The Polymorphism of Cordierite

During the course of a broader investigation on the cordierite‐spodumene join in the system Li 2 O–MgO–Al 2 O 3 –SiO 2 , a clearer picture of the relationships between the various reported polymorphs of cordierite has been developed. Three forms of cordierite have been recognized.*α‐Cordierite is the stable high‐temperature form commonly found in nature and in ceramic bodies. The other two forms are much less common and can be prepared only under special conditions. μ‐Cordierite, found by Rankin and Merwin, forms metastably by crystallization of glass in air below 900°C. and then reverts somewhat sluggishly to the α form when heated above this temperature. μ‐Cordierite has been found to be structurally analogous to β‐spodumene. Stable β‐cordierite forms hydrothermally below 830°C. from glass or from the two other crystalline forms. The equilibrium inversion temperature of β→α cordierite is given by Yoder as 830°C.