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1241 Views 5 Replies Last post: May 6, 2014 1:11 PM by Darrow RSS
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Apr 29, 2014 5:39 PM

Help identifying a rock please

Hello there.  This is my first post and I hope that someone can shed some light on something for me.


A few weeks ago I found an unusual looking rock, kind of out of place in a field.  It caught my eye and I picked it up and took it home.  I wondered if it was a meteorite which, whilst pretty rare in this country didnt seem like a wholly ridiculous thought.  I started to look on line and whislt there is lot sof information and so called guidance as to how to identify a meteorite it seems that pretty much every tick in a box also has a caveat attached so I am really no further forward.  I broke a little piece off to take a look inside.  I am hoping that someone with a more experience or even an expert eye might be able to either immediately identify it as a plain old rock or be unsure enough to suggest I get it checked properly.


This is the description:


It is far darker than the pictures suggest - more of a dark slate colour on the outside, which is smooth.  The inside is darker still and shiny (almost as if someone has painted glue over it and the light is reflecting. 

The inside is not solid - it is hard but it can be rubbed with a hard object and it will break into smaller parts - even 'dust'

As a rock it is not noticeably magnetic, howevert when flakes are broken off these are attracted to a magnet and looking through a magnifying glass you can see what look like little flecks of metal.

It weighs 336g and displaces around 125ml of water (this is very rough)

It will leave a dark greyish streak when rubbed on the underside of a mug



Any help and feedback would be much appreciated


Thank you in advance



    • Currently Being Moderated
      May 6, 2014 11:57 AM (in response to Darrow)
      Re: Help identifying a rock please



      I do not see any surface features indicative of meteorite (eg. no regmaglypts or fusion crust).


      The internal structure does not look remarkable; it does not show any of the special textures that some meteorites do.


      Thanks for determining the weight and volume, implying a denisty of ~2.7g/ml. Rocks of that density include granite and many sandstones. It is not dense enough for a mafic igneous rock, such as dolerite or basalt; nor a stony meteorite (and certainly not a metallic meteorite). Here is a handy table of densities.


      Thanks for noting the streak. If some grains from the rock are magnetic but the rock as a whole is not particularly so, then I think the specimen is composed of multiple minerals. As such its streak is probably not very useful (more useful with single mineral determination).


      I don't think it is man-made. It does not look like slag, and I can't imagine a reason for any reconstituted stone including magnetic components..


      In principle I might suggest your specimen is piece of magnetite sandstone. ...I imagine it comprised of only a small amount of magnetite, such that as a whole it is not noticeably magnetic, but selected grains rubbed off would be. But that is not a common rock type, at least not in the British Isles. You don't say where you found it; if it was elsewhere, then maybe that is a bit more of a possibility. (Here's a paper on sedimentary magnetite deposits in Canada.)

      Update 6may14: see my next posting, where I correct my misreading of the original description, the magnetic particles being paramagnetic, and hence not necessarily pointing to something like magnetite.


      There's only a limited amout of determination one can do from description and photographs.

      To get a better ID, the specimen would need to be thin-sectioned and inspected under a petrological microscope. That would enable a first-pass determination of the minerals and textures. The magnetic mineral(s) might need to be inspected using an ore microscope, equipped with an indentation hardness tester. That should get pretty close to a definitive answer, though other tests could be used if there was still ambiguity.


      In summary: I don't think it is a meteorite; I don't think it is man-made; we have insufficient data to determine what it is.



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        • Currently Being Moderated
          May 6, 2014 11:55 AM (in response to Darrow)
          Re: Help identifying a rock please




          The local bedrock geology is Palaeogene London Clay Formation and Bagshot Formation, comprising clay, silt and sandstone (~34 to 56 million years old)


          I must apologise for an oversight in my previous posting:

          You mentioned the particules were attracted to a magnet (they are paramagnetic), whereas I had assumed that they were magnetically attractive in themselves. There is a wider range of paramagnetic minerals. You may have just a piece of ironstone (iron-rich sandstone). But in that case, the question remains: what is the paramagnetic mineral? Pyrite is known to occur disseminated in some of the London Clay strata (link above), but pyrite is only very weakly paramagnetic; perhaps it is pyrrhotite.




          Further work:

          I suggest you make use of the Royal Holloway's outreach initiative; they state:

          "The mission and unifying theme of our Outreach programs is to improve public and student understanding of our planet and Earth sciences"

          (contact at the foot of that page)

          Hopefully, that may get you an 'in', and enable you to pose your question/specimen to one of the staff in the Earth Science dept., if you ask nicely. Then they might thin-section your specimen and give you an assessment.


          You have Egham Museum, which has a collection of artefacts, but I don't know they would be able to help (only if they recognized it as some sort of ore related to a past industry, perhaps)




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