Structure of sanidine-type KGaSi3O8: Tetrahedral-site disordering in potassium feldspar
Kimata, Mitsuyoshi; Saito, Shizuo; Shimizu, Masahiro
European Journal of Mineralogy Volume 7 Number 2 (1995), p. 287 - 294
published: Mar 29, 1995
manuscript accepted: Nov 8, 1994
manuscript received: May 11, 1993
ArtNo. ESP147050702004, Price: 29.00 €
Abstract The structure of hydrothermally synthesized sanidine-type KGaSi3O8 has been refined at room temperature with single-crystal X-ray intensities to R = 0.028. The space group is C2/m, Z=4 and the lattice parameters are a = 8.660(1), b = 13.102(2), c = 7.229(1)Å, ß = 116.06(1)(°). Mean T-O distances and Ga site occupancies are: T1O [1.667(2)Å, 0.280(1)], T2O [1.647(2)Å, 0.220(1)]. Thus the expanded crystal-chemical formula can be written in the form K(Ga0.28Si0.72)2(Ga0.22Si0.78)2O8. This is the first report, on the basis of single-crystal refinement, of a disordered Ga/Si distribution in galliosilicate feldspars. The framework of feldspar and paracelsian structures are composed of T12O7 and T22O7 pyroanions with eclipsed configurations (T-O-T angles = 140 ±2°). In the former structure T12O7 groups are linked at right angles to T22O7 groups, while in the latter these two pyroanions are corner-shared parallel to each other. Thus electrostatic environments for the extraframework (M) sites differ from each other. The selection of the crystal-structure type for the compounds with chemical formulae MT4O8 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 determine the preference of feldspar over paracelsian structure types.