4.3.1. The Effect of Sr Concentration in Solutions on Crystal Morphology

Regarding all variations of strontium content in solution (0 ≤ Sr/(Sr + Ca) ≤ 100%), weddellite is represented by dipyramidal and/or dipyramidal-prismatic crystals, and whewellite by round spherulites (Figure 2).

As strontium content in the solution increases, the size of the weddellite dipyramidal crystals grows as well: **Dp** varies from ~5–6 μm in pure calcium crystals to 30 μm at Sr/(Sr + Ca) = 35% (Figure 2a–d). Accompanying a further increase of strontium concentration in the crystallization medium, the formation of two weddellite crystal generations is clearly observed. Larger ones are mainly of dipyramidal pattern and smaller ones of dipyramidal-prismatic configuration (Figure 2e). The size of larger weddellite crystals increases with the increase of strontium concentration in the solution, from **Dp** = 30 μm at Sr/(Sr + Ca) = 35% to **Dp** = 200 μm at Sr/(Sr + Ca) = 80–95% (Figure 2f). The size of smaller weddellite crystals also increases with the increase of strontium content, from **Dp** = 15 μm at Sr/(Sr + Ca) = 35% in solution to **Dp** = 50–60 μm at Sr/(Sr + Ca) = 80–95% in solution. Tetragonal prism faces of smaller weddellite crystals are well developed and the size of these faces along the prism edge also gradually increase with the increase of strontium content in the solution, from **Pr** = 3–4 μm at Sr/(Sr + Ca) = 5% (Figure 2b) to **Pr** = 20 μm at Sr/(Sr + Ca) = 70–80% (Figure 2e,f). The aspect ratio between the prism and dipyramid faces with the increase of strontium content in the solution increases from 0.1 (at Sr/(Sr + Ca) = 5%) to 0.56 (at Sr/(Sr + Ca) = 50%) and then it starts to decrease to 0.1 (at Sr/(Sr + Ca) = 95%).

Shown above, the first and last syntheses of the studied series (purely calcium and purely strontium solutions) are characterized by the formation of weddellite crystals in the form of a tetragonal dipyramid, which indicates optimal conditions for the precipitation of calcium and strontium oxalate dihydrate. The middle members in the weddellite series exhibit tetragonal prism faces in their pattern. The ratio between linear sizes of the prism and dipyramidal faces increases with the increase in strontium content in solution from 0.1 (at Sr/(Sr + Ca) = 5%) to 0.56 (at Sr/(Sr + Ca) = 50%), and then decreases to 0.1 (at Sr/(Sr + Ca) = 95%). Accordingly, with an increase in strontium in the crystallization medium from 5 to 50%, the initial prism growth rate increases significantly and then (at a ratio Sr/(Sr + Ca) ≥ 65%) begins to decrease. The slowdown in the growth rate of weddellite prism faces in the middle of the calcium-strontium series is most likely due to the deviation of the crystallization conditions from optimal.

Kuzmina et al [25] demonstrates that the crystallization of whewellite in the form of spherulites is common for solutions containing citrate ions and indicates a more rapid growth of whewellite (compared with weddellite) under the conditions of a higher supersaturation of the solution. The average diameter of whewellite spherulites is dependent on the content of Sr2<sup>+</sup> cations in the solution. It first increases from 4 μm (at Sr/(Sr + Ca) = 5%, Figure 2b) to 6 μm (at Sr/(Sr + Ca) = 35–50%, Figure 2d), then it decreases to 1–2 μm (at Sr/(Sr + Ca) = 70–80%, Figure 2e,f) amid the decrease in the total amount of whewellite.
