bill, polymorphous (in the chemical sense) is not quite right, but perhars may trigger some memories.
Edit: pleomorphism may be synonymous.

Edit: The forms are called allotropes and the transformation is called a reconstructive transformation. For tin, the equilibrium termperature for the transformation is (at normal atmosphreric pressure) 13.2 degrees C.
http://www.msm.cam.ac.uk/Teaching/pastpapers/1998-IA.pdf, at bottom of page 5