In Chapters 7, 11, and 14, we provided systematic treatments of minerals according to their most common origin, such as igneous, sedimentary, and metamorphic. This classification omits a number of minerals that are of major economic importance, such as some native elements, sulfides, and a few silicates, as well as barite, fluorite, and some gem minerals.
The minerals in the prior three mineralogical chapters include a large number of silicates, with fewer carbonates, oxides, and sulfides. Many of these are of economic importance and are referred to as industrial minerals. An industrial mineral includes any rock, mineral, or other naturally occurring substance of economic value, exclusive of metallic ores, mineral fuels, and gemstones. In this chapter we include a variety of metallic ore minerals consisting of native elements and sulfides. The term ore mineral refers usually to a metallic mineral that is part of an ore and is economically desirable, in contrast to those minerals that are part of the gangue. Gangue is the valueless rock or mineral aggregate in an ore deposit that is not economically desirable but cannot be avoided in mining.
We also discuss several nonmetallic minerals that are of economic importance but could not be included in the mineral classification used prior. We close the chapter with examples of several important gem minerals.
The minerals discussed in this chapter are commonly found in either hydrothermalveins or pegmatites. Hydrothermal deposits result from precipitation of ore and gangue minerals in fractures, faults, breccia openings, and other spaces, by replacement or open-space filling, from water-rich fluids ranging in temperature from 50° to 700°C but generally below 400°C and ranging in pressure from 0.1 to 0.3 GPa; the fluids are of diverse origin. Pegmatites are exceptionally coarse-grained igneous rocks (Fig. 9.15), usually found as irregular dikes (Fig. 10.5(A)), lenses or veins, commonly at the edges of batholiths. Their most common overall composition is that of granite, with grains 1 cm or more in size. Pegmatites represent the last and most hydrous portion of a magma to crystallize and, therefore, may contain high concentrations of minerals that occur only in trace amounts in granites and are concentrated in the hydrous-rich residue.
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