- © The Mineralogical Society Of America
Micas have been long known to play a crucial role in most petrologic and petrogenetic processes, both in magmatic and metamorphic environments. Understanding the crystal-chemical and thermoelastic behavior of this mineral family with variations in pressure (P), temperature (T) and composition (X) is crucial to provide a reliable basis for further interpretation and prediction of phase equilibria, transformations and most reactions occurring in a variety of rocks. In addition, the development of geothermometers and geobarometers has extensively made use of compositional (Fe/Mg+Fe ratio, K/Na+K ratio, Tschermak substitution, etc.) and structural (cation ordering, polytypic occurrence, etc.) properties of micas, or of their occurrence in assemblages at equilibrium with other phases. A few studies by in situ high-pressure, high-temperature and high-pressure–high-temperature experiments are available (see e.g., Chapter 8 in Reviews in Mineralogy and Geochemistry, Volume 41). The objectives of these studies are to determine structural and thermoelastic properties of micas and to understand the microscopic mechanisms ruling their responses to thermobaric stress.
Following these objectives, we partition such investigations into two categories:
Those which do not lead to a breakdown or permanent change of the structure of the reactant phase or of its chemical composition.
High-pressure studies (single crystal, SC, and/or powder diffraction, PD) within the elastic boundaries of response are required for the determination of P-V equations of state, of compressibilities of bond lengths, of polyhedral volumes and similar parameters for any other structural building units. These studies lead to the understanding of compression mechanisms and to the estimate of the PV-contribution to the Gibbs energy for correct interpretation of stability fields.
High-temperature studies (SC, PD) are aimed at determining bulk/polyhedral/bond-length thermal expansions, or at obtaining insight into cation rearrangements triggered by temperature. Results from these studies produce information on the structural response to heating, on the bulk/tensorial thermal-expansion …