(Mg, Fe)2, SiO4. Mn++, Ni++, Co++ and others can also substitute for these elements in olivine, at least to some degree. In a given mineral, the exact distribution of the elements it contains depends on the composition of the melt or solution from which it crystallized, the oxygen fugacity, the sulfur fugacity, the temperature, the pressure, and so on. From the above, it can be seen that separating elements from natural materials falls into two parts: first, separating the minerals in the rock from each other (physical separation), and second, separating the desired element(s) from the ore mineral (chemical separation). It is difficult to underestimate the difficulty of both physical and chemical separations. Physical separation, although comparatively cheap, is generally inefficient because different minerals cannot be broken apart exactly at their mutual grain boundaries. Minerals are commonly intimately intergrown in the rock simply by virtue of the way they crystallized, and crushing the rock invariably generates particles in which pieces of one mineral grain are attached to another. This has been encountered, for example, in crushing and grinding eucritic meteorite simulants of lunar materials, which resulted in low grade ilmenite concentrates. To some degree such separations can be improved by finer grinding. However, if the crushed grain size is too small, surface effects begin to dominate the physical properties of the particle so that simple physical separations may no longer work. For example, large grains of gold can be separated from silicate grains by extremely primitive means (e.g., panning), because of their much greater density. Such density separations do not work, however, with micron-sized gold. On the Moon, an example of such a problem is in separating native (metallic) iron from raw lunar regolith. As pointed out by many people, the properties of iron and the background silicates differ greatly, and it should be possibly by relatively simple means to separate the iron, even though it makes up only about 0.5% of the regolith. However, much of this iron is present as extremely fine grains dispersed in glass generated by meteorite impact and separating such grains from the glass will be extremely difficult. Chemical separations are nearly always more expensive than physical separations. First, they are typically more energy intensive, because they commonly involve heating or (in the case of electrolysis) electricity. Second, they also nearly always involve a physical separation step, too. Chemical separation is based on the differential partitioning of an element into two (or more) separate phases. Examples include the reduction of iron oxide by carbon, in which a solid (Fe metal) separates from a gas phase (CO), or copper smelting, in which a Cu-rich metal phase and a silicate phase (slag) form immiscible melts. Commonly the new phases created by the chemical extraction procedure must then be separated. Moreover, side reactions are ubiquitous in chemical processes. Commonly the original concentrate to be treated is itself impure, because it was made by a physical process that does not yield very clean separations. Additionally, the ore mineral itself may be compositionally variable; e.g., sphalerite, the major ore of zinc, has
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