『Abstract
　Black rock-coatings occur in proximity to smelters and roast yards of the Greater Sudbury area, Ontario, Canada and contain information about the past interactions between surface minerals, and gaseous and particulate atmospheric components, many of which were pollutants. Rock-coatings were collected from various locations within the Sudbury area and are characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron microprobe analysis, infrared spectroscopy and X-ray photoelectron spectroscopy. Acidic fumigations and rain, the result of vast quantities of SO2 released from smelting, increased the chemical weathering rate of exposed rocks in the Sudbury area. Non-stoichiometric dissolution of the silicate minerals under acidic conditions resulted in the accumulation of silicic and the subsequent formation of a silica-gel type coating. The silica gel transformed overtime into amorphous silica, opal (opal C and opal-CT) and cristobalite. Dissolution of the underlying rock and also of metal-bearing particles by sulfuric acid resulted in the in situ formation of metal−sulfate-rich layers on the interfaces between the atmosphere and the silica-rich coating (atmosphere-coating interface, ACI) and between the silica-rich coating and the underlying rock (rock-coating interface, RCI). These metal−sulfate-rich layers contain nanometer aggregates of Fe-Cu-sulfate-hydroxide, goldichite,mereiterite, guildite, butlerite and antlerite. The silica-rich matrix also contains a mix of detrital grains from adjacent rocks and soils (feldspar, quartz, hematite, chlorite, montmorillonite) and non-dissolved smelter-derived nano- to micro-size particulates (metal−silicates, metal−oxides, C-spheres). The apparent disequilibrium between the embedded particles and the Fe−Cu-sulfates suggests that trapped nanoparticles were encapsulated into pores which prevented their equilibration with acidic metal−sulfate-bearing fluids. An XPS depth profile indicates a gradual transition from lower to higher concentrations of metals from the coating surface towards the metal−sulfate-rich layer on the ACI, which suggests that the outer surface on the coatings is currently leached on an angstrom scale by surface waters.』

1. Introduction
　1.1. History of the environmental pollution in the Greater Sudbury area
2. Experimental
　2.1. Sampling and sample preparation
　2.2. Scanning electron microscopy (SEM), electron microprobe analysis (EMPA) and X-ray diffraction (XRD)
　2.3. Focused ion beam (FIB) and transmission electron microscopy (TEM)
　2.4. X-ray photoelectron spectroscopy (XPS)
　2.5. Infrared spectroscopy (IR)
　2.6. Laser ablation inductively coupled mass-spectrometry (LA-ICP-MS)
3. Results
　3.1. Representative sample description : RY1, F1 and CC3
　3.2. Composition of the silica- and metal−sulfate-rich layers
　3.3. Depth-scan analyses of the metal−sulfate-rich layer on the ACI
　3.4. Silica-phases in the silica-rich matrix
　3.5. Embedded sulfates
　3.6. Embedded detrital particles
　3.7. Embedded spherical particles
4. Discussion
　4.1. Origin of amorphous silica and cristobalite
　4.2. Thickness of the silica layer with location and type of underlying rock
　4.3. Properties of silica gel
　4.4. Origin of the major elements in the metal−sulfate-rich layer
　4.5. Formation of metal−sulfate-rich layers in the black coatings
　4.6. Other models for the formation of sulfate-rich layers in silica coatings
　4.7. The surface of the metal−sulfate-rich layer on the ACI
　4.8. Occurrence and stability of the observed sulfate minerals
　　4.8.1. Fe³⁺4SO4(OH)10
　　4.8.2. Goldichite, mereiterite, butlerite, guildite and antlerite
　4.9. Role of sulfates in the formation of the coatings
　4.10. Origin of the embedded particles
　　4.10.1. Origin of chlorite and montmorillonite
　　4.10.2. Origin of hematite
　　4.10.3. Particulates emitted from smelters and ore-refinement processes
　4.11. Disequilibria between phases in the coatings
5. Summary and conclusions
Acknowledgments
Appendix A. Supplementary data
References