Two crinoid columnal questions !

[David91959]

Hello,

I have many crinoid columnals that I collected in gravel (I suppose it was river/creek gravel collected and sold by a "sand and gravel" company) in Illinois many years ago.  I have two questions that may be obvious to more seasoned fossil hunters/students. 

 

1) Exactly how did the crinoids actually grow, meaning, how did the stems' diameter expand horizontally in size as time went on, since the stems were mostly composed of hard calcium carbonate/lime. In other words, it seems that once they are "hardened" or "frozen" into a certain size, how can they get bigger? Was there growth tissue on the OUTSIDE of the stem that kept adding on calcium to the inside, like tree trunks creating wood and bark? (I can understand the growth of whorled type seashells, but I can't get this through my head.) 

 

2) Many of the columnal discs/segments I have found are virtually solid silica, some with a glossy almost pearly luster, I suppose would be classified as flint, chert or chalcedony. I assume from what I've read that this is from replacement of the original calcium with silica over a long period of time.  This may seem simple-minded or thinking too deeply, but do researchers claim to understand how the silica could actually have "moved" or migrated into the spaces where the calcium used to be? I can't visualize how this would be possible. Like, one molecule at a time gradually moves through solid rock? Does anyone understand why this seems so hard for me to visualize?  Or is this believed to occur because the original calcium actually changes it's molecular characteristics without moving?  Any input or comments would be greatly appreciated! 

[Mark Kmiecik]

Hi, David, and welcome to the forum. I would love to answer your questions but I honestly don't have the time to do that much typing and probably wouldn't do a very good job of explaining. I'm not blowing you off here. To do the topic justice I'd have to spend at least a couple of hours typing. There is no "simple" answer. You have asked a couple of very profound questions that deserve serious reply.

 

In reference to question #1, just Google crinoid and read the Wikipedia info. They still exist nowadays in various forms and there is a segment on growth. For question #2 Google "permineralization" and "petrification" and read the Wiki again. This will give you a generalized description of the processes involved. If you really want to know all the variations and details of the different processes that can achieve similar results you will need to read several books, which if you are genuinely interested won't be as boring as they seem to others.

 

P.S. -- Sedimentary rock is rarely "solid". Most of the time it is quite porous and allows solutions of various minerals to permeate and fill gaps and hollows formed within either by putrefaction or similar degradation of organic matter and/or dissolution and possible replacement of mineralized materials like shells.

[DPS Ammonite]

Crinoids can increase the diameter of the columnals.

link

 

 

 

 

 

[Mark Kmiecik]

DPS Ammonite's link didn't seem to post up, so here's my try at it. https://pdfs.semanticscholar.org/0a62/f2b2e4e6fc66f782005de7d2589fcd6d4d8c.pdf

 

It's about three pages of words I had to look up in order to understand, but was worth the time.

[David91959]

Thank you very much, Mark and DPS Ammonite!!