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896 Views 14 Replies Last post: Feb 2, 2014 9:29 AM by MikeHardman RSS
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Jan 29, 2014 5:43 PM

Black rock identification

Hi,

 

This rock caught my eye as it was very diiferent in shape and color to all the other ones I had ever seen.

I have never see such a uniformely dark rock. It has a matte finish, almost as if it were covered with some kind of plastic.
There is a very small part that chipped off, giving way to a more glittery, mettalic aspect. Don't know if you will be able to see this on the third picture (grey fleck.

photo 3.JPG

photo 1.JPGphoto 2.JPG

photo 4.JPG

 

I am just very curious to know what kind of rock this is as it is simply a mistery to me.

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    Jan 29, 2014 7:45 PM (in response to TCharrin)
    Re: Black rock identification

    A piece of coal maybe? Where did you find it?

    A

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        Jan 29, 2014 9:39 PM (in response to TCharrin)
        Re: Black rock identification

        Very interesting!

        Is it the 'coal' they use to carve model trains out of?

        Could be called Cannal coal.

        If it is you can use it for jewelery but only carbochons, it won't carve into the more intricate shapes like crosses.

        Come's from the very edge of the coal seam?

        Straight edges will polish up very nicely.

        Fantastic find!

         

        Tabfish

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        Jan 29, 2014 9:41 PM (in response to TCharrin)
        Re: Black rock identification

        Impurities on coal may give the metallic flecks, and cause the magnetic effect. Other than that, or jet (lignite) i'm all out of ideas! Hopefully someone who knows more will help you out

        A

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      Jan 30, 2014 8:25 AM (in response to TCharrin)
      Re: Black rock identification

      OK, thanks.

       

      Assuming the specimen is homogenous (that it is composed of just one substance):

       

      - if it is an element:

        - boron (2.34g/ml) is too light, and not found as native boron

        - strontium (2.6g/ml) is correct for density but the wrong colour (yellowish 'silver')

        - aluminium (2.7g/ml) is too heavy, and the wrong colour

         - scandium (3.0g/ml) is too heavy

       

      - if it is a mineral, considering just the black ones:

        - cronusite (2.51g/ml) is coal black, but too light (and probably too soft (1.5)); streak black

         - melanovanadite (2.55g/ml) is black, and probably too soft? (2.5, finger nail); dark red-brown streak

         - bariandite (2.7g/ml) is black, but usually fibrous or bladed; streak black

         - Wilhelmramsayite (2.75g/ml) is black or a dark leaden grey, and weakly ferromagnetic, but very rare (only from one place in Russia, I think)

         - the more-common magnetic minerals, such as magnetite, are much denser

         - coal is much less dense (~1.5g/ml)

         - jet similarly (~1.32g/ml)

       

      - if it is a meteorite:

         - its density is too low to be a metallic one (need to explain the magnetic character)

       

      - if it is industrial slag:

         - its density is too low to be largely metallic (need to explain the magnetic character)

       

      So I think it is either non-homogenous rock or industrial slag, including iron and/or nickel (to explain the magnetic character). Your specimen does not look like slag, but it could have been abraded and polished somewhat, thus altering its appearance. Many black rocks, such as basalt, are too dense, and not sufficiently magnetic.

      Petrified wood perhaps? The density and appearance could be right; but the magnetic character is a problem. That might be explained if the specimen includes (fine-grained, black) pyrite, which is weakly paramagnetic. Pyrite itself is too dense (~4.9g/ml), so the specimen would have to contain a significant proportion of less dense minerals as well. Common silicate minerals are around 2.6g/ml (the same as the overall density), so it would have to be something else - and carbon compounds (coal, roughly speaking) would do.

      Related: coal balls might be the right density, but I don't think they would show any magnetic character.

       

      I suspect it fell out of your BBQ, having been amongst the 'coals'. Those can contain inadvertent components such as lumps of metal or rock, and I suppose slag. Curiously, when fires occur in coal mines, combustion metamorphism can occur that results in the creation of magnetism, though usually weak (example ref.)

       

      So after all that, a clear ID remains elusive (to me, at any rate; other folks, please chip-in!).

      We'd really need to thin-section the specimen and analyse it under a microscope - to see and ID its minerals and textures. That could show cellular structures, indicative of mineralized or permineralized plant material.

       

      Without that mineralogical data, if I really had to guess, I'd suggest pyritized coal.

       

      Mike

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          Jan 30, 2014 10:04 AM (in response to TCharrin)
          Re: Black rock identification

          Try making a cheeky visit to a local (all things relatve) university earth science dept, and asking if somebody can help. You might find a tech or student who'll pass comment and/or thin-section it and inspect it under a microscope. If you can't see a suitable-looking unsuspecting soul, ask to see the departmental secretary, crawl on knees and ask if there was anybody who could help you.

           

          Keep us posted.

           

          Mike

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              Jan 30, 2014 9:32 PM (in response to TCharrin)
              Re: Black rock identification

              Local Lapidary club or geological society could help you id your specimen.

               

              Tabfish

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              Feb 1, 2014 9:05 PM (in response to TCharrin)
              Re: Black rock identification

              Rub the specimen against a piece of chalk, if it leaves a brown streak - light or dark then it could be Jet or Cannal coal.

              Just a quick thought, pick up a rock of a similar size to your specimen and see if your black rock is lighter of heavier.

               

              Tabfish

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                  Feb 2, 2014 9:29 AM (in response to TCharrin)
                  Re: Black rock identification

                  The streak test needs to be done on an unglazed porcelain tile (or the back thereof).

                  If done on chalk, many minerals will leave no streak - because they are harder than the chalk.

                  If it was a porcelain tile that you used, your specimen has a scratch hardness greater than that of the tile (7).

                   

                  Scratch hardness is measured again the Mohs’ scale, for which various common materials can also be ascribed:

                    1 Talc               none

                    2 Gypsum         fingernail

                    3 Calcite           copper coin

                    4 Fluorite           iron nail

                    5 Apatite           glass

                    6 Orthoclase      penknife blade

                    7 Quartz            steel file, porcelain tile

                    8 Topaz             sandpaper

                    9 Corundum       none

                  10 Diamond         none

                  The scratch hardness of a specimen can be gauged by trying to scratch it with those common materials.

                  For instance, if a steel file scratches it but not a penknife blade, it has a scratch hardness of about 6-7, and therefore might be orthoclase or quartz or another mineral with similar scratch hardness.

                  This scale is approximate, for various reasons, eg. minerals can have varying hardness depending on their crystal structure and the direction in which they are scratched, pressure of scratching, condition of the mineral, particular hardness of the scraching implement (steels, especially, vary).

                  (I state 'scratch hardness' because there is another measure of hardness called 'indentation hardness', which is tricker to assess; need a suitably equipped ore microscope.)

                  More on mineral hardness - http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness

                  You might like to try assessing the scratch hardness of your specimen.

                   

                  Also, you could try putting a drop of acid on it (try vinegar to start with, as it is the most-readily available), and observing it (perhaps closely, using a magnifying glass). If it fizzes, it probably contains some carbonate.

                  Have a read of this - http://geology.about.com/od/mineral_ident/ig/acidtest/

                   

                  Mike

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