The thin green rock veins in the Sierra

[CAMERONM]

Over the years I have noticed a thin green vein of rock, usually only about 1/8", in many places in the Sierra, usually at higher altitudes. Sometimes you see the vein over a few feet, but never as long as the long and wider white ones. Oftentimes the rock shears off to expose it as a green face, and it can be picked off fairly easily. It is distinctly different from all the other geological phenomena by how thin it is, by its color, how it seems to be less strong than the surrounding rock, and how it looks more "organic". What could it be? Some brief period where everything green died? I attach some photos.

[wildhiker]

Look like epidote veins to me. See: https://geology.com/minerals/epidote.shtml
-Phil

[bobby49]

I agree. That thin slab of green was what many of the old gold prospectors were looking for since they felt the green led to proper conditions for gold.

[giantbrookie]

Yes those are most certainly epidote veins and they are common throughout the Sierra. The photos show two different sorts of examples: commonly the rock will part along the vein leaving one surface covered with sparkling green/yellow-green epidote crystals. In other cases faulting took place along the vein horizon giving the surface a local polish and grooves.

[CAMERONM]

A few more. The first might show epidote in the split area, but many of the rocks in this area also had this flaky skin that sort of appears like caked mud.

The next two seem to me to be where a crack allows organic material to invade, and then it is exposed when the rock opens.

[giantbrookie]

The flaky eggshell-cracked skin reflects the weathering of the rock. Water that permeates into the rock reacts with some of the minerals in the rock form clay minerals that occupy a larger volume. This ends up cracking up the rock and the process works from the outside-in. The odd mud-crust-like cracking apparently results from the physical properties of the rock itself and the distribution of clay minerals within the rock. The same sort of process affects all rocks (rock being physically broken up by chemical weathering that forms clays that sort of pry the rock apart) but the nature of the host rock and distribution of the clay minerals will differ and break up the rock in a different way---many granitic rocks, for example, sort of disintegrate into sand instead of cracking and flaking like the rock you show.