SOLID-SOLUTIONS AND FELDSPAR-PARACELSIAN DIMORPHISM IN THE (CA,SR)[(AL,GA)(2)SI2O8] SYSTEMS

A number of terms was synthesized, at constant composition intervals, sufficient to adequately illustrate the title fields. The syntheses we re performed both at atmospheric pressure and hydrothermally, in the f ormer case by reaction in the solid state or by crystallization from t he melt and in the latter case mostly under the standard conditions of 1 kbar and 500 degrees C. The identification of the polymorphs was by room- and high-temperature X-ray powder diffraction, using Guinier-ty pe cameras. The whole of the results shows that at room pressure the e ffect of the expansion of the tetrahedral framework, induced by the gr adual proxying of GaO4 for AlO4 group, is of enlarging the paracelsian stability field and of confining the feldspars more and more to high temperatures. Ail this wholly conforms with the previous results for t he analogous (Sr,Ba) systems. On the contrary it has been found that, given a certain tetrahedral framework, the substitution of the smaller Ca for Sr cation re-stabilizes the feldspar form in its triclinic mod ification, while the analogous Sr for Ba substitution was found to cau se a, coherent, increase of the P-form field. The paracelsians constan tly result significantly denser than the corresponding feldspars which are triclinic, excluding the Sr-richest terms that are monoclinic. Sy nthetically, the terms CaAl2Si2O8, SrGa2Si2O8 within the present syste ms and BaAl2Si2O8, SrGa2Si2O8 in the (Sr,Ba) systems, represent antipo dal compositions as regards the stability of the F- and P-forms respec tively. The P-type framework is better matched, in all instances, by t he Sr cations.