Chemical control of the commensurate-incommensurate phase transition in synthetic melilites
Röthlisberger, Francois; Seifert, Friedrich; Czank, Michael
European Journal of Mineralogy Volume 2 Number 5 (1990), p. 585 - 594
published: Oct 4, 1990
manuscript accepted: Apr 27, 1990
manuscript received: Feb 19, 1990
ArtNo. ESP147050205004, Price: 29.00 €
Abstract The transition temperature, Tc, from the unmodulated high-temperature melilite to the incommensurately modulated structure strongly depends on the chemistry of these phases. In Ca-rich compounds, Tc was found to vary from below -170°C in Ca2Mg0.4Fe3+0.6Al0.6Si1.4O7 to above 650°C in Ge-rich members of the composition Ca2Zn(Si,Ge)2O7. It is shown that Tc increases with increasing ionic radii of the two types of tetrahedrally-coordinated cations by increasing the dimensions of the tetrahedral sheet. The concept of a misfit between the size of the interlayer cation and the size of the tetrahedral sheet also explains the absence of an incommensurate phase in Ba- and Sr-melilites.