Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: perryman on 15/08/2012 22:52:30
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Hello, I am referring really to the long one at the back, i found it in a field (chalky/clay) in selborne, hampshire, uk.
I have always found these things but they have almost always been roughly spherical and not larger than the medium sized one in the photo.
This new one is an emasculating 6.5 inches long, and has clearly part of a larger, broken rock.
I love these rocks and would love to know more about how they form and what they are made of... I've searched the net for something similar but to no avail and i am none the wiser.
http://i740.photobucket.com/albums/xx47/perrryman/P1060386.jpg
http://s740.photobucket.com/albums/xx47/perrryman/
Please advice! Cheers, sean. I include links to my account with photobucket, rather than include photos in the post as the "insert image" button function doesn't seem to work for me... unless this works (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fi740.photobucket.com%2Falbums%2Fxx47%2Fperrryman%2FP1060386.jpg&hash=a963c28a9226de3d32b71867ad5e86b2) cheers!
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Ok the photo worked... there are more pics available in the second link
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Rather than a simple concretion, perhaps you have some kind of a cast.
The round one looks similar to what we call Thundereggs (http://en.wikipedia.org/wiki/Thunderegg), which is apparently closely related to [url-http://en.wikipedia.org/wiki/Geode]geodes[/url], although it doesn't seem to have the agate layers.
Have you found any hollow ones?
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I have found material similar to that. What I found turned out to be gypsum and it is a possibility that is what it is. Can you perform a hardness test? If you can then perform the test on the crystals inside the rock in the photo. You have a very nice specimen of something.
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The iron oxide concretions found in the Navajo Sandstone exhibit a wide variety of sizes and shapes. Their shape ranges from spheres to discs; buttons; spiked balls; cylindrical hollow pipe-like forms; and other odd shapes ... many more also occur as isolated concretions, which range in diameter from the size of peas to baseballs. The surface of these spherical concretions can range from being very rough to quite smooth.
https://en.wikipedia.org/wiki/Moqui_marbles#Iron_oxide_concretions_.28Moqui_marbles.29
Navajo "Pipes"
... They range in size from less than 1 cm to greater than 50 cm in diameter and their walls are as much as 1 cm thick. They extend through the Navajo Sandstone for more than several meters. They are regarded as having formed as the result of the dissolution of siderite cements and the redistribution and oxidization of the released iron by groundwater flow. Hematite also occurs as other masses of diagenetic "ironstone" that exhibit a wide and amazing range of bizarre shapes, which can be described as both "strangely shaped stones" and "rusty scraps". Strangely shaped stones, pipe-like features, and other concretionary masses have been observed and described from the Navajo and other Jurassic sandstones in Utah and adjacent American states.
The reported composition of the rusty scraps, 30 percent ferric oxide and large amounts of silicon dioxide and calcium oxide, is consistent with the hematite masses found in the Navajo and other Jurassic sandstones in Utah and elsewhere in the Southwestern United States. The ferric oxide and large amounts of silicon dioxide is what a person would expect iron oxide cemented sandstone to consist of. Calcite and other carbonate cements and concretions are typically associated with the hematite masses in the Navajo and other sandstones and sedimentary rocks
https://en.wikipedia.org/wiki/Baigong_Pipes#Navajo_.22Pipes.22
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To bigblock:
Thanks, I'm glad I didn't lug the thing back for nothing, it was right on top of a huge hill in Selborne we call the Hanger. I really don't think it's gypsum, I think that stuff is quite soft and this is hard, very heavy. Also if it was gypsum i imagine the tractor that ploughs the field and uncovered it would have at some point in the past broken it up...
The small one on the right in the photo however is softer and more silvery and i brushed a lot of soft white crystals off it which had collected over the last couple of years in my room, maybe that was gypsum?
To CliffordK:
well i'm not exactly sure what a cast is in geological terms, but the radial arrangement of the crystal formation (seen in the photos in the second link) suggested to me that wasn't a cast... but what do I know...?
I have never found any hollow ones and all i have found have been opaque.
To RD:
thanks for the info on Navajo pipes, i do certainly see the resemblance but this specimen is definitely solid.
Cheers for all the replies, please keep them coming, i'd love to solve this mystery!
Sean
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To RD:
thanks for the info on Navajo pipes, i do certainly see the resemblance but this specimen is definitely solid.
The pipes were once solid too, but the core has gone leaving the iron oxide shell,
same with the spherical Moqui Marbles (https://en.wikipedia.org/w/index.php?title=Moqui_marbles&redirect=no) ...
with core ...
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http://www.rocksandminerals.com/specimens/moqui.htm
without core ...
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https://secure.flickr.com/photos/usinside/2929975246/
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2Fforum%2Findex.php%3Faction%3Ddlattach%3Btopic%3D44633.0%3Battach%3D16518%3Bimage&hash=55847e857ec3c969b9a307074d8847a4)
http://www.thenakedscientists.com/forum/index.php?topic=44633.msg391502#msg391502
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perryman,
The pdf file that this link points to does a great job at explaining the formation of iron concretions.
http://www.geology.utah.gov/online/pdf/pi-77.pdf
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Thanks for the help guys, cheers, Sean.
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The bottom picture is called a "Gump" It is from a box of chocolates :-)
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Hi, Perryman.
The specimen you said was softer; can you scratch it with your fingernail? If not, it is almost certainly not gypsum.
Is it hard enough to be marcasite? That could account for the appearance of "soft white crystals". Pyrite, especially if it is of organic origin, sometimes degenerates into melanterite, I've had some nice specimens turn into nasty heaps of dust. I've never seen marcasite do it, but it's essentially the same stuff as pyrite, so why not?
If you don't have any joy elsewhere, you might be able to contact Bob Markham, former geologist at Ipswich Museum, at http://www.geosuffolk.co.uk/ I have always found him very willing to help with local geological stuff.