I picked this up while walking my dog in the foothills of the Catalina Mountains in 2009. It (the rock, not my dog) was transported by stream action from bedrock in the Catalinas. I like this sample because it displays the essence of “banded gneiss” even in a small specimen. Most banded gneisses can only be appreciated in very large outcrops, or in canyon walls.
This banded gneiss displays black layers rich in biotite and hornblende, and light layers rich in quartz and feldspar. During metamorphism heat and pressure led to lit-par-lit (leaf by leaf) textures, created in part by differential melting.
Age & Formation
Banded gneiss from the Santa Catalina Mountains, Tucson, Arizona. Age of original rock, and age of metamorphism, are quite uncertain