The eye of the petrographer, the mind of the petrologist

To bring meaningful numbers out of metamorphic rocks, one must first understand what the numbers represent. An idealized objective in metamorphic studies is to demonstrate that coexisting minerals approached equilibrium. In a few rocks this can be done reasonably, for example, in coarse-grained nodules from kimberlite that were rapidly quenched from mantle conditions. More typical, and more interesting, are rocks showing disequilibrium by mineral zoning and reaction textures at various scales. These provide a view, if correctly interpreted, into a sequence of events and P-T conditions. Features in metamorphic rocks in central Massachusetts, including schists bearing strongly zoned garnets produced by four different types of reactions, and a newly described occurrence of wollastonite marble provide opportunities to explore these ideas. -from Author