Green/Black rock...What is it?

[georgiagirl23]

I have a rock that is green and black. The black part has an oily feel and sheen that make it look purplish. Any ideas what it is?

 

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[Jim in Idaho]

May be some sort of ultramafic. I would look the area over carefully. That type of rock is often associated with gold, platinum, silver, etc.

Jim

 

[DDancer]

*hrm* its possible Jim.
What I see and with whats described I'd put it down as a bit of weathered serpentine. The weathered bits are the greens. Some serpentine's as they weather morph into other similar mineral groups. The oily feel is common for that mineral group as are the colors. My thoughts. Nice colors though

 

[Jim in Idaho]

Thanks, DD. One question...do the serpentines weather into that purplish color?
Jim

 

[DDancer]

I've seen green and orange weathering~ well the orange is closer to yellow and blue's *but I've always got to wonder when I see blue...*. Usually its just green. I'm not sure what to attribute the purple hue to with black and dark serpentines. In my thought is most likely an indicator for high manganese content though that mineral is generally black however I've seen specimens that have a dark red to purple toning.

 

[woof!]

If it's heavy for its size, it's definitely ultramafic. If it readily deflects an ordinary compass, the black part includes a lot of magnetite. The green and the oily feel suggest greenstone metamorphic. Serpentine usually has a "slickensides" surface and the structure is usually somewhat fibrous, it's pretty distinctive stuff if you're familiar with it. However there are lots of other related greenstone minerals and the rocks that contain them can intergrade with serpentine and other minerals. ......If it's not metamorphic or maybe even if it is, the green could be from olivine. I recently collected ultramafic volcanics with olivine and also a purple surface sheen fairly similar to what you're describing. I don't know the mineralogy of the purple surface sheen, although I'm pretty sure it's a product of weathering processes.

If it's metamorphic, its geological setting is probably favorable for valuable minerals. In California, the Sierra Nevada greenstone belts adjacent to granite are associated with hydrothermal gold. In the high pressure low temperature Coast Range greenstone belts, very little gold but there is mining of mercury, asbestos, and chromium. And some nice rockhounding (esp. garnet and actinolite).

 

[Tnmountains]

Great responses ! Welcome to the forum GeorgiaGirl.

 

[MrLee]

My immediate thought was also serpentine, but I may be prejudiced since I live in Cali. It looks like some of my plasma agate from Clear Creek. Wonderful material and prized for using as knife butts. It's always difficult to identify something from a photo, but the commenters definitely help you narrow down the possibilities.

 

[georgiagirl23]

Ultramafic?

May be some sort of ultramafic. I would look the area over carefully. That type of rock is often associated with gold, platinum, silver, etc.
View attachment 1109290
Jim

Thanks Jim!
I am leaning towards some type of peridotite. I have found some peridotite in the immediate area, will post pictures later.

 

[rrhobdy]

Hi Georgia Girl... Thanks for posting pictures so that I could magnify them. The rock appears to be metamorphic, a silica (quartz grain) sandstone with imbedded iron pyrite (heavily eroded, thus the yellow stains), what appears to be green crystals is really green stained silica sand. The black is probably a manganese oxide. Thanks to little bits of rock crumbling onto the fur. If you turn rock on edge, I bet it will show layering. The green coloring probably comes from trace copper.

Bob