Читать книгу Earth Materials - John O'Brien - Страница 125

As noted earlier (Chapters 1 and 3), different minerals can have the same chemical composition, but different crystal structures. This ability for a specific chemical composition to occur in multiple minerals, each with a different crystal structure is called polymorphism. The resulting minerals are called polymorphs. In most cases, the crystal structure or form taken by the mineral is strongly influenced by the environment in which it forms. Polymorphs therefore record important information concerning the environments that produced them. Many polymorphs belong to very common and/or economically significant mineral groups, such as the examples summarized in Table 4.13.

The polymorphs of carbon can be used to illustrate how environmental conditions during growth determine which crystal structure a chemical compound possesses. Figure 4.36 is a phase stability diagram for systems composed of pure carbon. This phase stability diagram clearly indicates that diamond is the high pressure polymorph of carbon, whereas graphite is the low pressure polymorph. If we add geotherms, lines showing the average temperature of Earth at any depth, to this diagram, we can infer that diamonds are the stable polymorph of carbon at pressures of more than 3.5 GPa, corresponding to depths of more than 100 km below the surface whereas graphite is the stable polymorph of carbon at all shallower depths. Inferences must be tempered by the fact that Earth's interior is not pure carbon and temperature distributions with depth are not constant. Nonetheless, it is widely believed that most natural diamonds originate at high pressures far below the surface of old continental shields in which they most commonly occur. If graphite is the stable polymorph of carbon at low pressures, why do diamonds occur in deposits at Earth's surface where pressures are low? Obviously, as diamonds rise toward Earth's surface into regions of substantially lower pressure, something keeps the carbon atoms from rearranging into the graphite structure. What keeps the transformation from unstable diamond to stable graphite from occurring?

Table 4.13 Important rock‐forming mineral polymorphs.

Chemical composition	Common polymorphs
Calcium carbonate (CaCO3)	Calcite and aragonite
Carbon (C)	Diamond and graphite
Silica (SiO2)	α‐quartz, β‐quartz, tridymite, cristobalite, coesite, stishovite
Aluminum silicate (AlAlOSiO4)	Andalusite, kyanite, sillimanite
Potassium aluminum silicate (KAlSi3O8)	Orthoclase, microcline, sanidine
Iron sulfide (FeS2)	Pyrite, marcasite

Figure 4.36 Phase stability diagram showing the conditions under which graphite, the low pressure polymorph of carbon, and diamond, the high pressure polymorph of carbon, are stable beneath continental lithosphere.