What is isomorphism in mineralogy ?

Isomorphism : definition

Isomorphism is the property of certain minerals (then called isomorphic) to have the same crystalline structure despite different chemical compositions. These minerals are also said to be isostructural : they have X-ray diffraction patterns showing similarities.

Many minerals are isomorphic : aragonite and cerussite, rutile and stishovite, monazite and crocoite, etc...