*3.1. X-Ray Powder Di*ff*raction*

The results of PXRD (Table 1, Figure 1) showed that, in the absence of strontium in the solution, the obtained precipitate was represented almost exclusively by weddellite (Figure 1a).

**Figure 1.** Typical XRD patterns of precipitates (1—weddellite, 2—whewellite), obtained from solutions with different atomic ratio Sr/(Sr + Ca), %: **a**—0; **b**—5; **c**—35; **d**—70; **e**—100.

Accompanying the addition of a small amount of strontium to the solution (Sr/(Sr + Ca) ≤ 30%), the precipitate became dominated by calcium oxalate monohydrate whewellite (Figure 1b). Weddellite content ranged from 23 to 34%. Concurrently, at Sr/(Sr + Ca) = 25–30%, traces of calcium oxalate trihydrate caoxite (Ca[C2O4]·3H2O) were recorded in the precipitate (Table 1). Along with an increase of strontium content in the solution, the amount of whewellite in the precipitate decreased, while weddellite content increased, and at a ratio of Sr/(Ca + Sr) ~40%, these oxalates were present in the precipitate in almost equal amounts (Figure 1c). A further increase of strontium content in the solution (40% < Sr/(Sr + Ca) ≤ 80%) saw weddellite gradually start to prevail (Figure 1d). Accompanying a higher strontium content in the solution, the precipitate became close to monophasic—it is almost solely represented by weddellite (Figure 1e).

Along with the increase of strontium in the solution (and, consequently, in the formed crystals), the parameters of the weddellite tetragonal unit cell increased: *a* from 12.341 to 12.770 Å, *c* from 7.356 to 7.529 Å (Table 2). Linear parameters of monoclinic whewellite also increased: *a* from 6.289 to 6.396 Å, *b* from 14.576 to 14.860 Å, *c* from 10.120 to 10.367 Å (Table 2). The irregular fluctuations of the angle β value increased after Sr/(Sr + Ca) ≥ 40% (Table 2).

The average CSD size in Sr-containing weddellites varied from 162 to 71 nm and was smaller than 269 Å in Ca-weddellite. The minimum values of average CSD size (smaller than 100 Å) were observed at intermediate Sr/(Sr + Ca) ratios in solutions of 50–70%. Regarding Sr-containing whewellite, the average CSD sizes were from 57 to 28 Å, with an increase of strontium in the solution (Sr/(Sr + Ca) from 15 to 75), i.e., no less than two-times less than in weddellite (Table 2). Generally, the CSD size of the whewellite crystals gradually decreased with an increase in Sr content in the solution (up to the ratio Sr/(Sr + Ca) = 75%).


**Table 1.** Weddellite content in oxalate precipitate and strontium concentration in solution and in synthesized oxalates (via EDX).

\* Caoxite (2 wt%) is present in the precipitate.

**Table 2.** Oxalate unit cell parameters and average coherent scattered domain (CSD) size.


*3.2. Scanning Electron Microscopy (SEM) and Energy-Dispersive X Ray (EDX) Spectroscopy*

SEM data on the phase composition of the synthesized calcium oxalates support the XRD data (Table 1, Figure 2).

**Figure 2.** SEM images of formed (Ca,Sr)[C2O4]nH2O (n = 1, 2) crystals, synthesized from solutions with different Sr/(Sr + Ca), % ratio: **a**: 0, **b**: 5, **c**: 20, **d**: 35, **e**: 70, **f**: 80.

SEM images clearly show that weddellite dipyramidal crystals represent calcium oxalates synthesized from strontium-free solutions, the dipyramidal base edge (**Dp**) of which does not exceed 5–6 microns (Figure 2a).

Found in the precipitate obtained from a solution with a ratio of Sr/(Sr + Ca) = 5%, small spherical spherulites of calcium oxalate monohydrate whewellite (diameter ~4–5 μm) prevail, among which there are individual weddellite crystals and their intergrowths (Figure 2b). According to EDX spectroscopy, strontium content in weddellite (Sr/(Sr + Ca) = 4.2%) is close to that in the solution and is two times greater than in whewellite (Sr/(Sr + Ca) = 2.1%). Weddellite crystals are defined by dipyramidal habit (**Dp** ~10–25 μm). Not very well-developed prism faces appear on smaller crystals (rib length between prism faces **Pr** = 3–4 μm, Figure 2b inset 1). The value of the average **Pr**/**Dp** ratio (which describes the degree of prism face development compared to dipyramid) is only 0.1. There also are twin intergrowths which form "quadruplets" consisting of two intergrown tetragonal dipyramids with a common fourth-order symmetry axis, with each dipyramid being rotated relative to each other around this axis of symmetry by 45◦ (Figure 2b inset 2).

Accompanying an increase in—ratio in the solution up to 30%, whewellite continues to prevail over weddellite (Figure 2c). According to EDX, Sr content increases in both phases (up to ratio Sr/(Sr + Ca) = 16.0% in weddellite and up to 11.8% in whewellite), but it decreases relative to the solution. The size of whewellite spherulites increases (average diameter ~6 μm, maximum 15 μm). Weddellite average crystal size also increases (**Dp** ~20–25 μm). While the number of weddellite crystals of dipyramidal-prismatic pattern increases, the prism face continues to develop: **Pr** ~10 μm, the **Pr**/**Dp** ~0.35.

When the Sr/(Sr + Ca) ratio in the solution reached 35–40%, the number of crystals of whewellite and weddellite became comparable (Figure 2d). According to EDX, Sr content continued to increase in both phases (up to Sr/(Sr + Ca) = 20% in weddellite and up to 11–13% in whewellite) and decreased relative to the solution. The size of whewellite spherulites was relatively small (5–6 microns in diameter, but reached 20 microns). Weddellite crystals also increased in size (**Dp** ~15–30 μm). All weddellite crystals had prism faces and the prism faces continued to develop (**Pr** ~10–20, **Pr**/**Dp** ratio ~0.40–0. 45).

When the ratio in the solution Sr/(Sr + Ca) ≤ 40%, weddellite began to prevail over whewellite. Two generations of weddellite crystals were observed in the precipitate: large dipyramidal (**Dp** up to 80 μm) and smaller dipyramidal-prismatic (**Dp** ~25 μm, **Pr** ~15–20 μm) (Figure 2e). The **Pr**/**Dp** ratio in weddellite crystals continued to increase and reached 0.56 at Sr/(Sr + Ca) = 50% in solution, and then began to decrease to 0.33 at Sr/(Sr + Ca) = 70%. The diameter of spherulites of whewellite decreased to 3–5 microns. According to EDX, the Sr/(Sr + Ca) ratio reached 55.0% in weddellite and 35.5% in whewellite.

Reaching a Sr/(Sr + Ca) ratio of 80% and more in the solution, weddellite crystallized almost solely, and was represented by large dipyramidal (200–250 μm) and smaller dipyramidal-prismatic (30–50 μm) crystals (Figure 2f). Attaining Sr/(Sr + Ca) = 85–95%, the development of the prism face slowed (**Pr** remained ~20 μm, the **Pr**/**Dp** ratio decreased to ~0.1). The Sr/(Sr + Ca) ratio in weddellite via EDX increased from 78 to 100%, approaching the value of this ratio in solution.

According to the EDX data, the Sr/(Sr + Ca) ratios of 29.7–45.2 in weddellite and of 17.4–31.0 in whewellite correspond to the minimum CSD size for both phases.
