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 Ca6(Fe1.53+Al0.3Mn0.22+)∑2.0{[B(OH)4]1.2(SO4)2.3}∑3.5(OH)12·25H2O, indicates that H2O(g) is lost at about 135°C, SO3(g) is lost at about 1349°C, and the residue melts at about 1154°C. In sturmanite, a polymorphic transition occurs at about 627°C. For ettringite, approximately Ca6Al2(SO4)3(OH)12·26H2O, 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) Å, V 2355.2(8) Å3 for the hexagonal supercell, and a 11.147(3), c 10.918(5) Å, V 1174.9(9) Å3 for the subcell. The unit-cell parameters for ettringite are a 11.223(1), c 21.474(2) Å, V 2342.2(5) Å3 for the hexagonal supercell, and a 11.229(1), c 10.732(2) Å, V 1171.9(3) Å3 for the subcell. The volume of sturmanite is only slightly larger than that of ettringite.
