reduced with primitive technology by using carbon. As discussed, though, even Fe is not separated from ordinary rock but from ore (Other long-known metals, such as Cu, are easier to reduce but rare. Due to their chemistry, however, they become concentrated into sulfide deposits - yet another example of the value of natural concentration processes in a planet!). In addition, however, the costs of crushing the ore to separate the ore mineral in the first place is always a major part of the cost! Many people, including the present author, have commented that the pervasive comminution of the lunar regolith by meteorite bombardment is an advantage for mining operations in that, at least initially, no such rock crushing will be required. However, this "natural milling" is compromised by (a) the poor size sorting in the regolith, and (b) the pervasiveness of impact-generated glass. The latter is more important, because it complicates beneficiation: small, heterogeneous grains become cemented together by glass, so that a clean separation becomes more difficult. The smaller and rarer the grains, the more such glass cementation will be a problem; much native Fe, for example, is intimately associated with silicate glass [Morris, 1980]. It's appropriate here to digress on the structure and composition of common rocks. As already mentioned, most rocks consist largely of silicate minerals, and silicates are difficult to deal with. In part this is due to the high energy of the Si-O bond, and in part to the high oxidation potential of the metals commonly found in silicates. High oxidation potential is only part of the problem, however. Silicates are in general "giant molecule" structures based on a tetrahedron of four oxygen atoms surrounding a single Si atom. These tetrahedra can share vertices to make complex ring (beryl, tourmaline), chain (pyroxenes, amphiboles), and sheet structures (micas, clay minerals), and even 3-dimensional networks (silica minerals, feldspars, feldspathoids, zeolites). To preserve charge balance, metal ions fit into interstices in such silicate frameworks. (In the 3-D frameworks, replacement of Si atoms by Al gives the structure a net negative change, which is balanced by included metal ions. A fully polymerized 3-D structure with no Al substitution is just silica, SiO2). A further complication is that the identity of the interstitial metal ions is not critical; if nearly the same size, many can substitute free for each other. Ferrous iron (Fe2) and Mg2 commonly substitute for one another in geologic materials, for example. Such substitution introduces a great deal of compositional variety into silicate minerals. Thus, extracting elements from silicates is hampered not just by the high energy of the metal-oxygen bonds but by the polymerization of the silicate backbone; for sheer kinetic reasons, the molecules resist being broken up. Polymerization persists even in molten silicates; the viscosity of lavas is largely a consequence of their silica content, which determines their degree of polymerization: the more silica, the more viscous. It is hardly surprising, then, that raw silicates are seldom economic sources of elements on Earth, even though they are the reservoir of most material in the planet's crust. Only a few rare metals (e.g., Li, Be, Zr) are routinely extracted from silicates. Furthermore, beneficiation of such silicate ores is difficult because
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