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Thread: What is this rock?

  1. #1 What is this rock? 
    Forum Freshman jsloan's Avatar
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    I've got a roundish rock, about 4cm diameter, that consists of a light grey dolomite core surrounded on the outside by a darker, red-black layer about 3mm thick. The outer layer looks rusty, but is not magnetic nor does it have any fine loose particles which rub off. The rock fractures into irregular blocks when hit with a hammer. Both the dolomite and the outer layer are about H=6 on the Mohs scale.

    I've done the acid test. The dolomite reacts with vinegar when powdered, but there is no reaction by the outer layer.

    Thanks


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  3. #2  
    Reptile Dysfunction drowsy turtle's Avatar
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    If the grey layer had a hardness of 6, then it is not a dolomite. Dolomite has a hardness between 3.5 and 4 on the Moh's hardness scale.

    Are they distinct layers, or is there a gradation between the two? Is the rock crystalline, granular or smooth in texture? Where did the rock come from?

    My initial guess would be that the rock is largely a single mineral, with haematite staining in the outer layer. The reaction with acid may suggest carbonates in the inner layer; the outer layer's failure to react with acid may indicate a change in chemical composition of the environment in which the rock formed (haematite-based matrix rather than carbonates?).


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  4. #3  
    Forum Freshman jsloan's Avatar
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    Sorry, I gave the wrong hardness; it is actually around 3.5, on both the lighter inside and darker outside layers. The texture is microcrystalline. I picked it up while looking among the rocks in a gravel pit in central Alberta, Canada.

    Is it possible the outside minerals include hematite, even if shavings from that surface aren't attracted to a magnet? One thing I'm wondering, and haven't figured out, is what other mineral(s) than hematite might make up the outside layer.

    There is some gradation and color change toward the outer areas of the dolomite; but beyond that the outside coating/layer is about 2-3 mm thick and looks more like a crust than part of the rock it surrounds. I've mananged to put some pictures of the rock online, if that helps:

    Inside, showing freshly fractured face:

    http://www.thelensflare.com/image-fu...rock_61488.jpg

    Outside of the rock:

    http://www.thelensflare.com/image-fu...rock_61490.jpg
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  5. #4  
    Forum Cosmic Wizard SkinWalker's Avatar
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    looks like a bit of limestone that butted against a layer of clay with hematite in it to me. Hematite can color clay/shale without presenting an obvious magnetic property.

    It's always difficult to say via photos, however.
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  6. #5  
    Reptile Dysfunction drowsy turtle's Avatar
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    I'm not so sure about the outer layer being clay; I would have said it's some sort of mineral precipitate rather than a clastic sediment. Possibly gypsum?
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  7. #6  
    Forum Freshman jsloan's Avatar
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    I did some reading on hematite, and some more testing of the sample, and that's what I now believe the outer mineral is. I hadn't realized that hematite is not always attracted to a magnet. So, I think this rock is a hematite concretion.

    Thanks for all your help!
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  8. #7  
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    Quote Originally Posted by jsloan
    I did some reading on hematite, and some more testing of the sample, and that's what I now believe the outer mineral is. I hadn't realized that hematite is not always attracted to a magnet. So, I think this rock is a hematite concretion.

    Thanks for all your help!
    This is definitely a good explanation. Texturally it's an obvious weathering rind, and if it's a chunk of dolomite, liberated Fe ions have simply been oxidized locally.
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