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2162 Views 14 Replies Last post: Mar 15, 2013 9:22 PM by Ash RSS
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Mar 8, 2013 6:17 PM

Two fossils for id

Can anyone please tell me what fossils these are? They were both found on Hornsea south beach. The first one measures 5cm along the longest edge and 2cm deep. The second one is 2mm long and is in a rock that is 1cm long and magnetic. Thanks

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    Mar 8, 2013 9:31 PM (in response to Ash)
    Re: Two fossils for id

    Ash,

     

     

    It seems your attachments did not arrive in the order you intended, but never mind.

     

     

    The stripy specimen

     

    This is just a piece of rock, possibly a grey limestone but I really can't be sure just by looking, whose main features are veins, probably of quartz or calcite. It is a striking example, for sure, but in general such things are not rare. Your specimen is noteworthy in the multiplicity and parallelism of the veining.

     

    Veins such as these arise when the rock is stretched, and because it is relatively brittle*, it breaks, giving rise to cracks. That would happen deep underground, where the rock is saturated with fluid. One could say the fluid was water, which might be right loosely speaking, but that would miss the fact that it has chemicals dissolved in it. And those chemicals are what crystallized and filled the cracks with calcite, quartz, or other minerals, turning them into veins. In practice, the process of crack opening and vein filling may happen in parallel or in phases - which can affect the internal structure of the veins.

     

    Depending on the depth underground, and the physical and chemical conditions there, the fluid could be more highly concentrated and richer in metallic chemicals, perhaps. That could cause veins to be composed of ore minerals.

     

    Also, if you were to see the rock on a larger scale, you might find the veins were localized and arranged in patterns. Some of these are called en echelon tension gash arrays. Here's an example that shows two en echelon tension gash arrays, themselves arranged in a meaningful pattern http://www.nvcc.edu/home/cbentley/field_trips/index.htm (search the page for 'en echelon tension gash arrays').

     

    Veins are where geochemistry, mineralogy, petrography, structural geology and mining meet. There is much of interest to discover.

     

    * Brittleness and ductility in rocks depends very much on how quickly the deformation occurs, the nature of the rock and its conditions. Some rock put under stress over millions of years could almost flow (especially if warm and wet), or given greater stress and/or colder conditions it could develop brittle fractures.

     

     

    The magnetic specimen

     

    Tricky. I suspect iron (perhaps steel-making) waste. It may not be slag as such; it may be reject smelted metal. Your tiny 'fossil' could be a piece of different metal within it; metal which did not melt.

    We could do with the opinion of a mining or metallurgical expert...

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        Mar 9, 2013 7:21 AM (in response to Ash)
        Re: Two fossils for id

        Stripy one:

        The linear indentation separating the 'two halves' is just a narrow band of finer sedimentary particles.

         

        Magnetic one:

        The lack of sag and flow marks and lack of bubbles leans me away from low-metal slag.

        The lack of any surface expression of crystal structure leans me away from ore, but only slightly.

        I can't rule out weathered meteorite, but it doesn't 'feel' right. (For general reference re meteorites - http://epswww.unm.edu/iom/ident/index.html.)

        The lack of apparent structure leans me towards metal or high-metal slag; the apparent pattern of variation (your 'crystal' and the scattered dark lumps), ditto.

        We need some more info...

        By magnetic, do you mean steel/iron is attracted to it, or just that a magnet is attracted to it?

        Also, it would be instructive to break it - to see a fresh surface. If that was bright and shiny, that would be metal or ore; it might also show some useful fracture or crystal surfaces and/or maybe bubbles (though I could see no sign of bubbles on the surface).

        Density would help distinguish between metal and ore (weigh it and calculate its volume by water displacement). There is a useful table of densities in the meteorite link above.

        There are other things to try, such as streak test.

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            Mar 10, 2013 6:25 AM (in response to Ash)
            Re: Two fossils for id

            OK, we're making progress.

             

            The clear streak means it is not haematite (red-brown) or magnetite (black). See http://meteorites.wustl.edu/id/streak.htm

            I should check with you: did the streaking scratch the tile or the specimen? (If the tile, it indicates the specimen is harder than the tile, and the clear colour is not indicative of the specimen.)


            By all means leave breaking it until after you've determined its density.

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              Mar 10, 2013 7:20 AM (in response to MikeHardman)
              Re: Two fossils for id

              Also - your picking the specimen up with a magnet does not tell us if the specimen is itself magnetic or not (a magnet can pick up another magnet as well as ferro/nickel material). Does your specimen attract a steel paper clip?

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                Mar 11, 2013 1:50 PM (in response to Ash)
                Re: Two fossils for id

                Ah good - thanks Ash.

                That makes the 'fossil' look a bit more like a fossil or crystal than metal.

                And the surface generally looks more granular, rock-like.

                ...Suggesting iron ore / ironstone, though the lack of streak is a bit confusing.

                If it had been truly magnetic, that would have suggested magnetite or pyrrhotite.

                I await your density detemination and later photo of broken section.

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                    Mar 13, 2013 6:24 AM (in response to Ash)
                    Re: Two fossils for id

                    Ash,

                     

                    The photos are not showing (broken image-icon).

                    Please could you try editing your post and specifying the images again; they may be best as attachments rather than direct links.

                     

                    Density:

                    You can make an approximation by imagining the specimen to be a sphere and gauging its diameter (use cm).

                    Its radius is half that, then its volume, V, is given by  \!V = \frac{4}{3}\pi r^3

                    If the mass, M, is in grammes,

                    the density (M/V) will be in g/cc, which is numerically the same as the density figures (g/ml) in the table at

                    http://epswww.unm.edu/iom/ident/index.html

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                      Mar 13, 2013 10:38 PM (in response to MikeHardman)
                      Re: Two fossils for id

                      There's one thing about the Holderness coast - If you like collecting minerals this is the place to be!

                      Why?

                      You can find material from Scotland to the Scandinavian country's all along the Holderness.

                       

                      Tabfish

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                        Mar 15, 2013 9:18 PM (in response to Ash)
                        Re: Two fossils for id

                        Well, from the general colour, it might be granular magnetite. But being only paramagnetic, and the lack of black streak suggest otherwise. It would be nice to see some evidence of isometric crystal form (as pertains to magnetite), but that can be masked by an absence of free crystal faces. I get an impression there may be grains of a light-coloured somewhat elongate mineral species mixed with the black grains, which confuses interpretation.

                         

                        It would be very useful to know the density...

                        That could help me find candidate minerals.

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