The surface structure and composition of layered silicate minerals: novel insights from X-ray photoelectron diffraction, K-emission spectroscopy and cognate techniques

Abstract

Angle-resolved X-ray photoelectron spectra of freshly exposed cleavage planes of muscovite, lepidolite, phlogopite, and both natural and Pb-exchanged vermiculite are interpreted to yield both quantitative elemental analyses relating to the outermost 100 Å of the crystals and a wealth of structural information, the latter from consideration of phenomena resulting from diffraction of the photo-emitted electrons. It is demonstrated that X-ray photoelectron diffraction can be used to differentiate between equivalent, near-equivalent, and non-equivalent sites occupied by two (or more) elements either in one single crystal or in crystals of closely similar structure, even when the element(s) concerned comprise only a small fraction of the crystal and when the sub-lattice lacks both long- and short-range order. These studies are complemented by extensive chemical analyses, by energy-dispersive X-ray (K-emission) analyses performed in an electron microscope (an elementary calibration procedure for which is outlined) and by X-ray diffraction studies. The muscovite and lepidolite are shown to cleave in regions exhibiting typical bulk composition, whereas the phlogopite and vermiculite both cleave in regions rich in aluminium, and deficient in magnesium (relative to their silicon content). The principal interlayer cations in the vermiculite, potassium and calcium, could be entirely replaced with lead by prolonged refluxing in lead nitrate solution: the lead and calcium are shown to retain their hydration spheres whereas potassium coordinates directly (without hydration) to the layer oxygen, as in the true micas. Some structural implications of these data are evaluated and discussed.