The Mineral Cordierite: Structure and Reactions in the Presence of Fluid Phases

Abstract

The complex framework silicate cordierite with a structure similar to that of beryl forms Al/Si‐ordered orthorhombic as well as disordered hexagonal polymorphs and varieties as a function of temperature and composition. Even in the stability range of the ordered cordierite initial crystallization leads to the disordered form that becomes gradually ordered via a modulated structure. A miscibility gap as a two‐phase loop exists between orthorhombic Mg‐cordierite of the composition Mg2[Al4Si5O18] and the new hexagonal, Bc‐bearing cordierite solid solutions formed by the substitutions Be[4]+Si for 2 Al[4]. (Na,K)‐bearing cordierites with the alkalies present in the structural channels are due to the substitution Alk+ + Al[4] for Si[4]. These metastable phases are hexagonal, probably because of the deviating Al/Si ratios that are not conducive to ordering. Neutral gas molecules and atoms like H2O, CO2, N2, and Ar can occupy the structural channels, H2O changing its dipole orientation depending on the presence or absence of Na in these channels. The pressure temperature stabilities of cordierite end members are mainly a function of the nature of their octahedrally coordinated atoms with PT‐ranges decreasing in the order Mg > Fe > Mn > Co; but there is also a stabilizing effect by gas channel occupants. Disordered cordierites form metastably at generally higher temperatures and lower pressures than their stable, ordered counterparts.