Reduction spot, Red Hills, near Tucson, Arizona

Description
This rock is faceted by fracture (joint) surfaces. The white circle is a ‘reduction spot’ exposed on one of the fracture surfaces. The full 3D geometry of the reduction spot is probably a near-perfect sphere, though tectonic deformation of this siltstone (red) may have created slight internal deformation, transforming the sphere to an ellipsoid. Reduction spots can occur by the billions in certain red-bed siltstone formations. ; They form at depth as groundwater moves through the porous, permeable siltstone. If there are fossil plant fragments, or tiny specks of sulfide, within the siltstone, a local chemical-reduction environment will form in contact with the ground water, thus reducing the red hematite. The red rock becomes bleached, in the same way that bleach in a washing machine can produce reduction blemishes on blue jeans. Reduction spotting moves outward from the reduction agent, and where the siltstone is perfectly homogeneous in porosity and permeability, the reduction front migrates outward in all directions and at a common velocity, thus creating the sphere of white.
Whereabouts

I collected this in the late 1980’s in the Red Hills in the Tucson Mountains, while engaged in a class mapping project with graduate students.  I like the mystery of such spots, and particularly enjoy having such an ‘undeformed’ reduction spot as a reference to how things may have looked before a deformation creates strongly ellipsoidal reduction spots.

Age & Formation
Recreation Red Beds Formation of Jurassic age, ~180 Ma.