Original paper
Low-Temperature Crystal Structures of Orthoclase and Sanidine
Kimata, Mitsuyoshi; Saito, Shizuo; Shimizu, Masahiro; Iida, Ikuo; Matsui, Tomoaki
Neues Jahrbuch für Mineralogie - Abhandlungen Band 171 Heft 2 (1996), p. 199 - 213
31 references
published: Dec 12, 1996
DOI: 10.1127/njma/171/1996/199
ArtNo. ESP154017102004, Price: 29.00 €
Abstract
The crystal structures of orthoclase (Itrongay, Madagascar) and sanidine (Volkesfeld, Eifel, Germany) have been refined in space group C2/m at 121 K using singlecrystal X-ray diffraction data, compared with the 296 K structures. Crystal data for the orthoclase are as follows; chemical formula of K0.946▢ 0.054Fe0.0044Al0.902Si3.054 at 121K, a = 8.574(1), b = 13.006(1), c = 7.191(1)Å, β = 116.07(1)°, V = 720.2 (1)Å3; C2/ m; R = 4.%, and at 296 K, a = 8.600(1), b = 13.005(1), c = 7.193(1) Å, 0 = 116.03(1)°, V = 722.9(1)Å3; C2/m; R = 2.8 %, while those for the sanidine; K0.820Na0.157Ba0.015Fe0.006▢ 0.002Al1.020Si2.980O8, at 121 K, a = 8.514(1), b = 13.018(2), c = 7.183(1)Å, 0 = 116.03(1)°, V = 715.4(2)Å3; C2/m; R = 3.8 %, and at 296 K, a = 8.534(1), b = 13.010(1), c = 7.176 (1) Å, β = 115.99(1)°, V = 716.2(1) Å3; C2/m; R = 3.5 %. Thermal contraction in orthoclase and sanidine is extremely anisotropic, with the a-axis shortening much greater than the other axes. The framework structure of these monoclinic feldspars consists of two kinds of tetrahedral-type pyroanions: T12O7 parallel to c-axis and T22O7 to b-axis. Furthermore the feldspar framework accommodates the orderly array of the pair formed by two K-polyhedra (K2O16) sharing their oneedge with each other, closely parallel to the a-axis. The K2O16 pyroanions accommodating two K atoms in (010) were of a staggered type, and their deformation synchronous with shortening of K-OA2 bond distances accounts for extremely thermal anisotropy in the feldspar framework. A linear extrapolation of the experimental temperature factor B to zero at 0 K confirms that finite intercepts of B = 0.3 Å2 for K-cation in orthoclase and 0.5 Å2 in sanidine are the presence of zero-point motion. This demonstration provides the fundamental explanation for appearance of not thermal energy but static disorder effected by Al/Si disordering, or dynamic energy resulting from a collision between K and O atoms while collecting the reflection data under the Earth’s gravity.
Keywords
Orthoclase • sanidine • crystal structure • X-ray diffraction • low temperature • anisotropic contraction • pyroanion • deformation of K-polyhedron