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connorp

Colorful Stromatolites

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connorp

I've always wondered why many specimens are extremely colorful. Many seem to be unrivaled in terms of color with other fossils. Any thoughts?

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Missourian

Stromatolites are actually biogenic sedimentary structures. The laminae can vary in composition and microstructure, which could favor deposits of iron oxide to form. The coloration would be more akin to sedimentary strata than with most body fossils, which are often made up of uniformly 'drab' minerals like calcite. There are always exceptions to the rule, though.

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xonenine

my thought is to hope that Roger will treat us to an instructive, entertaining, and informative answer. :)

I am a big fan of Mookaite and Stromatolites in general too :)

http://www.thefossilforum.com/index.php?/topic/45281-microbialites-stromatolites-and-thrombolites/

there is a pretty fascinating PDF available, as well as many others

Edited by xonenine

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connorp

Stromatolites are actually biogenic sedimentary structures. The laminae can vary in composition and microstructure, which could favor deposits of iron oxide to form. The coloration would be more akin to sedimentary strata than with most body fossils, which are often made up of uniformly 'drab' minerals like calcite. There are always exceptions to the rule, though.

I expected iron oxides to be the cause of the common reds, especially considering the iron rich oceans stromatolites formed in. But what would cause such elaborate blues and greens such as below?

post-14060-0-96320400-1418954431_thumb.jpg

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connorp

Will this suffice?

We’re all familiar with the concept that iron influences mineral colours towards yellows, reds, browns and black (depending on its oxidation state) or that copper produces greens and blues , but the amounts present are quite large in most cases. The exotic elements produce an even wider array of colours when present at much lower amounts. If it were not for impurities, most crystalline minerals that were naturally formed in geologic conditions would be colourless. It’s those trace impurities which give them such a wide variation in colour, arising from complex crystal lattice substitutions. The elements responsible for these colour effects are typically the “transition metals” (those in groups 3 to 12 in the periodic table) which – apart from the familiar iron, copper etc also include the less familiar manganese, vanadium etc and the outright exotics such as molybdenum and zirconium.

In many cases, the colourations produced by these minerals are further influenced by heat and sometimes also radiation, arising from proximity to naturally-occurring radioactive elements and their isotopes in the Earth’s crust. Amethyst for example is no more than a purple variety of quartz with the purple colour coming from a combination of these effects (iron and other transition element impurities plus natural radiation). Heat it up and it turns yellow and then colourless. Irradiate it and it turns back to purple.

What we know from modern observations of the kinds or organisms which produced stromatolite fossils (principally cyanobacteria aka blue-green algae and allied forms) is that their cells are veritable powerhouses of enzymatic processes which depend on trace amounts of exotic elements for their chemistry. You can only do so much with carbohydrates produced by photosynthetic conversion of carbon dioxide and the ancestral forms of these bacteria were in any case likely chemosynthetic, living close to the ocean crust and relying on hydrothermal vents for both warmth and mineral supply. As a process, photosynthesis alone doesn’t provide the materials necessary to produce complex organelles, semi-permeable membranes and such. These kinds of organisms continue to need exotic trace elements as part of their primary nutrient intake with a much higher dependency than advanced multicellular organisms.

Although we don’t know for sure what original colouration stromatolite-formers might have exhibited, this kind of chemistry would undoubtedly have produced quite a spectrum of colours. Today we have an array of blues, greens, yellows, browns, reds and purples. The kinds of pigments responsible are organic – dihydroporphyrins (chlorophylls), phycobiliproteins, carotenoids, phycoerythrins and phycocyanins among others. Many of these materials are either heat-labile or have poor degradation resistance or both. So, the original pigmentation rarely survives the fossilisation process. Trace amounts of porphyrin-like compounds have been isolated from the Swaziland Sequence in South Africa which contains micro-structures that resemble fossil algae and dates to about 3.3 billion years. Trace amounts of chlorins and Nickel/Vanadium-chelated porphyrins have been isolated from two pre-Cambrian algal limestones, but they couldn’t be ruled out as modern contamination. Bacteriochlorophylls a and c have been isolated from organic matter within silicified Yellowstone stromatolites, but they are of comparatively very recent age.

Nevertheless, although the colours we see in fossil stromatolites are not completely original to the specimen, there are circumstances in which there may be at least some remaining pigment or a derivative of it. I think I may have shown this before, but this specimen shows seasonal growth bands of the red alga Solenopora jurassica Brown 1894 and is from the Jurassic Bathonian at Foss Cross Quarry, Chedworth, Gloucestershire in England (around 170 mya):

attachicon.gifSolenopora.jpg

Although it’s not a stromatolite as such, and might be more properly re-classified these days as a chaetetid sponge (ancestral to the corallinales), the genus definitely contains algal taxa. It’s known colloquially as “beetroot stone”. The pink colouration is not present in the surrounding sedimentary rock and is believed to be residual from the original organism, produced by boron-containing hydrocarbons.

More usually, what we see in fossil specimens as colouration is produced by the interaction of biological and sedimentary processes, together with subsequent chemistry and mineral exchange. That is, those exotic elements are released during the decomposition of the original organic compounds and, during diagenesis, become incorporated in the crystal lattices of the silicates and carbonates which replace the original material.

Amazing explanation. Thank you!

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ashcraft

As I understand it, stromatolites form in two ways. The limestone based types are formed from localized changes in pH as CO2 is removed from the water as it is used during photosynthesis, allowing the dissolved carbonates to precipitate. The other types form as the biofilm "captures" sand particles, which are later cemented and/or compressed. Color, as previously mentioned, can come from products of life processes "staining" the matrix being accumulated by the mat. Think how tea stains its container over time.

Another potential cause of color is oxygen/iron interaction. As the earth was becoming oxygenated, some evidence suggests that the oxygen content in the atmosphere "pulsed", sometimes higher, sometimes lower in concentration. This caused the iron that was associated with the material captured by the mat, to form different oxides, which exhibit strikingly different colors. The banded iron formations (BIFs) are thought to be an example of this.

Brent Ashcraft

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connorp

Speaking of, here's a nice one I just picked up. Collenia undosa, Biwabik Formation, Minnesota. It was deposited about 1.9 billion years ago.

post-14060-0-54981900-1419343825_thumb.jpg

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kevinnix

Here's another one, only a youngster at 512 to 502 myo, from Victoria river district in Northern Territory, Australia.

Formation = Antrim plateau volcanics.

Species = conphyton basalticum (conophyton refering to conical rings formed in the growth habit)

I have written in this forum and other Australian mineral forums before about this particular specimen, it's ability to take

a sparkling mirror finish when polishing with wet diamond polishing is amazing, a photo won't show it, only a video where

the specimen is moved in the light.

Thanks for the interesting information provided by various people in this thread.

post-15697-0-47811100-1419806734_thumb.jpg

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connorp

Here's another one, only a youngster at 512 to 502 myo, from Victoria river district in Northern Territory, Australia.

Formation = Antrim plateau volcanics.

Species = conphyton basalticum (conophyton refering to conical rings formed in the growth habit)

I have written in this forum and other Australian mineral forums before about this particular specimen, it's ability to take

a sparkling mirror finish when polishing with wet diamond polishing is amazing, a photo won't show it, only a video where

the specimen is moved in the light.

Thanks for the interesting information provided by various people in this thread.

attachicon.gifimage.jpg

Wow. Amazing specimen. Did you collect that yourself? Edited by connorp

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kevinnix

No, I haven't found my own stromatolite yet, this slice comes from a 3 kilogram chunk I purchased at a very reasonable price, I sliced it into 4 slabs, 3 are dressed up nice and polished to a nice finish, 1 learned how to fly while I was sanding it, it landed badly on a cement pathway and now lives its life as 7 smaller pieces, all polished up nicely.

Cheers, Kevin

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connorp

Here's a paper I started reading recently. I'm only a quarter of the way through but it is very informative and very interesting IMO. Deals with the classification of stromatolites.

http://pubs.usgs.gov/pp/0294d/report.pdf

Also here's some pictures that help in visualizing the form genera in the above paper.

post-14060-0-94742600-1421790059_thumb.jpg

post-14060-0-36707800-1421790072_thumb.jpg

post-14060-0-65770800-1421790077_thumb.jpg

Edited by connorp

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