Abstract

Angular, oriented microtubes are abundant in oligoclase in medium-grained gneiss from southeastern Pennsylvania. The microtubes are isolated, elongate voids that extend through the thin section with rectangular to square cross sections ∼2 μm on a side.

The microtubes occur in planar sets of from 5 to 40 equal-sized individuals, spaced 2 to 5 μm apart; single occurrences have never been observed. The walls are either parallel or perpendicular to [010] of the host. Oligoclase also contains open intragranular cracks with zigzag morphology and matching, stair-step–shaped walls. We suggest that angular, oriented microtubes form by displacement and healing along zigzag intragranular cracks. This hypothesis explains unique properties of microtubes: regular spacing, equal size, rectangular cross section, crystallographically controlled orientation, and existence in sets. Angular, oriented microtubes are perhaps indicators of fracturing under conditions of low in situ confining pressure followed by release of shear stress along microcrack planes.

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