which is an essential raw material for Portland cement. Despite its ubiquity, though, not just any limestone deposit will do; the purity of the deposit is extremely important. Lime to be used in cement manufacture must contain little magnesia (MgO) [e.g., Boynton, 1966, p. 98], but the double carbonate dolomite (Mg3Ca(CO3)) is a ubiquitous impurity in limestone, and indeed commonly a rock-forming mineral in its own right. Other common impurities such as phosphates, fluorite, and minerals containing heavy metals can also severely affect the quality of the cement [e.g., Bye, 1983, p. 16], Economic separation of such impurities is impossible, particularly at the scale that would be required. We thus arrive at the somewhat paradoxical result that the purity of a deposit of a substance that is both common and required in high volume is especially important! This is an important point that will be stressed again and again. Another example is elemental Si, for manufacture of IC chips. Although Si is the second most common element in Earth's crust, and silicates are dominant components of most rock types, Si is not recovered from just any rock; relatively rare deposits of nearly pure silica (SiO2, most commonly occurring as the mineral quartz) are exploited instead. The salient point is that separation of a desired element or compound is expensive. Not only are capital costs high, but maintenance costs are extremely high. Rocks are hard, and handling them in volume is rough on machinery, particularly in the enormous volumes necessary for resource extraction. It is highly cost-effective to let Mother Nature do as much of the separation as possible, and hence it is highly cost-effective to invest a great deal in seeking high-concentration deposits first. Searching for mineable deposits is called "exploration" for a reason! Obviously, the necessary concentration of a desired element for an economic deposit is a function of the element's value. As examples we may take iron versus gold. Traditionally, iron ores are essentially pure oxides, either magnetite (Fe2O2) or hematite (Fe2O2). These are reduced with C (charcoal or coke) at high temperature to yield Fe metal and CO, a process whose basic form goes back to antiquity. The weight percentage Fe in such ores is about 70%. Since World War II, lower-grade rocks containing iron silicates - "taconite" - have been exploited by the development of new beneficiation procedures that first separate the Fe oxides from the silicates [e.g., Park & MacDiarmid, 1970, p. 405]. Still, an iron ore has —30% Fe and an enormous volume. By contrast, gold ores can contain only a few parts per million Au [e.g., Percival et al., 1988]; the high value of the gold, of course, makes such low concentrations economic. Other metals occurring with the gold may - or may not - be economically recoverable as by-products. In the Fortitude deposit near Battle Mountain, Nevada, for example, Cu is not economically recoverable although it is much more abundant than gold in the deposit [Wotruba et al., 1988]. It is commonly pointed out that, because of the high costs of transport to space,
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