Mechanics of the Feldspar Framework; Crystal Structure of Li-Feldspar

• Published in: Journal of solid state chemistry Vol. 121; no. 1; pp. 12 - 23
• Main Authors: Baur, Werner H., Joswig, Werner, Müller, Gerd
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 05.01.1996; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Inorganic compounds; Physics; Salts; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Aluminium silicates; Inorganic compounds; Crystal lattices; Synthetic mineral; Mechanics; XRD; Experimental study; Lithium Aluminosilicates; Lithium silicates; Quaternary compounds; Crystal structure
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1006/jssc.1996.0003

The crystal structure of LiAlSi3O8was determined by single crystal X-ray diffraction, with 813 observed structure factors, refined toR1 = 0.079. Li-feldspar has the smallest unit cell volume of any feldspar with known crystal structure. Comparison of LiAlSi3O8with various alkali feldspars, including the most open structure of RbAlSi3O8, illustrates the noncollapsible nature of the feldspar-type aluminosilicate framework. Noncollapsible frameworks in which theT–O–Tangles (whereTis 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. TheT–O–Tangles serve as hinges between the rigid SiO4and AlO4tetrahedra. In monoclinic alkali feldspars, the antirotation of theT–O(A2)–TandT–O(C)–Tangles balances the framework. Oxygen atom O(A2) is located on a mirror and connects the aluminosilicate double crankshafts of TO4tetrahedra 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. IndividualT–O–Tangles can vary in the feldspar framework by up to 27° without having a strong effect on the unit cell volume and without any significant effect on the overall meanT–O–Tangle. The cell constantarelates to the short interatomicM–O(A2) distance (whereMis an alkali atom) and theT–O(A2)–Tangle. The unit cell volume (V) depends primarily ona, whilebandcchange little with variation ofV. Thus theT–O(A2)–Tangle 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.