Читать книгу Earth Materials - John O'Brien - Страница 127

Some polymorphs are characterized by structures that, while different, are similar enough that the conversion of one into the other requires only a rotation of the constituent atoms into slightly different arrangements without breaking any bonds. Transformations between polymorphs that do not require bonds to be broken and involve only small rotations of atoms into the new structural arrangement are called displacive transformations and tend to occur very rapidly under the conditions predicted by laboratory experiments and thermodynamic calculations. Polymorphs involved in displacive transformations rarely occur as metastable minerals far outside their normal stability ranges and so may preserve less information about the conditions under which they and the rocks in which they occur originally formed.

Alpha quartz (low quartz) is generally stable at lower temperatures than beta quartz (high quartz). Although α‐ and β‐quartz have different structures, the structures are so similar (Figure 4.37) that the conversion of one to the other is a displacive transformation. It is not at all unusual, especially in volcanic rocks, to see quartz crystals with the external crystal form of β‐quartz but the internal structure of α‐quartz. These quartz crystals are interpreted to have crystallized at the elevated temperatures at which β‐quartz is stable and to have been diplacively transformed into the α‐quartz structure as they cooled, while retaining their original external crystal forms.

Figure 4.37 The closely similar structures of α‐ and β‐quartz.

Source: Courtesy of Bill Hames.

Other transformations between silica polymorphs are reconstructive. For example, the transformations between the high‐pressure minerals stishovite and coesite and between coesite and quartz are reconstructive. Therefore, both stishovite and coesite can be expected to exist as metastable phases at much lower pressures than those under which they are formed. Their preservation in rocks at low pressures allows them to be used to infer high pressure conditions, such as meteorite impacts, long after such conditions have ceased to exist.