**5. Conclusions**

To clarify the patterns of Sr incorporation into calcium oxalates (whewellite and weddellite) the (Ca,Sr)[C2O4]·nH2O (n = 1, 2) solid solutions were synthesized and studied by complex methods (PXRD, SEM and EDX). It was shown that phase and elemental composition of synthesized solid solutions, as well as the morphology of their crystals, is strongly relevant to Sr concentration in the solution. The presence of two series of solid solutions, isomorphous (Ca,Sr)[C2O4]·(2.5 − x)H2O) (sp.gr. I4/m) and isodimorphous Ca[C2O4]·H2O(sp.gr. P21/c)–Sr[C2O4]·H2O(sp.gr. P 1) was experimentally proven for the first time. The causes of a difficult incorporation of strontium ions into calcium oxalates (especially into whewellite) were discussed. Morphogenetic regularities of their formation were revealed.

The results of this work show the possibility of diverse ionic substitutions occurring at calcium sites of calcium oxalate crystal structures, which opens a new page in the crystal chemistry of oxalic acid salts. Among the oxalates found in nature, biofilm biominerals formed as a result of the interaction of metabolism products of lithobiotic community with bedrock are highly favorable to exhibit ionic substitutions at a global scale. The regularities of these ionic substitutions are determined by both the mineral and elemental composition of the underlying rock and the species composition of the microorganisms inhabiting them. The applied value of the obtained patterns lies in the field of the development of biotechnologies which use the oxalate microbial crystallization as a source of bioremediation for environments contaminated with toxic elements.

**Author Contributions:** Conceptualization (A.V.R. and O.V.F.-K.); Investigation (A.V.R., M.A.K. and A.R.I.); Methodology (A.V.R., O.V.F.-K. and M.A.K.); Visualization (A.V.R. and A.R.I.); Writing—original draft (A.V.R., M.A.K. and A.R.I.); Writing—review & editing (A.V.R. and O.V.F.-K.).

**Funding:** This research was funded by the Russian Science Foundation (grant 19-17-00141 to A.V.R., O.V.F.-K., M.A.K., A.R.I.).

**Acknowledgments:** The laboratory researches were carried out in Research Resource Centers of Saint Petersburg State University: XRD measurements—in the X-ray Diffraction Centre; SEM investigations—in the Interdisciplinary Resource Center for Nanotechnology and Centre for Geo-Environmental Research and Modelling (Geomodel).

**Conflicts of Interest:** The authors declare no conflict of interest.
