The Spinel-Silica Reaction Succession: A Study of Incompatible Mineral Phases

Spinel and quartz are an example of an incompatible mineral pair on which few petrographic observations have been made. At Cortlandt, New York, emery bodies are cut by quartz veins, and the following reaction products were formed as a result of the interaction between spinel and silica: sapphirine, garnet, sillimanite, and cordierite. Their stability relations are expressed in an appropriate diagram. Where quartz and spinel occur in the same thin sections, they are invariably separated from each other by reaction rims bordering the quartz. The reaction between spinel and increasing amounts of silica yielded sapphirine, garnet plus sillimanite, and cordierite, in that order. In addition to the spinel, the sapphirine and probably the garnet (pyrope-almandite) are incompatible with quartz.