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Pyroxene solid solution (CaNa)Fe(Ge,Si)2 O6 : the niches of topological stability of C2/c-II and C2/c-III structures. Discontinuity of (b, c, beta) - V trends at the Ca-Na substitution in the solid solution (CaNa)FeGe2O6.

The pyroxene structures C2/c-II (Hd) and C2/c-III (Aeg) have distinct niches of topological stability [Nov0319E, Nov-2004, Nov7EIZV], what is predetermined by the important difference in properties of their M2 cations - Ca has in pyroxene structures "coordination number" eight [Zhang-97], and Na is six coordinated [Cam-73, Origl-2003].

The discontinuity of Ca-Na trend proves, that, in real, the end members of solid solution (CaNa)FeGe2O6 give no perfect solid solution. The limited miscibility of (CaNa)FeGe2O6 was determined, in real, both by NGR and X-Ray powder diffraction methods [Nov-99b, Nov0320E, Nov7EIZV].

At low concentration of Na (x(Na)=0 - 0.4) the Ca-Na trend UCP-V practically coincide with the trend Fe-Mg of the solid solution Ca(Fe,Mg)Ge2O6 and illustrate just the elastic response to decreasing of the "average" size of M1 polyhedra due to submission of Fe2+ ions by Fe3+, as it takes place at the submission of Fe2+ ions by Mg in the solid solution Ca(Fe,Mg)Ge2O6.

At x(Ca)=0.6-0.8 one can propose the appearance of some distinct intermediate structure - their NGR spectrum demonstrate quite different, specific distribution of local fields.

As for two samples, having the nominal composition x(Ca)=0.2, their structure could not be identified definitely as the aegirine structure (C2/c-III) - the additional studies are needed.