What Are These Cristals On The This Sea Urchins?

[Dromiopsis]

Hello!

15 years ago, I have found this broken sea urchin (Gymnocidaris?) in the kimmeridgian of La rochelle (France) on the atlantic coast(Pointe du Chay).

On the surface, there are cristals of calcite exactly on the tubercules...

My question is, are they previous spines in place recrystallized in calcite cristal?

Thanks in advance

D

Edited September 26, 2012 by Dromiopsis

[CH4ShotCaller]

Looks like calcite crystals to me. Not sure if they're from the spikes or just formed because so much organic matter was present that surrounded the host upon it's demise. It would be cool if the calcite formed at each spike!

Edited September 26, 2012 by CH4ShotCaller

[Dromiopsis]

Looks like calcite crystals to me. Not sure if they're from the spikes or just formed because so much organic matter was present that surrounded the host upon it's demise. It would be cool if the calcite formed at each spike!

Hello CH4ShotCaller,

Thanks for your answer, I have already example of the case you explain (please see picture of an Echinocorys where the Bryozoa fragments have been partially transformed into calcite crystals.

But in that case, each crystals begin on each tubercules...not ramdomly...

Thanks again

D

Edited September 26, 2012 by Dromiopsis

[Coco]

Hi,

In La Rochelle area, sea urchin spines and tests are in calcite. I don't think that the calcite crystals are spines, because the sea urchin spines are in calcite, but they keep all their decoration (structures and reliefs). Here, we distinguish crystals of calcite very well trained(formed).

Coco

[Dromiopsis]

Hi,

In La Rochelle area, sea urchin spines and tests are in calcite. I don't think that the calcite crystals are spines, because the sea urchin spines are in calcite, but they keep all their decoration (structures and reliefs). Here, we distinguish crystals of calcite very well trained(formed).

Coco

Hello Coco,

thanks for your answer.If it's not an epigenesis of spines, why should we have a nucleation only from tubercules? the crystals grow in the same direction as spine.

D

Edited September 27, 2012 by Dromiopsis

[Plax]

perhaps the tubercles were points of crystal growth because they protruded into a cavity. the crystal growth partially filling the cavity in the other picture shows origin on one side and growth in one direction, the most protuberant part of the cavity wall may have been the best place for the crystal to start, have also seen crystal growth follow unusual paths in other situations, attached pic is a Eutrephoceras with crystals following the siphuncle route from our local Peedee Fm (Maastrichtian), the siphuncle aperture is filled with crystals and growth connects to the next opening

[Dromiopsis]

perhaps the tubercles were points of crystal growth because they protruded into a cavity. the crystal growth partially filling the cavity in the other picture shows origin on one side and growth in one direction, the most protuberant part of the cavity wall may have been the best place for the crystal to start, have also seen crystal growth follow unusual paths in other situations, attached pic is a Eutrephoceras with crystals following the siphuncle route from our local Peedee Fm (Maastrichtian), the siphuncle aperture is filled with crystals and growth connects to the next opening

Hello Plax,

Thanks for the explanation, it's true that the growth of a crystal is not easy to understand...

By the way, very nice picture you add in the post!

I got one Orthoceras from the lower Ordovician of Öland, Sweden, completly crystallised inside

D