A concept for the classification of crystalline (metallo)borophosphates in terms of structural chemistry is proposed and the compounds known to date are classified in an overview. Similarities and differences with (alumo)silicates and Liebau's classification are discussed with respect to the observation that the different borate and phosphate complexes are not interconnected arbitrarily in borophosphates. By combination and extension of existing concepts for silicates and borates a hierarchical system based on oligomeric building units has been developed. The observed connection rules are rationalized and the strong influence of the composition on dimensionality and structural motifs of the formed anions is pointed out. Likewise the effect of OH groups is taken into account by grading anions according to the degree of protonation (ratio O:OH). A general distinction is made between tetrahedral and mixed coordinated borophosphates. Metalloborophosphates are treated separately as special cases of borophosphates. Finally, anion‐substituted compounds, border cases, and borate‐phosphates complete the overview.

In one ear …︁ : The outer shape and inner architecture of individual particles of a complex calcite–gelatine composite resemble the known characteristics of biogenic otoconia (“ear dust”; see picture). The relationships between the morphology of artificial and biogenic specimens are discussed.

The absence of mixing between Y3+ and Cu2+ results in a more idealized Kagomé lattice in YCu3(OH)6Cl3 compared to hebertsmithite and kapellasite.

Biogenic otoconia (ear dust) are composite materials of calcite with about 2 wt.-% proteins showing an average longitudinal size of about 10 μm. The tiny biomineral particles are situated in the inner ear (in the maculae) and act as sensors for gravity and linear acceleration. Our comparative study of calcite-gelatine composites (grown by double diffusion) and human otoconia is based on decalcification experiments, scanning electron microscopy, TEM and X-ray investigations in order to obtain a complete picture of the 3D structure and morphogenesis of the materials. Otoconia as calcite-protein composites display a cylindrical body with terminal rhombo-

Single crystals of BPO4 were grown by chemical transport reactions with PCl5 using a gradient from 1073 K to 973 K as well as with solvothermal syntheses in the temperature range between 413 K and 523 K using water, ethanol or 2‐propanol as polar protic solvents. The atomic arrangement of BPO4 was reinvestigated by means of single crystal X‐ray diffraction data and confirmed the earlier findings with significantly smaller standard deviations. Thermogravimetric investigations of powdered samples which show no extra lines in X‐ray diffraction diagrams revealed weight decrements which are attributed to water losses. The presence of protonated borate and phosphate species in a number of X‐ray pure solvothermally grown BPO4 samples is evidenced by infrared spectroscopy.