**1. Introduction**

Natural calcium oxalates dihydrous weddellite Ca[C2O4]·(2.5 − x)H2O and monohydrous whewellite Ca[C2O4]·H2O are widespread biominerals. They often form in the human body, for instance, a major part of the stones in the human urinary system consist of both [1,2]. These minerals also are found in salivary stones and other human pathogenic formations [3–5]. Besides, weddellite and whewellite often occur in crustose and foliose lichen thalli on the surface of Ca-bearing rocks and minerals [6–9]. Oxalate crystallization is a result of the interaction between metabolites of lichens and associated microscopic fungi with an underlying stone substrate in this case [10–14].

The crystal structure of monoclinic whewellite (space group P21/c) and tetragonal weddellite (sp.gr I4/m) of renal stones are well known [15–21]. Variations of weddellite unit cell parameters are well explained by the variable water content [18–20]. There is no evidence of ionic substitutions occurring at calcium sites of calcium oxalate crystal structures of renal stones. The complex multicomponent composition of the crystallization medium (of urea and of other human physiological liquids), however, allows for the probability of such substitutions [22]. The probability of Cd–Ca substitutions in synthetic whewellite is proposed by McBride et al., 2017 [23].

We found strontium impurities in weddellite and whewellite crystals in lichen thalli on Sr-bearing apatite rock via energy-dispersive X-ray (EDX) spectroscopy [9]. It allows us to suggest that Sr2<sup>+</sup> ions leach from fluorapatite and substitute Ca2<sup>+</sup> ions in oxalates. Synthetic strontium oxalates, monohydrate and dihydrate, have been reported by Baran, Sterling and Christensen, Hazen [7,15,24]. The tetragonal Sr [C2O4]·(2.5 − x)H2O is isotypic to weddellite and, similarly, contains a variable number of water molecules. The monohydrate, Sr[C2O4]·H2O, belongs to the triclinic crystal system (sp gr. P1). This suggests 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. P1).

To clarify the patterns of Sr2<sup>+</sup> ion incorporation into calcium oxalates (whewellite and weddellite), we synthesize (Ca,Sr)[C2O4]·nH2O (n = 1, 2) solid solutions and study the variations of their phase composition and crystal morphologies as well as in ion substitutions.
