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Minerals that take the crystal form of another, pre‐existing mineral are called pseudomorphs and are said to be pseudomorphic after the earlier mineral (Figure 4.39). Pseudomorphs can be produced in many ways. All require that the original crystal possessed a significant number of crystal faces (was euhedral or subhedral) at the time it formed. Some pseudomorphs are produced by replacement in which the atoms in a pre‐existing mineral are replaced by the atoms of a new mineral that retains the external crystal form of the original crystal. A common example is the replacement of pyrite (FeS₂) crystals by goethite (FeOOH) to produce goethite pseudomorphs after pyrite. Another common example is quartz (SiO₂) pseudomorphs after fluorite (CaF₂). Some pseudomorphs are casts produced by dissolution of the old mineral followed by precipitation of the pseudomorph to fill the cavity left behind. Other pseudomorphs are produced by the loss of a constituent from the original crystals. For example, the dissolution of carbonate ion from crystals of the copper carbonate mineral azurite [Cu₃(CO₃)₂(OH)₂] can produce native copper (Cu) pseudomorphs after azurite. Still other pseudomorphs are produced when the new mineral forms a thin layer or crust over the original crystal. The encrustation of the original mineral by the new mineral allows the new mineral to mimic the crystal form of the original mineral. Still other pseudomorphs form by inversion as when β quartz crystals are transformed into α quartz, as described in the preceding section.

Figure 4.39 (a) Hematite replacing pyrite; (b) chalcedony encrusting aragonite; (c) quartz cast filling an aragonite solution cavity.

Source: Photo courtesy of Stan Celestian, Maricopa Community College.

The properties of minerals and other crystalline materials are strongly influenced by their crystal structures and chemical compositions. These properties and the minerals that possess them are the subjects of Chapter 5.