Review on Chemistry of Water-Containing Calcium Carbonates and Their Transformations into Amorphous and Crystalline Carbonate Modifications

Calcium carbonate (CaCO3) is a dominant component of sedimentary rocks and biogenic structures, and is one of the most frequently studied inorganic compounds. It also plays a key role in preparing modern engineered materials. CaCO3 has three well-known polymorphs, calcite, aragonite, and vaterite, and four solvatomorphs with diverse crystallographic arrangements, hydration states, reactivity, and stability. Its solvatomorphs include the variable water-containing amorphous calcium carbonate (ACC—CaCO3·xH2O) and the crystalline monohydrocalcite (MHC—CaCO3·H2O), calcium carbonate hexahydrate (ikaite—CaCO3·6H2O), and the recently reported hemihydrate (CCHH—CaCO3·0.5H2O). Here, we review the preparation, crystal structure, and properties of these solvatomorphs and discuss their mutual transformations.