Svalbard Find (Fossil? Mineral? Bone? Meteorite?)

[stretchy54]

My son returned from Svalbard (400 miles N of Norway) last month and today he presented me with this "rock", and asked me if it was a fossil. He said he found it along a mixed field of glacial morraine, in one of the rare non-protected areas where rock hunting is permitted, close to Longyearbyen in Spitzbergen. He mentioned that there were no other "rocks" similar to this in the area he explored. The "rock" measures 3 1/4" wide, 2" long, and 1" thick and is rather hefty at 0.6 pounds. The hexagon pattern and the very smooth sides are quite unexpected to me. At first glance I thought some kind of bryzoa. Then I noticed the overall shape was clam-shell like. Under a magnifying glass I then thought it may not be a fossil, since I couldn't find anything beyond the general hex pattern . My son thinks it looks bone-like, but it is rather dense/heavy. Then we thought perhaps meteorite material. It's definitely a nice "rock" and am happy to have it as a gift. But does anyone have any insight as to what it may be? Thanks.

[Auspex]

It looks rather like a siderite nodule, and the high density would support this.

[painshill]

Yes, I would say it’s a siderite nodule too. Here’s one from Lough Allen in Ireland:

[pic from LoughAllenBasin.com website]

That crystal pattern is quite characteristic but siderite readily alters to limonite and goethite as pseudomorphs (ie the original crystal shape is retained) as well as having secondary minerals which can produce a wide range of colours (from pale yellowish through to black). As Auspex says the nodules are often heavy since they may have up to about 40% iron, but not usually magnetic unless they have experienced some geological heating.

Pure mineral siderite has a theoretically white (colourless) streak on the back of a porcelain tile but dirty red-brown from mineral alteration and impurities is often what you get.

[jpc]

Wow.. He got to go to Spitzbergen! Your rock is indeed a siderite concretion. Some of them typically flake off little bits like this one, leaving this sort of texture.

[Carl]

I'm pretty sure it's what is called a counter-septarium. It's when a mud nodule dries as cracks from the outside in rather than the inside out, which is more common (septarian nodule or septarium). Can't easily find images on the Web but you can check out Pp. 168-169 in Seilacher's Trace Fossil Analysis for images and details: http://books.google.com/books?id=6IlYZ7ZvkaIC&q=septarian#v=snippet&q=septarian&f=false