The glass transition temperature of anhydrous amorphous calcium carbonate

25 Amorphous calcium carbonate (ACC) is the least stable polymorph of calcium 26 carbonates. It has been identified to play an important role in nature (e.g., 27 biomineralization and speleothem formation), where it acts as a precursor for the 28 transformation to more stable polymorphs such as calcite. Further, the use of ACC in 29 technical applications requires a robust understanding of the material's properties. We 30 present the first study that reveals the existence of a glass transition for synthetic and 31 anhydrous ACC. The glass transition occurs at 339 °C. Such measurements are 32 impossible with conventional differential scanning calorimetry (DSC) due to the high 33 tendency of ACC to crystallize. Fast scanning DSC with heating rates of 500 °C/s and 34 higher, however, can be used to separate the endothermic glass transition signature from 35 the exothermic crystallization event since crystallization is shifted to higher temperatures. 36 This allows the detection and quantification of the glass transition for ACC. These 37 observations indicate that ACC is a structural glass and is especially significant because 38 the synthesis of ACC, precipitation from a solution followed by lyophilization, contrasts 39 with the more conventional and well-known route of glass formation the rapid cooling of 40 a melt. Moreover, we prove that a structural glass can be produced from a simple single-41 component carbonate system. 42