Porphyroblasts

Porphyroblast:  I’ve always thought that was such a neat word, maybe even interesting enough for a story.   Say it out loud three times (“pore-fur-o-blast, pore-fur-o-blast, pore-fur-o-blast”).  Now don’t you want to learn more, perhaps even own a few?

Porphyroblasts are those large recrystallized minerals that grow in the groundmass of a metamorphic rock, most typically in schists and gneisses.    In New York State, we immediately think of the bright red almandine-pyrope garnets in the gneissic rocks in the Gore Mountain area, but the truth is the metamorphic schists and gneisses throughout New York and New England often contain garnet porphyroblasts.    Unfortunately a lot of New York’s garnets are hosted in high-grade metamorphic gneiss and they don’t display crystal faces when the rocks are broken.  Nevertheless they are large, colorful and fun to collect.

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