literature; common elements are certainly significantly more concentrated in some places, and even deposits of rare elements cannot be excluded on the basis of present knowledge [e.g., Gillett, 1983, 1990; Haskin, 1983; Binder, 1988], In this paper, I will first review what makes a deposit mineable on the Earth; then I will apply these constraints to the Moon and discuss their implications. Fundamental Constraints An "ore" is any natural material from which a substance, usually a metal, can be won at a profit. It thus is an economic term only; mode of formation, mineralogy or chemistry are irrelevant. Three factors enter into whether a geologic deposit is an ore: (1) Concentration and amount (or "grade" and "tonnage", as the mining industry terms them). How much of the desirable element is there, and how much other stuff is it dispersed in? In a general way, of course, this double constraint is obvious; but it seems to be not so well recognized how vital the degree of concentration is. (2) Contrast of properties. This constraint is often overlooked by non-geologists: how easily (i.e., cheaply) the desired element(s) are separated depends strongly on their chemical and physical state in the prospective ore material. Partly, but only partly, this is a question of the energy needed to break the chemical bonds that must be broken. In fact, physical processes ("mineral dressing") are generally used first to concentrate ("beneficiate") the ore mineral(s) before any chemical separation is carried out, because physical processes are much cheaper. Thus, ease of separation also strongly depends on any contrast in properties between the ore mineral(s) and non-ore minerals ("gangue") in the prospective ore, the grain sizes of the ore mineral(s), and the chemical state of the ore minerals themselves. Last, the very toughness of the rock - how difficult it is to crush - is a major cost determinant. (3) Location. This constraint is again obvious, but paradoxically, it is somewhat less important than commonly thought. To be sure, ceteris paribus, the more convenient to markets and transportation infrastructure, the more valuable a potential ore deposit is, and certainly the overwhelming cost of transport to LEO is the major motivation for the use of extraterrestrial resources. Nonetheless, this cost is a two-edged sword, because it makes the capital and maintenance costs for resource extraction also extremely high. Each of these general constraints, and the trade-offs between them, is discussed in detail below. Grade and Tonnage Almost anything, if large enough, convenient enough, and pure enough, can be a valuable resource. Limestone, for example, is an abundant rock type on the Earth's surface. It is also in demand by the megaton for concrete manufacture: limestone is largely calcite, CaCO, and the carbonate is pyrolyzed in a kiln to lime:
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