Borophosphates (intermediate compounds of the system MxOy-B2O3-P2O5-(H2O)) contain complex anionic structures built of BO4, BO3, and PO4 groups and their partially protonated species, respectively. A first approach to the development of a structural chemistry of borophosphates is based on linking principles of the primary building units following the general Line of silicate crystal chemistry. The crystal structures of borophosphates are first divided into anhydrous and hydrated phases. Further gradings are based on the (molar) B:P ratios. The structural chemistry of borophosphate anions already extends from isolated species, oligomers, rings, and chains to layers and frameworks. Additional characteristics are integration of planar BO3 groups, preferred formation of 3-membered rings, and unusual branching of tetrahedral chains. P-O-P linking in borophosphates is not observed up to now

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