4.3.2. Sr-distribution in Oxalate System «Solution–Crystal»

The strontium concentration in oxalate monohydrate and oxalate dihydrate (0 ≤ Sr/(Ca + Sr) ≤ 100%) depends on strontium concentration in the solution following a third degree polynomial function (Figure 5):

Sr/(Sr + Ca)Solution = 0.0006[Sr/(Sr + Ca)wh] <sup>3</sup> <sup>−</sup> 0.0815[Sr/(Sr <sup>+</sup> Ca)wh] <sup>2</sup> <sup>+</sup> 4.3286[Sr/(Sr <sup>+</sup> Ca)wh] <sup>−</sup> 5.4929,

Sr/(Sr + Ca)Solution = 0.0001[Sr/(Sr + Ca)Wd] <sup>3</sup> <sup>−</sup> 0.0266[Sr/(Sr + Ca)Wd] <sup>2</sup> + 2,67[Sr/(Sr + Ca)Wd] <sup>−</sup> 7.1757.

Fitting with a third degree polynomial function is necessary due to the fact that the difference between strontium content in the crystallization medium and in synthesized solid solutions increases in the middle of the corresponding series.

The described distribution of strontium in the oxalate "crystal-solution" system indicates that the incorporation of strontium ions into calcium oxalates from solutions with close to equal amounts of Ca and Sr is difficult, which can explain the decrease of weddellite prism face growth rate and the lowering of CSD sizes for both weddellite and whewellite.

The difference in strontium content between the solution and whewellite is greater than between the solution and weddellite (Figure 5), which supports a proposal for a more difficult incorporation of Sr2<sup>+</sup> cations into whewellite than into weddellite. This effect can most likely be explained by the specific features of weddellite and whewellite crystal structures. This result also agrees well with the above conclusion that whewellite crystals grow more rapidly than weddellite crystals.

**Figure 5.** Sr/(Ca + Sr) ratio in the solution versus Sr/(Ca + Sr) ratio in the weddellite (•) and whewellite (•).
