*4.1. The E*ff*ect of Strontium Concentration in Solution on Phase Composition of the Precipitate*

The strontium content in the solution significantly affects the phase composition of the precipitate. Considering the absence of Sr2<sup>+</sup> ions in the solution of non-stoichiometric composition (Ca/C2O4 = 5) containing citric acid, tetragonal calcium oxalate dihydrate weddellite (Figures 1a and 2a) is obtained, which is unstable in the crystallization field of monoclinic calcium oxalate monohydrate whewellite [17,21]. The addition of a small amount of Sr2<sup>+</sup> ions (Sr/(Sr + Ca) = 5%) to this solution violates the conditions favorable for weddellite crystallization and leads to intensive crystallization of whewellite, the content of which in the precipitate exceeds the amount of weddellite by 2.6 times (Figures 1b and 2b, Table 1). Accompanying an increase in the strontium content in the solution, the amount of weddellite in the precipitate gradually increases and, at a ratio of Sr/(Sr + Ca) = 35–45%, matches the amount of whewellite (Figures 1c and 2d). A further increase in strontium content again creates conditions favorable for the crystallization of weddellite, the content of which in the precipitate begins to prevail (Figures 1d and 2e). Reaching an Sr/(Sr + Ca) ≥ 80% ratio in solution, the precipitate consists almost exclusively of weddellite. Thus, the presence of strontium in the crystallization medium can be favorable for the crystallization of both weddellite and whewellite, depending on the concentration.
