Tuesday, February 12, 2013

Extremely Unusual Agate

While working on the online rockhounding adventures I received an usual package in the mail.  It was a large binder notebook sent to me by Roy Howard from California.  I didn't get time to examine its contents until yesterday.

Many years ago Mr. Howard used to spend time agate hunting in southern California.  Subsequent to the agate hunting trips, he pulled out the specimens to examine them more closely.  While checking translucency using a flashlight he noticed that there was a weird inclusion in one of the pebbles.  Although the specimen was semi-translucent, he could not tell exactly what the inclusion was.  So he used lapidary equipment to cut the stone in half, making sure to not touch the inclusion.  Then he polished the surface in the half that contained the inclusion.  Roy couldn't believe what he was looking at.  The inclusion appeared to be a time-frozen glimpse of a root hatching out from the body of a fresh seed, a seed which still has its awn attached.

Pictured below are similar Chalcedony pebbles along with the two halves of the "organic agate" specimen. 

Once Mr. Howard realized that he may have something special, he began investigating the inclusion.  Over a period of time, he worked with experts from the Smithsonian, Penn State University, Iowa State University, Montana State University, and various granaries.  Many of the experts were not aware that organic material can form inclusions in agates.  Articles about this mysterious stone were published in the Lapidary Journal in November 1993 and Rock and Gem in November 1997. 

One professor, who is an expert in fossilization and microcrystalline quartz had a different opinion.  Correspondence sent to Mr. Howard from Dr. Peter Heaney from Penn State stated that "I am generally convinced that the object embedded within the chalcedony is a seed that has been partially replaced by hematite and goethite.  It is in many ways a spectacular specimen, because the morphology of the seed is so well preserved and the chalcedony is sufficiently transparent to reveal the embedded specimen.  At the same time, the inclusion of organic materials (such as plant tissues, algae, and bacteria) in microcrystalline chert and chalcedony is surprisingly common. The most ancient bacterial fossils are 3.5 billion years old, and they occur as organic films in chert.  ... The silicification of organic material occurs by the attachment of dissolved silica to the surface of the tissue, which accordingly serves as a mineral template.  Consequently, the tissue is not initially replaced during mineralization.  Rather, it undergoes 'permineralization' such that all void spaces within the tissue are filled with silica.  Much research has shown that fossilization can occur at room temperature, and thus embedded tissues undergo very little degradation during the fossilization process.  It is commonly assumed that the petrifying media are thick gels, but my research indicates that silicification occurs by low-viscosity fluids that contain silica polymers only a few nanometers in diameter.  This explains why silica is able to petrify the interior as well as the exterior surfaces of cells.  ...  Your wheat seed is a wonderful end-product of this process, and it owes its existence to the same events that generate all fossilized wood tissues."  What is most interesting about Dr. Heaney's conclusion is that at least for this specimen, it seems to indicate that  the chalcedony specimen with its inclusion formed in a fluid environment rather than a silica gel environment.

One of the grain experts was even able to identify the possible species as Festuca arundinacea, a tall grass similar to, and possibly an ancestor to the modern wild oat.  Experts estimate that the seedling in the chalcedony specimen is between 30 and 50 million years old.  That is when grasses are said to have first appeared on the continent.

Here are a couple of photos of wild oat.

One of the things I was curious about after reviewing the notebook is what we can learn about the probably temperature of chalcedony formation from the fact that the specimen has the remnant of a sprouted wild oat seed.  I examined seven articles that hypothesize the temperature of agate formation.  Of course, they are all over the map estimating agate formation temperatures ranging from 32 to 752 degrees Fahrenheit.  All of the articles provided a range with the average low temperature at 135 degrees and the average high temperature at 453 degrees Fahrenheit. 

Given that the seed actually germinated in the vesicle pocket prior to agate formation AND that the seed did not subsequently decay, it is most likely that the silica started to crystallize in the pocket soon after germination.  If much time had passed, or if the conditions were more extreme, then the seedling would have certainly been destroyed.  So this specimen gives us a great glimpse into what the temperature in the vesicle pocket must have been. 

I found three sources that estimated the range of temperature needed to germinate wild oat seeds.  The actual temperatures stated in these three articles range from 45 to 80 degrees Fahrenheit, with the averages at 52 to 69.  One of the articles clearly states that the germination potential declines significantly any time the temperature exceeds 91 degrees Fahrenheit. 

By then looking at all the numbers, it seems that there is incongruence between what most of the scientists feel was the temperature required for agate formation and what botanists know is the temperature required for wild oat seed germination.  Now it is possible that the species represented by this ancient seed may have been much different than modern species of wild oat – but not probable.  After all, it is still a seed made of organic material.  It would not have survived the higher temperatures that some scientists think facilitated agate formation.  So it appears that this specimen proves that most of the agate genesis hypotheses regarding agate formation temperature are not correct – at least not correct for the formation of these chalcedony pebbles.  To be sure, though, more research is required.
I like the name that Mr. Howard has given this specimen.  He calls it the "Three Kingdoms Gem."  When it is held, the animal holds the mineral; the mineral holds the vegetable.



Fallick, A. E., Jocelyn, J., Donnelly, T., Guy, M., and Behan, C., 1985, Origin of Agates in Volcanic Rocks from Scotland: Nature, v. 313, p. 672-674.

Götze, J., Plötze, M., Tichomirowa, M., Fuchs, H., and Pilot, J., 2001a, Aluminum in Quartz as an Indicator of the Temperature of Formation of Agate: Mineralogical Magazine, v. 65, no. 3, p. 407-413.

Moxon, T., 1996, Agate: Microstructure and Possible Origin: Terra, Doncaster, UK.

Moxon, T., 2002, Agate: A Study of Ageing: European Journal of Mineralogy, v. 14, p. 1109-1118.

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