the properties of the silicate ore mineral differ little from those of the silicate gangue. It is informative to contrast the terrestrial sources of two common metals with high oxidation potential: Al and Mg, the third and eighth most common elements in the crust, respectively. Both are essential constituents of many minerals, including some extremely common rock-forming silicates, and thus they are abundant in ordinary rock. Neither is recovered from ordinary rock, however. Aluminum ore is bauxite, a mixture of hydrated aluminum oxides that occurs as a soil in certain humid tropical areas. In such areas, abundant rainfall and warm temperatures cause extremely pervasive and deep surface weathering. Under such conditions, common rocks are ultimately rendered down to their least soluble constituent: aluminum. Other metals are removed as ions in solution, and even the silicate framework is eventually broken down, with the silica also leached away. Here again, processes on the Earth have concentrated Al naturally, and it turns out to be vastly more cost-effective to seek out those concentrated deposits rather than to mine ordinary rock. Bauxite is an iron-free form of laterite; if iron is also present in the original rocks, it becomes oxidized to the ferric state and remains behind as well, since it's also insoluble. (Such ferric iron accounts for the deep red color of most tropical soils.) To highlight a point of this paper, the reader should note that laterite, which contains both Fe and Al, is an economic source of neither. (Locally, however, aluminum-free laterites can be iron ores.) Although it is also abundant in silicates in ordinary rock, most magnesium is recovered from salt deposits or seawater. In either case, Mg in aqueous solution is concentrated and precipitated as the chloride, which is then electrolyzed. In this case, the ease of handling magnesium ion in solution has proved to be the critical economic factor. A little Mg is still made by reduction of the oxide with ferrosilicon, but the oxide is derived by calcination of dolomite, not from a silicate [e.g., Rosenqvist, 1983], Location The location of a potentially economic deposit is important, but typically less so than the constraints above. We can broadly divide economic deposits into "opportunistic" vs. "logistic" deposits, depending on the increasing importance of location. Like all such divisions, this one is not hard and fast but is nonetheless useful. Opportunistic Opportunistic deposits include those whose sheer size is so overwhelming that they are worth mining despite the extensive transport and infrastructure costs. This category also contains those commodities whose value is so great that they are worth seeking for themselves. One example is Au, as shown by the "gold rushes" from antiquity to present-day Nevada. The metal is sufficiently valuable that it's worth
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