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The properties of materials obtained during the thermal treatment of gypsum depend on structural defects in the multiphase gypsum-anhydrite system. Analysis of the crystal structures of gypsum (CaSO4·2H2O), bassanite (CaSO4·0,5H2O), ?-anhydrite (?-CaSO4) and natural anhydrite (?-CaSO4) allowed us to identify a common structural motif in each of these structures. This is an irregular hexagon formed by chains of ?Ca?SO4?Ca?SO4?. During successive thermal transformation of gypsum into anhydrite, this structural motif...