Common and Rare in the Earths Crust

Humans have at all times made use of the components of the earth s crust. As a consequence of the discovery that even rare materials may have technically interesting properties, there has been a drive to widen the exploitation of natural resources. New separation methods have made the extraction of desirable elements possible. Such rare elements are often obtained as by-products of the mining of common ores. Gold and platinum, for instance, are present in the anode sludge from electrolytic copper refining. The mineral molybdenite, a sulfide, is roasted at 800°C to the oxide from which molybdenum metal and its alloys are produced. During the roasting process the oxide of a rare element, rhenium, volatilizes and its compounds can be extracted from the gas phase as a by-product. Several examples of this type are treated in the different element chapters of this book. 

Rare elements, such as silver, with a low mean content in the earth s crust are well known if two criteria are fulfilled  

Mercury Less abundant than rare earth metals 

On the other hand we realize that there is a substantial difference between the mean content of an element in the earth s crust and its accessibility. Two reasons may be mentioned for common dements appearing to be rare  

Geochemistry takes up these issues and, to some extent, provides answers. This is why geochemistry is included in this book about the elements on earth and their discovery. 

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