substitutional solid solution

…case they are known as substitutional elements), or, if they are appreciably smaller than the matrix atoms, they may take up places between regular sites (where they are called interstitial elements).

…through the process of atomic substitution. This process involves the random replacement of an atom in a mineral by a foreign atom of similar ionic radius and valence, without changing the atomic packing of the host mineral. Atoms of copper, zinc, and nickel, for example, can substitute for iron and…

Substitutional solid solution is the most common variety. For example, as described above, in the carbonate mineral rhodochrosite (MnCO₃), Fe²⁺ may substitute for Mn²⁺ in its atomic site in the structure.