What phenomenon describes the substitution of one chemical element for another in a mineral's crystal structure?

The correct answer, isomorphism, refers to a phenomenon where one chemical element can be substituted for another within a mineral's crystal structure without significantly altering the mineral's overall structure. This substitution occurs when the elements involved have similar sizes and charge, allowing them to fit into the crystalline lattice in place of one another.

For example, in the case of certain minerals, magnesium can substitute for iron in the crystal structure, resulting in variations of the same mineral (like olivine). This versatility in elemental composition contributes to the diversity of mineral species while maintaining their structural integrity.

Other terms in the context are distinct and encompass different processes. Polymerization refers to the process where small molecules join together to form larger structures, which is not the same as substitution within a crystal lattice. Cation exchange describes a specific type of ion exchange process that typically involves the replacement of cations in a solution interacting with charged sites in a mineral, rather than the stable substitution that characterizes isomorphism. Saturation generally relates to the concentration of a mineral in a solution or the point at which no more of a substance can be dissolved, which does not directly pertain to the topic of structural substitution.