Medium-range order in disordered K-feldspars by multinuclear NMR

Abstract

The structures of K-rich feldspar, (K>Na)AlSi3O8, are currently described as >ideal> crystals with periodic average structures from Bragg diffraction maxima obtained by reciprocal-space techniques. Polymorphism is explained by variable substitutional disorder of framework Si and Al cations in tetrahedral T sites, and positional disorder of cavity alkali cations in a single M site. Here, high-resolution magic angle spinning multinuclear magnetic resonance spectroscopy, leading to 29Si, 27Al, and 23Na spectra at 9.4 as well as 27Al, 39K, and 23Na spectra at 19.6 T, has been used to investigate the >real> structures along the order-disorder series of K-feldspar crystals. The >ideal> and >real> structure coincides only in the perfectly long-range ordered triclinic end-member of the low-microcline structure. Long-range disordered structures (either with monoclinic or triclinic symmetry by X-ray diffraction) show non-random disorder at the medium-range scale, triclinic-like distortions with four sets of T sites for framework atoms, two sets of M sites for alkali atoms, and Al-O-(K,Na) multi-site correlations by NMR spectroscopy. The K-feldspar structures can be described by a medium-range structure using the number of Al atoms per four-membered rings of tetrahedra, with >...-2-0-2-0-...> chains for microcline and orthoclase where the Al-occupancies t1O > t2m > t2O ≈ t1m, and with >...-1-1-1-1-...> chains for valencianite and sanidine, in which t1O > t2m ≈ t2O ≈ t1m. Framework cations respect Loewenstein's rule (Al-O-Al avoidance), as well as some additional constraints of charge dispersion involving deficiency of Si atoms in Q4 (4Si,0Al), (1Si,3Al), and (0Si,4Al) environments, constraints which are particularly strong in valencianite. These >real> structure features cannot be described by >ideal> structures owing to the lack of resolving power of the reciprocal-space techniques.