The minerals belonging to the tobermorite group differ essentially by their basal spacing (approximately 14, 11, and 9 Å ), related to their water content; additionally, the phases with an 11 Å periodicity differ in their behaviour upon heating, which can be “normal” if they collapse to form a 9 Å phase or “anomalous” if they preserve their original basal spacing. The “anomalous” thermal behaviour of tobermorite 11 Å from N’Chwaning II mine (Kalahari Manganese Field, Republic of South Africa), which does not shrink at 300 °C, is accurately studied by means of thermo-gravimetric and differential scanning calorimetric (TG-DSC) analysis and in situ and ex situ X-ray diffraction experiments. In situ X-ray powder diffraction shows that at ~420 °C, tobermorite 11 Å transforms into a phase with a basal spacing of approximately 10.1 Å, namely “tobermorite 10 Å “. The TG-DSC study shows a two-step weight loss: the former occurs between room temperature and 300 °C, whereas the latter is observed between 500 °C and 600 °C. These two steps correspond to endothermic and exothermic processes, respectively, which are related to the dehydration of tobermorite and its transformation into tobermorite 10 Å. Above 1000 °C, tobermorite 10 Å transforms into wollastonite. Finally, ex situ X-ray diffraction studies, using both powder and single-crystal techniques, show that the transformation from the 11 Å to the 10 Å phase passes through an intermediate clinotobermorite-like phase.