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ssss1 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.
Order–disorder transformations
38high sanidinelow microcline
ssss1 Variations in the order of minerals.
Source: Klein and Hurlbut (1985). © John Wiley & Sons.
4.9.2 Pseudomorphs
replacement222castsloss of a constituent3322encrustationinversion
ssss1 (a) Hematite replacing pyrite; (b) chalcedony encrusting aragonite; (c) quartz cast filling an aragonite solution cavity.