Surface Chemistry of Feldspars

Abstract

Feldspars and other silicate minerals must have structural perturbations at the external surface that involve hydroxyl, water and various molecular and ionic complexes. Internal surfaces around voids and at twin boundaries and intergrowths must also have chemical properties that differ from the ideal bulk structure. Quantum chemistry, lattice dynamics, and electrostatics offer theoretical guidance for interpretation of experimental data. Ideally, the chemical type and structural position of each surface species would be measured without any degradation from the analytical probe. Some techniques can be used only in a vacuum, while others cause significant structural and chemical changes from heating, electrical charging, and momentum transfer. Each technique has restrictions on the type of sample, which may require special preparation. The scanning electron microscope/microprobe gives morphological information at sub-micrometer resolution from the low-energy secondary electrons, and chemical information at the micrometer scale from the high-energy scattered electrons and x-rays. Transmission electron microscopy gives structural and some chemical information for thinned specimens down to the nanometer scale. Conventional X-ray photoelectron and Auger electron spectroscopies yield quantitative chemical information from X-rays and photoelectrons, but have limited spatial resolution (horizontal, 0.n μm; vertical, 0.0n μm). Ion bombardment provides micrometer horizontal and subnanometer depth resolution of isotopes from mass spectrometer analysis of secondary ions and neutrals, but quantitative understanding of the destructive sputtering and knock-on processes is incomplete. Rutherford back-scattering and nuclear reaction analysis are useful, especially for light elements. Other techniques, involving infrared absorption, nuclear magnetic resonance and electron spin resonance have specialized applications. Rapidly growing are scanning techniques using a pointed probe which rides over surface species as mechanical, electrical and magnetic forces are measured. New techniques that use a brilliant, tunable and polarized X-ray beam from a synchrotron storage ring at near-surface incidence (absorption spectroscopy, fluorescence analysis, and diffraction) should probe the chemical and physical relations between surface species, with single-layer depth profiling as the aim. Information on particular elements, especially at coherent internal surfaces, is attainable from resonant X-ray scattering. The sparse information on feldspar surfaces is supplemented by observations on zeolites, clays and other oxygen-rich materials. Atomic-scale drawings of hydrated/hydroxylated feldspar surfaces are given to focus discussions of chemical and physical phenomena. Dynamic changes of surface chemistry as a feldspar is subjected to wetting/drying and heating/cooling cycles, to organic acids, and to metal-halide and other complexes are considered in preparation for the plenary lecture on weathering. Surfaces might become more stable as aluminol groups are replaced by silanol, as expected from experience with industrial zeolites. Absorption complexes on feldspar surfaces might have been involved in biological evolution. Feldspars from the Moon, other planets and meteorites should have unusual surfaces in response to high vacuum, intense radiation and particle bombardment. Potential chemical concentrations at twin boundaries and other internal surfaces are discussed. Ideas are given for systematic experimental and theoretical study of surface properties of feldspars, with emphasis on choice of natural and synthetic specimens which optimize the advantages of the techniques, and minimize the weaknesses.