Pseudomorphism

Pseudomorphism is the existence of a mineral that has the appearance of another mineral. Pseudomorph means false form. Pseudomorphism occurs when a mineral alters in such a way that its internal structure and chemical composition changed but its external form is preserved. These mechanism of pseudomorphisms can be explained as follows :-

• Substitution : In this mechanism chemical constituents are simultaneously removed and replaced by other chemical constituents during alteration. An example is the replacement of wood fibers by quartz to form petrified woof that has the outward appearance of the original wood, but is composed of quartz. Another example is the alteration of flourite which forms isometric crystals and is sometimes replaced by quartz during alteration. The resulting quartz crystal look isometric, and are said to be pseudomorphed after fluorite.
• Encrustation : If during the alteration process a thin crust of a new mineral forms on the surface of a preexisting mineral, then the preexisting mineral is removed, leaving the crust behind, we say that pseudomorphism has resulted from encrustation. In this case the thin crust of the new mineral will have casts of the form of the original mineral.
• Alteration : If only partial removal of the original mineral and only partial replacement by the new mineral has taken place, then it is possible to have a space once occupied entirely by the original mineral be partially composed of the new mineral.
  • In serpentine pseudomorphed after the olivine or pyroxene.
  • Anhydrite (CaSO₄) pseudomorphed after gypsum (CaSO₄.2H₂O)
  • Limonite after pyrite and Anglesite after galena.