*4.2. Sr-Ca Ionic Substitutions in Solid Solutions*

According to EDX, strontium is present in crystals of both calcium oxalate monohydrate and dihydrate (Table 1). The ratio Sr/(Sr + Ca) varies from 0 to 100% in weddellite and from 2.1 to 35.5% in whewellite. A narrower range of observed strontium concentrations in whewellite is explained by the synthesis conditions under which whewellite, unlike weddellite, precipitates only in solutions with an Sr/(Sr + Ca) ratio varying from 5 to 75% and always alongside weddellite. Altogether, strontium content in weddellite is always higher than in whewellite by 1.4–2 times (Table 1), which indicates that the entry of Sr into whewellite is difficult compared to weddellite.

It is known that, despite the significant difference in ionic radii (rCaVIII = 1.12 Å, rSrVIII = 1.26 Å, [28]), Sr2<sup>+</sup> ions easily replace Ca2<sup>+</sup> ions which occupy large cavities coordinated by 7–9 cations in various crystal structures, like in apatite [29,30]. This suggests that all (or almost all) strontium recorded via EDX isomorphically substitutes calcium in its positions in weddellite and whewellite. A simultaneous increase of the unit cell parameters of weddellite and whewellite with the increasing Sr content (Figures 3 and 4) confirms this assumption.

An increase in the unit cell parameters of weddellite, primarily parameter *a*, also occurs with the increase of zeolite water (Wz) [18,19]. An increase of zeolite water amount in weddellite (from 0.13 to 0.37 apfu), however, leads to an increase in the *a* parameter by only 0.049 Å [18]. Conversely, the contribution of strontium to the increase of the weddellite parameter *a* is an order of magnitude greater; the change in parameter *a* reaches 0.469 Å (Figure 3a). The dependence of unit cell parameters of weddellite on strontium content is not linear and follows a second degree polynomial function (Figure 3). Violation of the linear correlation between the strontium content and the parameter values can be due, first, to the fact that when the Sr/(Sr + Ca) ratio reaches ~40% in weddellite (~60% in solution) Sr2<sup>+</sup> ion entry into weddellite slows. Second, it is possible that at this strontium content, the amount of zeolite water in weddellite ceases to increase and a further slower increase in parameters occurs only due to an increase in strontium.

**Figure 3.** The increase of weddellite unit cell parameters: *a* unit cell parameter versus strontium (present study) and zeolite water [18] content, % (**a**); *c* unit cell parameter versus strontium content, % (**b**).

Seen at low strontium concentrations in whewellite (with Sr/(Sr + Ca) ratios in crystals varying from 2.1 to 12.7%; and in solution from 5 to 40%), there is a well-defined linear relationship between the values of linear parameters (*a*, *b*, *c*) and the content of strontium, while the angle β does not change significantly (Figure 4). The fluctuations of both linear parameters and angle β at higher strontium concentrations can be explained by desymmetrization (lowering the symmetry from monoclinic to triclinic), since the crystal structure of strontium oxalate monohydrate (Sr[C2O4]·H2O) is triclinic [7,24]. Desymmetrization may be associated with reciprocal turns of calcium polyhedra and oxalate groups, associated with the partial ordering of calcium and strontium atoms at the crystallographic sites. To the question of why desymmetrization is not at maximum in the middle of the series, further studies should be made.

**Figure 4.** Variations of whewellite unit cell parameters versus strontium content, %: *a* (**a**); *b* (**b**); *c* (**c**); β (**d**).

Thus, assumption of 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), received experimental confirmation. A detailed study of the desymmetrization pattern of the monoclinic whewellite structure with strontium incorporation via single-crystal X-ray diffraction analysis is now in progress.
