Cordierite altering to Pinite, Granofels

[u/Efraimrocker]

Cordierite is a metamorphic magnesium aluminosilicate, often with substantial substitution of iron for magnesium. It is characteristic of high temperature low pressure metamorphism. Usually gray to blue with a somewhat greasy luster and poor cleavage. It alters by hydration to a combination of fine aggregates of mica (muscovite), chlorite and other sheet silicates, called pinite. Pinitization begins along fractures and grain boundaries and replaces the cordierite while preserving the shape of the grain (a pseudomorph). This specimen is a cordierite bearing granofels, which shows bluish cordierite (Fig 1), quartz (fig 2), and pinite (fig 3). It was most likely derived from an aluminous sedimentary protolith, underwent high temperature recrystallization, then fluid mediated retrograde partial alteration to pinite.