STRUCTURE OF SANIDINE-TYPE KGASI3O8 - TETRAHEDRAL-SITE DISORDERING INPOTASSIUM-FELDSPAR

The structure of hydrothermally synthesized sanidine-type KGaSi3O8 has been refined at room temperature with single-crystal X-ray intensitie s to R = 0.028. The space group is C2/m, Z = 4 and the lattice paramet ers are a = 8.660(1), b = 13.102(2), c = 7.229(1)Angstrom, beta = 116. 06(1)(degrees). Mean T-O distances and Ga site occupancies are: T1O [1 .667(2)Angstrom, 0.280(1)], T2O [1.647(2)Angstrom, 0.220(1)]. Thus the expanded crystal-chemical formula can be written in the form K(Ga0.28 Si0.72)2(Ga0.22Si0.78)(2)O-8. This is the first report, on the basis o f single-crystal refinement, of a disordered Ga/Si distribution in gal liosilicate feldspars. The framework of feldspar and paracelsian struc tures are composed of Tl2O7 and T2(2)O(7) pyroanions with eclipsed con figurations (T-O-T angles = 140+/-2 degrees). In the former structure Tl2O7 groups are linked at right angles to T2(2)O(7) groups, while in the latter these two pyroanions are corner-shared parallel to each oth er. Thus electrostatic environments for the extraframework (M) sites d iffer from each other. The selection of the crystal-structure type for the compounds with chemical formulae MT(4)O(8) may be governed by two factors: M isotropic temperature factors and relative charge deficits of their M sites. The higher former and/or the larger latter determin e the preference of feldspar over paracelsian structure types.