However, Ti is difficult to separate from its oxide, especially under anhydrous conditions, both because it is refractory and because of the high strength of the Ti-O bond [e.g., du Fresne, 1983], Therefore, like aluminum, Ti will probably also not be extracted initially. Ilmenite is also the main Ti ore mineral on Earth; it is concentrated in certain placer deposits. Terrestrial smelting processes rely on abundant water, however, and thus are irrelevant for lunar conditions. As noted, Ti is also a possible by-product from electrolysis. Silicon As in Earth's crust, Si is the second most common element, after O, in the lunar crust. Unlike Earth, though, large deposits of essentially pure silica (SiO2) are unknown; the element nearly always occurs in more complex silicates. Thus, it is most likely that Si will be recovered as a by-product from electrolytic, pyrolytic, or chemical extraction of other elements, in one of the processes described above. Discovery of deposits of nearly pure silica could change this assessment, however. Crystals of cristobalite (one of the SiO2 polymorphs) up to 0.5 mm across occur in many mare basalts (D. Vaniman, personal comm., 1990), but separating such crystals economically is a challenge. Rare elements The conventional assessment that the Moon can be a source only of common elements is most probably premature. Although the Moon does not have the bewildering variety of geologic processes, many involving liquid water, that the Earth does, it nonetheless is a much more complicated body than sometimes recognized. The protracted igneous activity the Moon underwent during its early history may well have led to local concentrations of rare elements; a number of magmatic processes can form ores even under anhydrous conditions [Gillett, 1990]. Seeking such deposits is an additional rationale for further lunar exploration. It also should be noted that sources of potential reagents such as halogens or carbonates could make practical other extraction processes for metals [cf. Steurer, 1982, p. 88 ff]. Conclusions Terrestrial mining experience has much to offer in guiding the search for economic sources of materials off Earth. Fundamental lessons from this experience include: (1) the recoverable grade of a potential ore deposit is its most important economic characteristic, and (2) it is seldom cost-effective to commit to a mine without a great deal of exploration and metallurgical testing first. More generally, the value of a planet as a source of resources stems from the large-scale chemical fractionation caused by geologic processes. In effect, in local areas nature has carried out a great deal of the necessary separation already, rhe
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