Increased solubility of quartz following ferrous–ferric iron reactions
A knowledge of the solubility of quartz and silicates in natural waters is important to the understanding of many gelogical processes. Here we provide experimental data to support an earlier hypothesis1, that during the supergene enrichment of banded iron-formations, redox reactions involving iron result in a much more rapid dissolution of quartz than would be predicted from its known solubility in water. The process is relevant not only to iron ore genesis but also to the much wider field of chemical weathering— for example, to the genesis of laterites and soils from quartz-bearing rocks. In addition, the reactions may offer a mechanism by which quartz is pseudomorphed by iron oxides.
Deep-seated iron ores from banded-iron formation
Very large iron ore deposits have formed in many parts of the world, evidently by the supergene alteration of Precambrian banded-iron formation (BIF). Some of these deposits extend to great depths, ranging to 2,400 m at Krivoyrog1, beyond the likely reach of oxygenated water. We propose here, on the basis of studies in the Hamersley Ranges, Western Australia, a mechanism for deep-seated ore formation in which electronic conduction through the magnetite layers in BIF connects a cathodic region near the surface, where oxygen is reduced, to an anode at depth, where iron (II) from magnetite, carbonates and silicates is oxidized and precipitated as iron (III) hydroxyoxides. The electrical circuit is completed by ionic conduction through groundwaters. The model is based broadly on a mechanism suggested by Sato and Mooney2 to explain self-potentials associted with sulphide ore bodies. It is based more specifically on a model demonstrated by Thornber3 for the weathering of massive nickel–iron sulphide deposits at Kambalda, Western Australia, but with substantial differecnes in the physical situation and the reactions at depth.
MEASUREMENT AND PREVENTION OF CORROSION BY ELECTROCHEMICAL METHODS
THE ELECTROCHEMICAL THEORY OF AQUEOUS CORROSION IS BRIEFLY REVIEWED, AND THE APPLICATIONS AND LIMITATIONS OF LABORATORY ELECTROCHEMICAL TECHNIQUES ARE DESCRIBED. EXAMPLES OF ALLOY EVALUATION BY ANODIC POLARIZATION ARE DISCUSSED, PARTICULARLY IN ENVIRONMENTS SIMULATING INDUSTRIAL SERVICE CONDITIONS. APPLICATIONS AND LIMITATIONS OF ANODIC PROTECTION ARE OUTLINED TOGETHER WITH REFERENCES TO THE ENGINEERING OF ANODIC PROTECTION SYSTEMS. CATHODIC PROTECTION IS DISCUSSED WITH RESPECT TO THE APPLIED CURRENT AND ELECTROCHEMICAL POTENTIAL REQUIRED FOR ADEQUATE PROTECTION, CONTROLLED POTENTIAL CATHODIC PROTECTION, AND THE PERFORMANCE OF SACRIFICIAL ANODES. ELECTROCHEMICAL CORROSION RATE DETERMINATIONS ARE REVIEWED. THE POLARIZATION RESISTANCE (OR LINEAR POLARIZATION) METHOD IS DESCRIBED AS A USEFUL TECHNIQUE TO MONITOR CHANGES IN CORROSIVITY OF CERTAIN INDUSTRIAL PROCESS STREAMS. IN GENERAL, THE DISCUSSIONS ARE LIMITED TO THOSE ASPECTS OF THEORY AND PRACTICE WHICH MAY BE OF INTEREST TO THE PRACTICING ENGINEER CONCERNED WITH SERVICE CORROSION PROBLEMS AND MATERIALS SELECTION. /AUTHOR/