Pseudomorph
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In geology, a pseudomorph is a mineral compound resulting from a substitution process in which the appearance and dimensions remain constant, but the mineral which makes up the chief component of the compound is replaced by another.
Three kinds of pseudomorph exist:
A paramorph (also called allomorph) is a mineral changed on the molecular level only. It has the same chemical composition, but with a different structure. The mineral looks identical to the original unaltered form. This occurs, as an example, in the aragonite to calcite change.
An infiltration pseudomorph is a pseudomorph in which one mineral is replaced by another. The appearance of the mineral remains unchanged.
An incrustation pseudomorph results from a process by which a mineral is coated by another and the encased mineral dissolves. The encasing mineral remains intact, hence hollow.