New Insights into the Crystal Chemistry of Elpidite, Na(2)Zr[Si(6)O(15)]·3H(2)O and (Na(1+Y)Ca(x)□(1−X−Y))(Σ=2)Zr[Si(6)O(15)]·(3−X)H(2)O, and Ab Initio Modeling of IR Spectra

Elpidite belongs to a special group of microporous zirconosilicates, which are of great interest due to their capability to uptake various molecules and ions, e.g., some radioactive species, in their structural voids. The results of a combined electron probe microanalysis and single-crystal X-ray diffraction study of the crystals of elpidite from Burpala (Russia) and Khan-Bogdo (Mongolia) deposits are reported. Some differences in the chemical compositions are observed and substitution at several structural positions within the structure of the compounds are noted. Based on the obtained results, a detailed crystal–chemical characterization of the elpidites under study was carried out. Three different structure models of elpidite were simulated: Na(2)ZrSi(6)O(15)·3H(2)O (related to the structure of Russian elpidite), partly Ca-replaced Na(1.5)Ca(0.25)ZrSi(6)O(15)·2.75H(2)O (close to elpidite from Mongolia), and a hypothetical CaZrSi(6)O(15)·2H(2)O. The vibration spectra of the models were obtained and compared with the experimental one, taken from the literature. The strong influence of water molecule vibrations on the shape of IR spectra of studied structural models of elpidite is discussed in the paper.