Crystal-fiber gypsum vein, Rincon Mountains, Tucson, Arizona

Missing from this specimen is the host rock itself. What remains is just the gypsum vein filling. The lateral edges of this vein mark the contacts with the host rock. The line (surface) down the center of the gypsum vein can be thought of as the line (surface) of original fracturing of the host rock. The gypsum fibers progressively filled the fracture as the walls of the fracture opened. The direction of fiber growth is oblique to the fracture surface, revealing that there was both opening and lateral slip.

I collected this from a clay quarry located along the southern margin of the Rincon Mountains.  I like it because it is such a clear ‘inside’ view of crystal fiber veins and how they grow.  Gypsum is a lovely, satiny material, made even better when it grows like fibers in response to structural deformation.

Age & Formation
Host rock is Miocene fanglomerate of the Pantano Formation, ~26 Ma.