Mineralogy in two dimensions; scanning tunneling microscopy of semiconducting minerals with implications for geochemical reactivity

Abstract

The atomic structure and nanometer-scale morphology of the (001) surfaces of hematite and galena, exposed by fracturing in air or under oil (to prevent exposure to air) at room temperature, have been studied with scanning tunneling microscopy (sru), low-energy electron diffraction (ueo), and field-emission scanning electron microscopy (reseu). The (001) surface unit cell of hematite is hexagonal, and the surface diffraction pattern is consistent with the atomic arrangement and cell size of the equivalent plane in the bulk. However, sru has shown that the surface is not flat, but instead consists of undulations, pits, and ridges. The diameters of the undulations range up to several hundred angstroms at their base, with heights between 5 and 100 A. In places, the surface appears to be atomically flat; however, atomic-resolution images with the sru could not be obtained. The (001) surface unit cell of galena is square, and, as with hematite, the surface diffraction pattern is consistent with the atomic arrangement and cell size of the equivalent plane in the bulk. Direct srvr imaging of one-half of the surface atoms (probably S) confirms this result. Lower-resolution sru images again show that the surface is not flat, but consists of irregular undulations (up to 50 A in treignt;, ridges, and cleavage steps 20 to 50 A in height. We suggest that the undulations on both the hematite and galena surfaces are mostly the result of uneven fracture propagation due to less than perfect crystallographic control of breakage. The atomic structure and nanometer-scale morphology of mineral surfaces play a key role in mineral solubility and sorption reactions at the aqueous solution-mineral interface. srvr provides a unique means of studying both the structure and the finest-scale morphology of mineral surfaces in vacuum, in air, and under aqueous solution, and it can be used to directly observe and categorize energetically favorable surface sites that are involved in these processes. Electronic and vibrational tunneling spectroscopy, as an extension of sru imaging, is being developed and may be available in the future for identifying individual atoms and molecules on mineral surfaces.