Relationship between order-disorder and elastic phase transitions in framework minerals

Abstract

Two processes are important in the high-temperature behaviour of many framework silicates: (1) order/disorder of tetrahedral cations in the framework, (2) elastic instabilities of the framework itself. These processes frequently couple via a common strain. The nature and effect of such coupling is considered for albite, cordierite and leucite. There is a stark contrast in the dependence of macroscopic spontaneous strain in each of these minerals: while Q and Q_od are strongly coupled in albite, the high-temperature behaviour of Q in leucite seems independent of Q_od. Leucite analogues, such as K₂MgSi₅O₁₂, show strong strain—order/disorder interactions, however, and illustrate the importance of the size of the ordering cations on the interaction between Q and Q_od. Results for the kinetic controlled ordering of K-bearing cordierite emphasize the role chemical inhomogeneity has in enhancing the development of tweed microstructures.

Key Words:

• Ferroelasticity
• order-disorder
• kinetics
• feldspar
• cordierite
• leucite