Feldspars with trivalent non-tetrahedral cations; experimental studies in the system NaAlSi (sub 3) O (sub 8) - CaAl (sub 2) Si (sub 2) O (sub 8) - LaAl (sub 3) SiO (sub 8)

Feldspars with <= approx 45% trivalent cations on M positions were synthesized by heating gels of composition Ca (sub 8-2x) Na (sub x) La (sub x) Al (sub 16) Si (sub 16) O (sub 64) at 1300 degrees C and 1 bar. In the range 0 < x(gel) <= 3, these feldspars can be derived from anorthite by the substitution Ca (super 2+) + Al (super 3+) right arrow Na (super +) + Si (super 4+) . As x increases, the triclinic anorthite structure gradually becomes monoclinic at x(gel) = 3.05. Electron diffraction indicates transformation from P1 hrough I1 to I2/c with increasing x. (super 29) Si MAS NMR spectra indicate considerable Al/Si disorder, as well a slow long and medium range order of the Q (super 4) (4Al) units. The latter is due to distortion of the [SiO (sub 4) ] tetrahedra and considerable variation of the Si-O-T angles caused by rotation of the [TO (sub 4) ] tetrahedra as they adjust to the different M cations.