Veinlets of light green to white, fine acicular intergrowths of muscovite and chlorite occur in shear zones of chlorite-grade Pennsylvanian carbonaceous strata from the Portsmouth, R.I., area. Alleghanyan deformation produced a set of shear surfaces containing these intergrowths, oriented with {001} at a high angle to the bounding shear surface. Sander (1930) describes such structures as cross micas.

X-ray analysis indicates no interstratification. The intergrowths are well-crystallized muscovite (probably 2M1 and chlorite (trioctahedral and Mg:Fe ≃ 2:1). The phases are not readily distinguishable by light microscopy. However, heat treatment at 825°C destroys the chlorite structure and produces a brown, amorphous, expanded layer which accentuates the intergrowth relationship. Typically, discrete elongate euhedral fibers of muscovite and chlorite are intergrown parallel to [100] or [010]. Also, shorter and euhedral to subhedral fibers of chlorite are commonly observed as inclusions in muscovite.

The unusual elongation is believed due to crystal growth into an opening shear zone. Oriented seed crystals in the phyllite matrix initiated growth mainly along the a axis. Subsequent interference with neighboring crystals limited growth in the c and one other direction. Cross fibers developed through relatively unimpeded growth at the tips of crystals.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.