properties. Thus, any characterization of such propertiesis an average, and a range of values and area where they are valid must both be determined. The recognition, much less the significance, of this heterogeneity seems profoundly underestimated in many extraterrestrial resource scenarios. Rock types, for example, represent a range of compositions and textures, and a rock name also commonly implies a mode of origin. "Basalt" is a typical example. The name refers to a volcanic rock, or a class of volcanic rocks, relatively poor in silica and relatively rich in iron and magnesium (e.g., Carmichael et al., 1974). Basalts nonetheless have a wide range of compositions, some of which themselves have specialized names (e.g., "tholeiite"). Composition even varies within a single flow, depending on how the lava cooled. The term "basalt" also says very little about the rock itself. The texture could be massive or vesicular, the rock could be broken up or intact; the nature of any fracturing is unspecified; and chemical alteration, by weathering or volcanic fluids, might be present. Lunar mare basalts are no exception. As can be seen, there is also great variation in the nature of heterogeneities in rocks, including: the size and distribution of mineral or sedimentary grains; the nature and distribution of material between the grains; the presence of any layering (and the nature of the variation by which such layering is defined); the nature and degree of any fracturing or veining; the presence of any chemical alteration by weathering, hydrothermal fluids, or sedimentary brines. Such alteration in particular is commonly sporadic, following subtle vagaries in rock permeability, or reflecting the proximity of altering fluid; and such alteration is also an extremely common source of economic mineralization. The values of a potential ore depends greatly on the nature of such heterogeneities with respect to the distribution of the desired element. Is the element dispersed throughout the rock? If so, what is its grain size? Alternatively, is the element concentrated in veins or fracture fillings? Is it then related to later chemical alteration? In any case, what is - or are - the mineral(s) the desired element is contained in? Such heterogeneities, at many levels and of many types, are also the reason that human beings are still so deeply "in the loop" in mineral extraction. Even the seemingly simple process of digging out ore-bearing rock requires real-time judgment calls based on extremely subtle pattern recognition that also must occur in real time. Breaking up rock and digging it out is expensive, so only the rock that absolutely must be dug can be dug, if the mine is to be economic. Such tasks are done well by human beings, but they are so far impossible (not merely difficult) for machines. Hence, the consequences for mineral extraction of natural heterogeneity are profound. Machinery and techniques must be flexible, and processes must be robust. One example of the necessary flexibility comes from a copper plant in the senior author’s experience: if the crushed ore sent to the flotation mill was too high-grade, the flotation vat would overflow. Hence high-grade material had to be diluted first.
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