**Aleksei V. Rusakov \*, Mariya A. Kuzmina, Alina R. Izatulina and Olga V. Frank-Kamenetskaya \***

Crystallography Dept., Institute of Earth Sciences, St. Petersburg State University, 7/9, University emb., 199034 St. Petersburg, Russia; m.kuzmina@spbu.ru (M.A.K.); alina.izatulina@spbu.ru (A.R.I.)

**\*** Correspondence: alex.v.rusakov@gmail.com (A.V.R.); o.frank-kamenetskaia@spbu.ru (O.V.F.-K.); Tel.: +7-921-924-2229 (A.V.R.)

Received: 14 November 2019; Accepted: 6 December 2019; Published: 8 December 2019

**Abstract:** To study strontium (Sr) incorporation into calcium oxalates (weddellite and whewellite), calcium-strontium oxalate solid solutions (Ca,Sr)[C2O4]·nH2O (n = 1, 2) are synthesized and studied by a complex of methods: powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Two series of solid solutions, isomorphous (Ca,Sr)[C2O4]·(2.5 − x)H2O) (space group I4/m) and isodimorphous Ca[C2O4]·H2O(sp.gr. P21/c)–Sr[C2O4]·H2O(sp.gr. P 1), are experimentally detected. The morphogenetic regularities of their crystallization are revealed. The factors controlling this process are discussed.

**Keywords:** calcium oxalate; strontium oxalate; solid solutions; ionic substitutions; weddellite; whewellite; X-ray powder diffraction; scanning electron microscopy; EDX spectroscopy
