662 Å, b 101.37°, V 757.29 Å3; space-group symmetry P21/c; Z = 2), and a rinkite-type structure characterized by a low occupancy of the M(2) and M(3) sites

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Sturmanite, ideally Ca 6 Fe 3+ 2 (SO 4 ) 2.5 [B(OH) 4 ](OH) 12 •25H 2 O, is trigonal, P31c, Z = 2, with a 11.188(9), c 21.91(7) Å, V 2375.1(5) Å 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 [SO 4 ] 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 Ca 6 [Fe 3+ 0.6

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