Tausonite, srTi0 3 , is of a perovskite type, its unit cell is primitive, cubic, space group Pm3m. The single-crystal study does not reveal supplement reflection, which could indicate a pseudo-cubic symmetry of this mineral. The, pow der data from a diffractometer (DRON-3.CuK~ radiation) are identical to those of a synthetic srTi0 3 " The double-line uLi~oQ is solving well, the halfwidth of reflections is comparable to that of quarz in close regions 2 S. The lattice parameters were obtained from a leastsqare refinement -3.9048(1).1. Taking into account the "ideal" cubic structure of tausonite, unit cell parameters help calculate the distance between atoms. The atom sr is coordinated with 12 atoms 0, and the sr-o distance is 2.7611. The atom Ti is coordinated with 6 atoms 0, and the Ti-0 distance is 1,,9521. Evidently tausoni te is the first natural mineral in wl.ich the Ti-octahedra have the highest symmetry m3m. Considering the phase equilibria and thermodynamic properties of the amall crystalline particles seems to be of great interest for mineralogy and crystallography since many of the minerals may occ~ in a dispersed state or indeed have passed through a microcrystal stage during their growth history. From the surface Chemistry-state point equations are derived which describe the relationship between the shift of polymorphic equilibrium temperature and characteristics of size, habits and surface properties of crystals. These show a possibility for the phase size effect (PSE) in mineral crystals, that is to say the dependence of a phase transition point upon the crystal size. Experimental evidence for PSE in dispersed phases of Zns and HgS is obtained. It is shown that the influence of crystal size on the 3C--+2H transition in Zns, at temperatures much lower than 1293 K (transition point in the bulk) is caused by the difference in specific surface free energies between 30 and 2H. being about 0.2 J/m 2 • The variations of the 0& -J3 transition point in HgS under annealing is presumed to be connected with the different particle size as well. The crystal polytypism could also be connected with the peculiarities of phase equilibria of small crystalline particles. From this an original mechanism for ZnS polytypes and micro twins formations in natural environments is proposed. It is based on the conception of PSE and presupposes the spontaneous accommodation of structure to the crystal size, constantly enlarging during the growth stage. The physicochemical consideration of the ZnS polytypism allowed to suggest the preference of structures having a higher percentage of cubic packing especially at elevated temperatures of the ore formation. The pure 2H-form could preferably occur under low temperature conditions. There are structural si!i1ilarities between the two polymorphs. Both have alternating layers of carbonate and sulphate groups sandwiching the lead ions which are bridged by the hydroxyl groups. The difference between the structures lies in the relative positions of the cation and anion groups. A third poly...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.