Geopuzzle #3

I’ve been out in the field for the past couple of days (the South African weather having picked up just in time to completely fry me), and my travels have yielded me a deskcrop that I’m very proud of:


What do you think it is? Just to show that I’m not completely heartless, here’s a picture of the outcrop from which it was obtained:


This is potentially pretty difficult, so I may post another clue over the weekend. Go to it in the comments.

Categories: fieldwork, geology

Comments (6)

  1. Mathias says:

    Something magmatic and possibly related to ore comes to mind pretty quickly especially in South Africa. What comes to mind is Chromite (i once saw magnetite that looked like on the foto though), perhaps from an outcrop in the Bushveld. Those brighter spots in the specimen and outcrop could be serpentinite, weathered olivine, like you see it in layered intrusions. What bothers me is that I never saw chromite so massive and smooth, like it has been polished. Then again our universities collection isn’t exactly what I would call well equiped. The amount of what I think is serpentinite increases towards the top of the dark layer and continues right into the brighter layer ontop. I don’t think these are seperate layers looks more like a form of differentiation maybe. The bottom layer underneath seems different. The surface seems rough but smoother then further above.
    I also thought of coal before I saw the outcrop image. But…nahh!

  2. Garry Hayes says:

    I would second Mathias’ comments. I immediately thought magnetite, but the outcrop resembles layered dunite, so chromite is reasonable as well, although I have never seen a chunk of chromite this size.

  3. BrianR says:

    It does seem pretty heavy…a substantial rock indeed! Peridotite with a nice black crust on it? That’s a wild stab in the dark.

  4. Ken Clark says:

    How about Vivianite? It’s a nice iron ore with a crystal structure like that, pretty rare though.

  5. Ron Schott says:

    Yup. I’d go with chromite from the Bushveld intrusion, as well. Wasn’t the first thing that popped into my mind on seeing the upper photo, but the lower photo removes any doubt. Beautiful photo for teaching about LMIs.

  6. Andrew says:

    I concur with Mathias, with the addition of the magic word “cumulate” in front of the chromite.