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 w ith known crystal structure, Comparison of LiAlSi3O8 with various alka li feldspars, including the most open structure of RbAlSi3O8, illustra tes the noncollapsible nature of the feldspar-type aluminosilicate fra mework. Noncollapsible frameworks in which the T-O-T angles (where T i s 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 b y inserted cations. The T-O-T angles serve as hinges between the rigid SiO4 and AlO4 tetrahedra. In monoclinic alkali feldspars, the antirot ation of the T-O(A2)-T and T-O(C)-T angles balances the framework, Oxy gen atom O(A2) is located on a mirror and connects the aluminosilicate double crankshafts of TO4 tetrahedra into slabs parallel to (001), wh ile 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), an d finally O(Do) and O(Dm) (all in general positions) are in an equilib rium 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 wit hout having a strong effect on the unit cell volume and without any si gnificant 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 alkal i atom) and the T-O(A2)-T angle, The unit cell volume (V) depends prim arily 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 alkal i atom. (C) 1996 Academic Press, Inc.