allomerism

from WordNet (r) 3.0 (2006)

allomerism
    n 1: (chemistry) variability in chemical composition without
         variation in crystalline form

from The Collaborative International Dictionary of English v.0.48

Allomerism \Al*lom"er*ism\, n. [Gr. ? other + ? part.] (Chem.)
   Variability in chemical constitution without variation in
   crystalline form.
   [1913 Webster]