An interstitial atom is a point defect of a mineral, produced by an extra atom that is placed in its crystalline network. This causes a strong distortion in the proximity of the atom, but is attenuated with the distance. You can force the phenomenon: by irradiating the crystal to give it energy, we get an atom "right" at its network point to jump between the atoms that do stay in their network position.
In ceramic materials, which are composed of an ionic and a covalent part, it is normal to jump the cation, smaller than the anion: the set formed by interstitial atom and hollow is known as Frenkel defect.
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