Silica Replacement

[petroffski]

Greetings from a newbie,

After attending the Museum of Natural History today my brother and I came up with a question no one could answer.

Where does the silica come from when a coral or a plant gets fossilized in limestone?

Regards,

Peter

[Gatorman]

Welcome to the forum Peter, I cannot answer your question but I'm sure one of our other members can. The museum you went to was it in Florida?

[petroffski]

Thank you for the quick reply.

We visited the Museum of Natural History in Denver.

http://www.dmns.org/main/en/

Any help is greatly appreciated.

[ebrocklds]

after a quick search on google the only thing i came up with is that the limestone does not actually supply the silica, rather the water moving through the limestone matrix already has it in solution and it only effects the organic material replacing it with silica in the form of agate, opal, or calcedony. i am not sure this is 100% correct as it came from a book written in 1855. but it is somewhere to start.

i will keep looking.

brock

[ebrocklds]

by the way, welcome to the forum!

[petroffski]

Brock-

Thanks for the theory about the water carrying silica but I can't help but assume that there would be SOME trace of silica in the limestone if that were the case. On the other hand, how else could it get there? Also, I can see that the "water carrying the silica" thing replacing organic matter makes sense but aren't coral skeletons (for example) non-organic calcium carbonate?

I've wondered about this for quite some time and has always bothered me. I have some limestone bricks that I found in the Verde Valley here in Arizona with some wonderful, well preserved, detailed coral fossils that are 100% silica. They etch out perfectly from the limestone!! I have one block about 8 lbs. worth that has some nice layered coral embedded with a well exposed top layer of 80% exposed coral.

I couldn't help but notice that your "member image" looks like a therizinosaur claw. Am I close? I only think so because our local museum has a fantastic exhibit on the weird beast and that claw looks very familiar.

Thanks for your effort- What a great forum ya'll have here!!

Peter

http://www.musnaz.org/exhibits/therizinosaur.html

[ebrocklds]

Brock-

I couldn't help but notice that your "member image" looks like a therizinosaur claw. Am I close? I only think so because our local museum has a fantastic exhibit on the weird beast and that claw looks very familiar.

actually it is a very large Allosaurus fragilis claw that i found several years ago in eastern wyoming

as for the silica problem i am stumped as well.

brock

[Harry Pristis]

Greetings from a newbie,

After attending the Museum of Natural History today my brother and I came up with a question no one could answer.

Where does the silica come from when a coral or a plant gets fossilized in limestone?

Regards,

Peter

Welcome to the forum, Peter. Here is something I prepared to explain the phenomenon of agatized coral (the state gemstone) in Florida.

AGATIZED CORALS FROM THE TAMPA FORMATION OF FLORIDA

The Early Miocene (25mybp) Tampa Formation underlies much of Florida. It is composed of soft, highly-fossiliferous limestones intermixed with sand and clay., There are several widely-separated exposures of the Tampa limestones which produce silicified corals and mollusks, the best known of which is Ballast Point on Tampa Bay. Nearby exposures of the same formation, such as at Sixmile Creek on Tampa Bay, may produce only calcareous specimens.

Many of the corals, mollusks, and other taxa of the Tampa Formation with shells or skeletons of calcium carbonate have been subjected to complete or partial silicification. This replacement has produced specimens of considerable beauty and sometimes of faithfully reproduced pseudomorphs. Most often, the original calcareous structure has been partially or wholly dissolved, and the replacing silica obscures identification of the taxon.

The process of dissolution of the calcium carbonate and the precipitation of the cryptocrystalline quartz (chalcedony) in Ballast Point corals is described by Lund in his 1960 paper "Chalcedony and Quartz Crystals in Silicified Corals." Lund says of the corals:

"The silicified coral masses from Ballast Point are of varying sizes and shapes. Some are globose and range up to a foot in diameter, some are tubular, and others are irregular in shape. Many of the masses are hollow, and the preserved 'shell' is characteristically comprised of two distinct layers. The outer layer consists of replaced coral in which the features [may be] preserved in remarkable detail, and the inner part consists of either banded chalcedony or banded chalcedony over which quartz crystals have grown. Most of the hollow forms are lined with colloform chalcedony, a few are lined with small quartz crystals, and less commonly specimens are partitioned and lined with both kinds of material, each in a separate chamber."

The origin of the dissolved silica is plants and animals such as diatoms, radiolarians, and silica-secreting sponges, as well as other siliceous matter occurring in the matrix.

For more information see:

Lund, Ernest H., Chalcedony and Quartz Crystals in Silicified Coral; American

Minerologist (1960), volume 45, nos. 11-12, pp 1304-1307.

Weisbord, Norman E., New and Little-known Corals from the Tampa Formation of Florida; Bulletin No. 56, Florida Department of Natural Resources (1973).

-------Harry Pristis

[petroffski]

Thanks Harry!

My assumption that there is NO silica in the limestone matrix is clearly wrong. The actual mechanics of this silica (bound in other small sea creatures) finding its way into bigger fossil creatures, as opposed to staying put, makes my head hurt. But that's my problem. To be continued...

Thanks-

Peter

[ebrocklds]

i also read that not only does this process happen when the organic material disolves and then filled, but it can take place a particle at a time, with the dissolution and precipitation of silica happening almost simultaneously, leaving a very accurate and identifiable fossil, not just a stienkern. (exterior shape)

brock

[jkfoam]

Greetings from a newbie,

After attending the Museum of Natural History today my brother and I came up with a question no one could answer.

Where does the silica come from when a coral or a plant gets fossilized in limestone?

Regards,

Peter

This is a good question and one I have had an interest in for many years after collecting silicified fossils from the Permian of west Texas. The following quote is from an article I just read which sheds some light on this jubject.

"Siliceous limestones, where their silica is original and of organic origin, have contained skeletons of sponges or radiolaria. In the chalk the silica has usually been dissolved and redeposited as flint nodules, and in the Carboniferous limestone as chert bands. It may also be deposited in the corals and other organic remains, silicifying them, with preservation of the original structures (e.g. some Jurassic and Carboniferous limestones). "

I think the author says that the Radiolaria and sponges have skeletons that are made of silica and that these dissolve and the silica is redeposited in corals and other organic remains silicifying them. The link to the article I reference follows;

http://www.1911encyclopedia.org/Limestone

On another note, I have seen various analyses of limestone samples and the level or content of silica as SiO2 has varied from 0.1 Wt % to as much as 3.5 Wt %. I have always thought that the source of the silica came from a multitude of sources such as volcanic ash falls, lava flows, sand sediments washed into the oceans from rivers, hot springs, etc.

JKFoam