Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 31

Gemstones can be found in all three major rock classes but there are a few specific rock types that are most important. Kimberlites are arguably the most important rock type for gemstones, as these rare rocks represent the final stages of magmatism that bring diamonds from deep in the Earth towards the surface. They originate deep in the Earth’s interior and have distinct chemistry. Generally speaking, they are high in carbon dioxide (CO₂), potassium (K) and magnesium (Mg) and low in silicon (Si). Granitic rocks that have undergone considerable chemical fractionation are also important, as they become gradually more enriched in rare elements, such as beryllium (Be) and boron (B), which are essential components for several gemstones, such as aquamarine. Similarly, pegmatites that are enriched in rare elements are also very important for producing many varieties of gemstones, such as tourmaline. Sedimentary rocks of importance include limestones and evaporites. Although gemstones are not generally found in these rocks they are important protoliths for metamorphic gemstone deposits (such as marble‐hosted corundum). These concepts and specific rock types are discussed in greater detail in their relevant chapters.

Figure 2.13 Idealized schematics of fluid flow through the Earth’s crust with divisions based on diffuse permeation of fluids without fractures and fluid flow focused through fractures. Common quartz veins are generally characterized by some diffusional exchange with the wall rocks. Oliver & Bons (2001) / with permission of John Wiley & Sons.

Figure 2.14 Schematics of three fault types: reverse, normal, and strike‐slip. Reverse faults arise through compressional tectonics while normal faults arise from extensional tectonics and strike‐slip faults from transverse tectonic activity. https://www.nps.gov/subjects/geology/geologic‐illustrations.htm / Public domain.

Figure 2.15 Schematics of three folding patterns of layered rocks. Here, the folds comprise an axial plane that separates one limb from another for the anticline structures of varying dips, or tilts. The syncline structures will also have axial planes but were left out for clarity in this diagram. Earle (2015) / CC BY 4.0.