= Crystal polymorphism
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<Crystal polymorphism> is the existence of different <crystal structures> for the same chemical substance. Each <crystal polymorph> can have its own <mass density>, <melting> behavior and range of stability on a <phase diagram>. A different external shape of the same <crystal structure> alone does not establish a different <crystal polymorph>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/polymorph>[IUPAC Compendium of Chemical Terminology, fifth edition, entry “polymorph” (15225), definition and Note 2]{ref}
At fixed <temperature> and <pressure>, compare the <chemical potentials> of the possible <crystal polymorphs>. The lower value determines the stable bulk <thermodynamic phase>; equality permits <phase coexistence> when neither form is beaten by another <thermodynamic phase>. A higher-<chemical potential> form can persist through <metastability>, so finding it in a sample does not by itself locate an equilibrium boundary.
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