Pelitic Schist

What is Pelitic Schist?

Simply, it’s a metamorphic rock that used to be sedimentary. The original fine-grained sedimentary rock, likely mudstone or shale, has been heated and deformed (metamorphosed). Like a caterpillar turning into a butterfly, the new rock looks much different from the original with ‘upgraded’ minerals and texture.

This rock sample is part of a study of the Norumbega Fault System in Maine. Yvette Kuiper, a structural geology professor at the Colorado School of Mines, has spent multiple years completing research in the area. One of her previous students, Emilie Gentry, finished her master’s thesis characterizing the extent of the fault system. The Pelitic Schist design is one of many thin sections Emilie used for her research.

Under the Microscope

In cross-polarized light, the strong lines of the colorful micas, both biotite and muscovite, draw your attention across the design. Yellow to grey staurolite grains (porphyroclasts) are wrapped by the micas and quartz to create sigma clasts. Sigma clasts are named after the greek letter sigma (σ) that has a ‘tail,’ similar to the mica tails seen in the design. These textures highlight the metamorphic fabrics used to interpret the deformation of the rock.