Calcium carbonate occurs in nature in three crystallized modifications: Calcite (calcareous spar) - hexagonal crystallized Aragonite (pearls) - rhombic crystallized Vaterit - rhombic crystallized If calcium carbonate precipitates as a poorly soluble compound when the solubility product is exceeded from aqueous solutions, the precipitate is initially amorphous and then slowly changes to the crystalline form of calcite. When heated (burned), calcium carbonate decomposes to form calcium oxide (burned lime) and carbon dioxide: CaCO 3 → CaO + CO 2 This reaction is used for the production of burnt lime and for the production of carbon dioxide. Calcium oxide reacts with water in a strongly exothermic reaction to form calcium hydroxide = slaking of lime: CaO + H 2 O → Ca(OH) 2 Calcium hydroxide is the main component of lime mortar. Calcium carbonate is formed by reaction with the CO 2 in the air, which then leads to the corresponding strength: Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O Linguistically, the term "lime" therefore occurs for different chemical compounds: CaCO 3 Calcium carbonate - Lime CaO Calcium oxide - burnt lime, quicklime Ca(OH) 2 Calcium hydroxide - Hydrated lime, slaked lime Common names for naturally occurring calcium carbonate include limestone, chalk, and marble. Lime = Calcite = CaCO 3