『Abstract
　Naturally weathered feldspar surfaces in the Jurassic Navajo Sandstone at Black Mesa, Arizona, was characterized with high-resolution transmission and analytical electron microscope (HRTEM-AEM) and field emission gun scanning electron microscope (FEG-SEM). Here, we report the fist HRTEM observation of a 10-nm thick amorphous layer on naturally weathered K-feldspar in currently sightly alkaline groundwater. The amorphous layer is probably deficient in K and enriched in Si. In addition to the amorphous layer, the feldspar surfaces are also partially coated with tightly adhered kaolin platelets. Outside of the kaolin coatings, feldspar grains are covered with a continuous 3-5μm thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth and etch pits were also found on feldspar grains. These characteristics of the aged feldspar surfaces accentuate in the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems, and may partially contribute to the commonly observed apparent laboratory-field dissolution rate discrepancy. At Black Mesa, feldspars in the Navajo Sandstone are dissolving at 〜10⁵ times slower than laboratory rate at comparable temperature and pH under far from equilibrium condition. The tightly adhered kaolin platelets reduce the feldspar reactive surface area, and the authigenic K-feldspar overgrowth reduces the feldspar reactivity. However, the continuous smectite coating layer does not appear to constitute a diffusion barrier. The exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer (leached layer versus re-precipitated silica), which is uncertain at present. However, the nanometer thin layer can be detected only with HRTEM, and thus our study raises the possibility of its wide occurrence in geological systems. Rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface.』

1. Introduction
2. Geology, hydrology, and geochemistry settings
3. Sampling and experimental methods
　3.1. Sampling and size fractionation
　3.2. X-ray diffraction
　3.3. Electron microscopy and electron microscope analysis
4. Results and discussion
　4.1. Mineralogical compositions of Navajo Sandstone at Black Mesa
　4.2. Clay coatings
　4.3. Surface morphology of feldspar grains
　4.4. Possible implications of coatings on feldspar dissolution kinetics
　4.5. Amorphous surface layer
　　4.5.1. HRTEM observations
　　4.5.2. Comparison with previous studies
　　4.5.3. Postulation of the origin of amorphous layer at Black Mesa
　4.6. Paragenesis, surface features, and coating minerals
5. Conclusions and remarks
Acknowledgments
References

Fig. 1. Various hypotheses of the feldspar-water interfaces. 〔Zhu,C., Veblen,D.R., Blum,A.E. and Chipera,S.J.(2006): Naturally weathered feldspar surfaces in the Navajo Sandstone aquifer, Black Mesa, Arizona: Electron microscopic characterization. Geochimica et Cosmochimica Acta, 70, 4600-4616.から〕