Structures of quartz (10(1)over-bar-0)- and (10(1)over-bar-1)-water interfaces determined by X-ray reflectivity and atomic force microscopy of natural growth surfaces

The structures of prismatic (10 (1) over bar0) and pyramidal (10 (1) over bar1) growth faces of natural quartz crystals, and their modification upon annealing at T less than or equal to 400degreesC were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of similar to 0.1 to 1 mum-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (10 (1) over bar0) surface. Vertical relaxation of atomic positions (< 0.4 Angstrom for terminal oxygens and < 0.2 Angstrom for silicon and oxygen atoms fully coordinated to structural tetrahedra) is limited to a depth of 14 Angstrom. Electron density profiles for all measured interfaces are consistent with a single layer of adsorbed water, with no evidence for additional organization of water molecules into distinct layers extending into the bulk solution. Similar interfacial structures were observed for natural and annealed surfaces of identical crystallographic orientation, indicating that extensive reconstruction of the silica network at the quartz surface did not occur under the annealing conditions. Copyright (C) 2002 Elsevier Science Ltd.