Feldspar-paracelsian dimorphism of the synthetic compounds (Sr,Ba)[(Al,Ga)2(Si,Ge)2O8

Abstract

Abstract New results on the title dimorphism are given for the systems SrGa2Si208 – SrGa2Ge208, BaGa2Si2Og – BaGa2Ge2O8 and BaGa2Ge2Og – SrGa2Ge2O8. Their end terms as well as several intermediate terms were obtained by dry syntheses (reaction in the solid state or crystallization from the melt) or hydrothermally under 1.0 Kbar water pressure at 500° C. The proxying of Ge for Si, or of Sr for Ba, is accompanied by the shrinkage of the feldspar stability field which is progressively confined to the higher temperatures at room pressure. The composition limits for obtaining this form are SrGa2(Si0.75Ge0.25)2O8 and (Ba0.25Sr0.75) Ga2Ge2O8. The paracelsian form was obtained for all the Sr and Ba gallosilicates and germanates. The general effect of the hydrothermal conditions is of stabilizing this form. The phase transition and the stability relations of the two forms are considered for all the title terms: the feldspar/paracelsian transition is enantiotropic only within the intervals: Sr(Al0.50Ga0.50)2Si2O8 – SrGa2Si2O8, SrGa2Si2O8 – (Sr0.50Ba0.50)Ga2Si2O8 and BaGa2(Si0.50Ge0.50)2O8 –BaGa2Ge208. The correlations between the stability conditions, the densities, and certain structural parameters are examined.