DTA, TG, and XRD studies of sturmanite and ettringite

Thermal analyses (DTA and TG) were carried out on sturmanite and ettringite from the Kalahari Manganese Field, South Africa. The TG trace for sturmanite, approximately Ca-6(Fe1.53+Al0.3Mn0.22+)(Sigma2.0){[B(OH)(4)](1.2)(SO4)(2.3)}(Sigma3.5)(OH)(12).25H(2)O, indicates that H2O(g) is lost at about 135degreesC, SO3(g) is lost at about 1349degreesC, and the residue melts at about 1154degreesC. In sturmanite, a polymorphic transition occurs at about 627degreesC. For ettringite, approximately Ca6Al2(SO4)(3)(OH)(12).26H(2)O, the H2O(g) and SO3(g) are lost at about 149 and 753 C, respectively, and the residue melts at about 1176 C. Ettringite melts after the liberation of both H2O(g) and SO3(g), whereas sturmanite melts after the liberation of H2O(g). The loss of SO3(g) occurs at a considerably lower temperature. in ettringite than in sturmanite. Using powder X-ray diffraction, the unit-cell parameters for sturmanite are a 11.157(1), c 21.846(3) Angstrom, V 2355.2(8) Angstrom(3) for the hexagonal supercell, and a 11.147(3), c 10.918(5) Angstrom, V 1174.9(9) Angstrom(3) for the subcell. The unit-cell parameters for ettringite are a 11.223(1), c 21.474(2) Angstrom, V 2342.2(5) Angstrom(3) for the hexagonal supercell, and a 11.229(1), c 10.732(2) Angstrom, V 1171.9(3) Angstrom(3) for the subcell. The volume of sturmanite is only slightly larger than that of ettringite.