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Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, and South Africa in his first post-doc, he now works at the University of Edinburgh.
Anne Jefferson has a love of all things water-related and blends hydrology, geomorphology, geology, and climate change in her work. She has a Ph.D. from Oregon State University and is now an assistant professor at the University of North Carolina at Charlotte.
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Category: fieldwork • geology
Posted on: February 1, 2008 10:39 AM, by Chris Rowan
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.
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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!
Posted by: Mathias | February 1, 2008 12:02 PM
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.
Posted by: Garry Hayes | February 1, 2008 1:11 PM
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.
Posted by: BrianR | February 1, 2008 1:27 PM
How about Vivianite? It's a nice iron ore with a crystal structure like that, pretty rare though.
Posted by: Ken Clark | February 1, 2008 3:25 PM
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.
Posted by: Ron Schott | February 1, 2008 4:28 PM
I concur with Mathias, with the addition of the magic word "cumulate" in front of the chromite.
Posted by: Andrew | February 2, 2008 5:21 PM