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Quartz-fiber pressure shadow on fractured pebble, northwestern Arizona


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Age & Formation: 
Mazatzal Quartzite, Mesoproterozoic in age, ~1.5 Ga.
Geohistory: 
The purple object is part of a quartize pebble. The pebble was fractured and pulled apart in a tectonic strain environment that ‘demanded’ profound stretching. This occurred at significant depth under temperature and pressure conditions of low-grade regional metamorphism. As the pebble fragments slowly pulled away from one another, the intervening area continuously ‘filled’ with quartz-sericite crystal fibers, forming ‘beards’ growing from the fractured faces of the pebble. The fibers grew in the sheltered shadow region of the hard pebble fragments. The source of the quartz and sericite was pressure dissolution of the rock where hit by highest stress concentrations. The overall feature is a pressure shadow. Normally pressure shadows are tiny, and seen only through magnification, typically using a microscope.
Whereabouts: 
I believe I collected this in Sue Beard’s thesis area in northwestern Arizona near the Cottonwood Cliffs. This was back in ~1983-84. This is one of my favorite rocks, …remarkable for being a pressure shadow you can hold in your hand, and use to discuss how pressure dissolution operates.