Truth In High-Pressure Minerals? Crossing the Footsteps of Bernard W. Evans

Microscopic petrography and microanalysis are an attempt, beyond the intrinsic beauty of natural objects and experimental artefacts, to gain an insight into the relationships among phases, their evolution with time, and the relative weight of governing processes like crystal nucleation and growth, volume or grain boundary diffusion, dissolution, mass transport, etc. Phase-equilibrium data and thermodynamics offer a convenient and rigorous reference framework (or boundary case) to discuss the role and variations of intensive parameters in shaping rocks. Both approaches have their own beauty and may convey or disclose part of the truth. In some instances, however, they appear to lead to conflicting evidence. With a strong personal bias toward blueschist- and eclogite-facies rocks, I will present a few illustrations of such petrological experience in Mg-rich and Fe-rich systems. Several of them let us cross the track of B.W. Evans, who contributes as well to each of the petrographic, analytical, experimental and thermodynamic aspects of our present knowledge. The stability of glaucophane, deerite, magnesio-chloritoid, the occurrence and preservation of metamorphic diamond or aragonite, in the light of experimental and thermodynamic evidence, will be part of the landscape during this petrological quest with Bernard.