High-temperature crystal structure of sanidine Part II. The crystal structure of sanidine at 935°C
Kimata, Mitsuyoshi; Shimizu, Masahiro; Saito, Shizuo
published: Feb 22, 1996
manuscript accepted: Sep 21, 1995
manuscript received: Feb 9, 1995
ArtNo. ESP147050801004, Price: 29.00 €
Abstract The crystal structure of sanidine from Volkesfeld, Eifel, Germany, is refined using single-crystal X-ray intensities collected on an automatic four-circle diffractometer at 23 and 935 (± 5)°C, to R factors of = 0.046 and 0.055, respectively. The chemical formula is K0.789Na0.160Ba0.014Fe0.0030.034Al1.018Si2.987O8. The space group is C2/m, Z = 4 and the lattice parameters are a = 8.531(1) Å, b = 13.007(1) Å, c = 7.179(1) Å, β = 116.00(2)° and V = 715.9(1) Å3 at 23°C; a = 8.677(1) Å, b = 13.016(2) Å, c = 7.184(1) Å, β = 115.73(1)° and V = 730.9(1) A3 at 935°C. A comparison of the two structures leads to an equitable assessment of the following geometrical features: 1) extremely high thermal expansion of the a cell edge in comparison with little change in the b and c axes, 2) expansion of K-O bonds and 3) apparent shortening of T-O bond lengths. Structural adjustments to thermally relaxed stresses along the a-axis are principally triggered by the combined effect of longer M-OA1 and M-OA2 distances, and larger OC-OB-OC angles, related to the (100) plane formed by combination of T12O7 and T22O7 pyro-anions linked at right angles to each other. The former orientation is parallel to the c-axis and the latter to the b-axis. The thermal behaviour of the intertetrahedral O-Obridge-O angles within each tetrahedron in the pyro-anions accommodates compensating the adjustment to invariancy of the b and c axes.