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Question
Rock
Rock  
What type of rock is this?

Please check the attachment. Segments with fibre
vains to the center of the rock core.

Thanks

Answer
Jan:

What you appear to have is part of what is called a cone in cone structure.  The rock looks to be limestone, with veins of calcite threaded into the radial fractures around the circumference.

A suite of cone in cones looks


The formation of cone-in-cone structures has been attributed to:

1.Volume increase inversion from aragonite to calcite in which expansion of conical aragonite pushed cones apart and allowed for clay to intrude

2.Burial-induced pressure solution and clay layers remaining as insoluble residues

3.Fracturing of crystalline mineral composites that form in over-pressured chambers, with fractures forming from a decrease in pore pressure

4.Formation during early diagenesis by expansive mineral growth (force of crystal growth), in which the cones are produced by the growth of cone-shaped aggregates of fibrous calcite, the clay layers originate as the crystals displace and disturb the original clay-rich sediment.

5.Gillman and Metzger[7] proposed that their cone-in-cone structures were formed as a result that as fibrous aragonite grew, it displaced the still plastic clay materials. This is very similar to the displacive crystal growth mechanism proposed above in point number four. The displacive crystal growth mechanism tends to be the more popular and widely used explanation for cone-in-cone formation.


Sounds like a lot of scientific gobbledy-gook.  Suffice to say that they form from diagenesis, or post burial changes in the original sediment due to secondary mineral crystal growth or recrystalization.  Think of a layer of sediment, clay or semi liquid limestone goo deep in the earth buried under layer upon layer of sediment above it.  The pressure and temperature increase caused thing to start to change and recrystalize.  This sets up a situation where the structure of the rock, once fairly ordered and horizontal to go haywire when pressures build within the layer due to the chemical and physical changes and it is still confined in the layer.  Minerals originally in the sediments dissolve, and recombine to make new minerals and fluids being rich in leftovers crystalize forming veins.  Later the layer solidifies preserving the caotic new structure.

If an ice cream cone were a cone in cone structure, you would have a piece of the top cut cleanly off across the long axis of the cone, so it looks round.

They are often mistaken for fossils.  The first time I saw some in Montana, USA, I thougth they were fossilzed bone, and I got pretty excited, since the area was know to have a lot of T-rex fossils.  Alas it was only cone in cone in silty limestone.

Here are some pictures of other cone in cone structures from different angles:

http://www.bing.com/images/search?q=cone+in+cone+structrues&FORM=HDRSC2

http://www.bing.com/images/search?q=cone+in+cone+sediment+structrues&qs=n&form=Q

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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