FELDSPARS WITH TRIVALENT NON-TETRAHEDRAL CATIONS - EXPERIMENTAL STUDIES IN THE SYSTEM NAALSI3O8 - CAAL2SI2O8 - LAAL3SIO8

Feldspars with up to about 45% trivalent cations on M positions were s ynthesized by heating gels of composition Ca8-2xNaxLaxAl16Si16O64 at 1 300-degrees-C and 1 bar. In the range 0 < x(gel) less than or similar to 3, these feldspars can be derived from anorthite Ca[Al2Si2O8] by th e substitution 2Ca2+ --> Na+ + La3+; at x(gel) greater than or similar to 3 this substitution is accompanied by the substitution Ca2+ + Al3 --> Na+ + Si4+. As x increases, the triclinic anorthite structure gra dually becomes monoclinic at x(gel) = 3.05(3). Electron diffraction in dicates transformation from P1BAR through I1BAR to I2/c with increasin g x. Si-29 MAS NMR spectra indicate considerable Al/Si disorder, as we ll as low long- and medium-range order of the Q4 (4Al) units. The latt er is due to distortion of the [SiO4] tetrahedra and considerable vari ation of the Si-O-T angles caused by rotation of the [TO4] tetrahedra as they adjust to the different M cations.