Which of the following best describes isomorphous substitution in clay minerals?

Question

BlackTom AI · Accepted Answer

Question restatement: Which of the following best describes isomorphous substitution in clay minerals?

Option A: Substitution of one exchangeable ion by another
This describes a surface or exchangeable-ion exchange process, not isomorphous substitution within the mineral lattice. Isomorphous substitution occurs in the crystal framework itself, not merely swapping surface ions, so this option is incorrect.

Option B: Substitution of an exchangeable ion by another of lower valence
This option implies a change among exchangeable ions at the surface or interlayer, and it also emphasizes valence difference. Isomorphous substitution, however, is a lattice-substitution phenomenon where ions occupy lattice sites, and the key feature is size compatibility rather than simply a valence change at the surface. Therefore, this statement does not capture the core lattice-substitution concept and is incorrect.

Option C: Substitution of a cation by a cation of similar size
This description aligns with the classic definition of isomorphous substitution in clay minerals: a cation in the crystal lattice is replaced by another cation of comparable ionic radius, allowing the structure to remain intact while altering some properties (like charge balance and lattice energy). This option correctly identifies the essential criterion of size compatibility in isomorphous substitution and is correct.

Option D: Substitution of an ion by a like valence ion
While some substitutions can occur with similar valence, isomorphous substitution is not defined by valence alone; it is defined by the ability to substitute within the crystal lattice without distorting the structure, which typically relies on similar ionic size. Stating “like valence” is too restrictive and can be misleading, making this option incorrect.

In summary, option C best captures the defining characteristic of isomorphous substitution in clay minerals by emphasizing substitution within the lattice by a cation of similar size, whereas the other options describe processes related to surface exchange or misstate the key criterion (size) for lattice substitution.

Which of the following best describes isomorphous substitution in clay minerals?Single choice

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