Quartz-fiber pressure shadow on fractured pebble, northwestern Arizona

&lt;p&gt;The purple object is part of a quartize pebble. &amp;nbsp;The pebble was fractured and pulled apart in a tectonic strain environment that ‘demanded’ profound stretching. &amp;nbsp;This occurred at significant depth under temperature and pressure conditions of low-grade regional metamorphism. &amp;nbsp;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. &amp;nbsp;The fibers grew in the sheltered shadow &amp;nbsp;region of the hard pebble fragments. &amp;nbsp;The source of the quartz and sericite was pressure dissolution of the rock where hit by highest stress concentrations. &amp;nbsp;The overall feature is a pressure shadow. &amp;nbsp;Normally pressure shadows are tiny, and seen only through magnification, typically using a microscope. &amp;nbsp;&lt;/p&gt;

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.

Age & Formation

Mazatzal Quartzite, Mesoproterozoic in age, ~1.5 Ga.