Mechanics of the feldspar framework; Crystal structure of Li-feldspar

The crystal structure of LiAlSi3O8 was determined by single crystal X-ray diffraction, with 813 observed structure factors, refined to R1 = 0.079. Li-feldspar has the smallest unit cell volume of any feldspar with known crystal structure, Comparison of LiAlSi3O8 with various alkali feldspars, including the most open structure of RbAlSi3O8, illustrates the noncollapsible nature of the feldspar-type aluminosilicate framework. Noncollapsible frameworks in which the T-O-T angles (where T is a tetrahedrally coordinated atom; here Si and/or Al) antirotate when a tetrahedron rotates upon compression or expansion of the framework are mechanically stable and can maintain open pores without propping by inserted cations. The T-O-T angles serve as hinges between the rigid SiO4 and AlO4 tetrahedra. In monoclinic alkali feldspars, the antirotation of the T-O(A2)-T and T-O(C)-T angles balances the framework, Oxygen atom O(A2) is located on a mirror and connects the aluminosilicate double crankshafts of TO4 tetrahedra into slabs parallel to (001), while atom O(C) is in a general position. In triclinic alkali feldspars, the pairs of angles around atoms O(Bo) and O(Bm), O(Co) and O(Cm), and finally O(Do) and O(Dm) (all in general positions) are in an equilibrium transmitted through oxygen atom O(A2). Despite its stability, the noncollapsible feldspar framework is extremely flexible. Individual T-O-T angles can vary in the feldspar framework by up to 27 degrees without having a strong effect on the unit cell volume and without any significant effect on the overall mean T-O-T angle, The cell constant a relates to the short interatomic M-O(A2) distance (where M is an alkali atom) and the T-O(A2)-T angle, The unit cell volume (V) depends primarily on a, while b and c change little with variation of V, Thus the T-O(A2)-T angle is the linchpin of the framework mechanics. A further consequence of the noncollapsibility of the framework is that hydrogen feldspar does not collapse upon itself, even though it lacks an alkali atom. (C) 1996 Academic Press, Inc.