largely underappreciated in many extraterrestrial resource scenarios. Although it has been stated that the extreme cost of access to space makes even ordinary materials in space "as valuable as gold," this inaccessibility is a two-edged sword: it also makes capital and maintenance costs extremely high - and such costs - for a mine are high even in the best of circumstances. In the authors’ opinion, many of the scenarios advanced for the development of extraterrestrial resources do not give proper regard to mining experience on Earth. Taking into account the experience of handling natural materials on the Earth, although it will not guarantee the success of extraterrestrial resource extraction, should at least increase its chances of success greatly. In this paper, we will first sketch out the vagaries of natural materials, which make them difficult to deal with, and then describe briefly the approaches that have been found to work in mine development on the Earth. The Complexity of Natural Materials Compositional complexity. Virtually all natural materials, including rocks and the debris derived from them, are compositionally complex. High-purity material is exceedingly unusual even after the early stages of concentrating or smelting an ore mineral. Rocks and rock debris are furthermore not just mechanical mixtures, but chemically complex as well. Rocks are made of minerals, which are chemically distinct phases that nonetheless typically vary - sometimes substantially - in composition. Most minerals are based on oxygen as the anion, with cations of metallic elements fitted in the interstices between the oxygen atoms. (Anions are much larger than cations, of course, due to the smaller effective electric charge of their nuclei.) A great many minerals are silicates, which are based on a tetrahedron of four O'2 anions tightly bound to a small Si+4 cation; in turn, these tetrahedra can occur in isolation, or share vertices to form complex ring, chain, and sheet structures, or even form three-dimensional networks. To maintain charge balance, other cations fit into interstices between the oxygens in the silicate framework. Other minerals are true oxides, in which the cations fit among close-packed oxygen atoms without clear association into radicals such as the SiO4 4 tetrahedron. Finally, although sulfide minerals are insignificant volumetrically in the crust of both the Earth and Moon, they are extremely significant economically, because their physical properties make them easily separable from silicates and oxides, and are as compositionally varied as silicates and oxides. Sulfides are usually non-polar compounds whereas silicates and oxides are polar compounds. Compositional variation in minerals comes about because the cation site is typically not sensitive to anything but the size and charge of the contained cation. Thus, if two cations have similar size and charge, they can substitute for each other. Mg++ and Fe++, for example, are nearly the same size and thus replace one another freely in most minerals, as in olivine, a silicate mineral whose composition is
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