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Thread: What is it?

  1. #1 What is it? 
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    I have in my possesion a rather dense almost black rock, it was found many years ago in a wood. it is of matt appearance and has a surface texture similar to pencil lead/graphite. It is approx 60mm diameter and has one or two dimples. Measuring across it's diameter yields an electrical resistance of approximately 7.2K ohms. I have ruled out graphite as it's density is approximately 4.85 when compared to water(1.0) It has been suggested to me it may be a meteorite but there is no sign of a fusion crust[which I know are fragile and lost easily], a second possibility is some form of 'slag iron' but I have no samples to compare it with. Upon close inspection it appears very uniform with occasional 'gold' coloured crystals, which I believe are iron pyrites. It's only recently I re-discovered it, any other possibilities?


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  3. #2  
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    The high density suggests to me that its not a typical bit of rock, but is rich in dense minerals. It's probably not a sulphide coz they have typically have lower resistivities at that density (although my first reaction by your description was that it might have been Pyrrohite, a quick check reveals a resistivity about 3 to 7 orders of magnitude too low).

    I don't know, you got a picture?


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  4. #3  
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    unfortunately not, hence the detailed description, it really does look like a huge lump of dark pencil lead, but graphite's density is only about 2.2
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  5. #4  
    Forum Freshman nexus6's Avatar
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    How about Lignite

    Jet:
    Jet is a semi-precious stone, a hard black variety of lignite that takes a brilliant polish. Lignite is otherwise known as brown coal. Chemistry 40-60% Carbon
    Paul C

    Tyrell: The light that burns twice as bright burns half as long - and you have burned so very, very brightly.
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  6. #5  
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    THis is way more dense than lignite. - but thanx
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  7. #6  
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    What is its streak? Does it pass or fail a scratch test? Are there fisible grains or crystals? How big are the grains? Metallic or non-metallic?

    If it has a dark red streak it could be hematite.

    http://www.mindat.org/min-1856.html
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  8. #7  
    Forum Sophomore DarcgreY's Avatar
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    It could also be magnitite.
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  9. #8  
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    Quote Originally Posted by DarcgreY
    It could also be magnitite.
    Now that's is a good one, Density of Magnetite = 5.05 from Google, very similar to mine. Many different shades and colors etc mostly dark, so that's good, resistivity, can't find data.

    Against: - My rock is totally non-magnetic, even using a very high power magnet suspended on a long string - no deviation - none.

    ANy advance on Magnetite?
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  10. #9  
    Forum Sophomore DarcgreY's Avatar
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    That rules out magnetite then, not sure what else fits the description.

    If it's non-magnetic that should rule out ferrous minerals shouldn't it?

    I'll get my minerals book out and see if I can find a match.
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  11. #10  
    Forum Sophomore DarcgreY's Avatar
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    It looks like Hematite is a better fit after all as it's non-magnetic and has similiar specific gravity.
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  12. #11  
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    I have managed to take a couple of pics but how can I show them here?
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  13. #12  
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    goto www.imageshack.us/ upload it there and copy the pic link(usually last on page)

    I take it you can go from there?
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  14. #13  
    Forum Sophomore DarcgreY's Avatar
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    Here's a picture of two samples of hematite:



    You can post images by having them hosted somewhere and linking to the URL.

    This is the site I use:

    http://www.imageshack.us/

    edit-snipped
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  15. #14  
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    [/img]



    Thanks CC!

    The lower picture shows where we cut a lump off.

    Looks very close to the Left hand sample in your Picture!
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  16. #15  
    Forum Sophomore DarcgreY's Avatar
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    It's streak should tell you if it's Hematite, like UBigBobby says, it should be dark red.
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  17. #16  
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    heres the wiki on hematite

    http://en.wikipedia.org/wiki/Hematite
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  18. #17  
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    I have done a streak test on the back side of an unused wall tile - result very dark grey/black.

    Hmmmmmm
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  19. #18  
    Forum Professor captaincaveman's Avatar
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    just noticed this also, didn't know there were so many


    http://en.wikipedia.org/wiki/List_of_minerals



    hey billco, look in the koran. i heard all the answers are in there :wink:
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  20. #19  
    Forum Sophomore DarcgreY's Avatar
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    It could be Ilmenite which has an iron-black to brown black streak.
    The specific gravity is a bit low at 4.5 to 4.7 though, it's weakly magnetic also.

    Ilmenite-FeTiO3
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