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Ooids

 
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micro_pix



Joined: 11 May 2012
Posts: 141

PostPosted: Mon Jan 29, 2018 8:58 am    Post subject: Ooids Reply with quote

The advice to tourists is “take only pictures and leave only footprints” but you always seem to bring some sand home in your suitcase, so I had a look at it under crossed polarisers.

I know virtually nothing about sand (or geology) and found it interesting so I thought I’d share what I found out.

These sand grains are from the island of Great Exuma in the Bahamas – islands sitting on huge submerged carbonate (limestone) platforms. The sand grains on the beaches are spheroidal sedimentary grains of mostly calcium carbonate and are called “Ooids”. These ooids are around 0.2 – 2.0mm across, they grow from a seed particle in layers, in water that has a high saturation of calcium and carbonate ions and high turbulence (waves on the shoreline).

The sub-surface tunnelling apparent on the grains below is caused by Endolithic Cyanobacteria , they can penetrate and colonise the interior of solid limestone, including ooids. Apparently (when the bacteria have gone) the microborings are filled with "aragonite cement" which has a different crystal structure of CaCO3 to the rest of the ooid - which maybe why I can see the tunnels under cross polars in transmitted light. Sedimentary rock formed from ooids is called “Oolite” (Egg Stone). In certain circumstances the ooids accrete, bound by the microbial mats of the cyanobacteria, to form Stromatolites.

It is so very different in form and formation from the sample of sand from my local beach in the UK.

Ooid sand grains 4X objective Incident light.


Ooids (Crossed polarisers, transmitted light, 4X objective, stacked, cropped)


A single grain in cross polarisers transmitted light to show the tunnelling of the bacteria. 10 X Objective, Stacked.
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micro_pix



Joined: 11 May 2012
Posts: 141

PostPosted: Sun Feb 04, 2018 11:00 am    Post subject: Reply with quote

I've updated the above to include a photo in incident light and updated the text.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19326
Location: Richland, Washington State, USA

PostPosted: Sun Feb 04, 2018 11:53 am    Post subject: Reply with quote

I missed these first time through.

Fascinating material, very well photographed! I knew nothing about any this type of sand grains or the bacteria that colonize them.

--Rik
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Beatsy



Joined: 05 Jul 2013
Posts: 1391
Location: Malvern, UK

PostPosted: Sun Feb 04, 2018 2:05 pm    Post subject: Reply with quote

Remarkable. I particularly like the detailed patterning on the last one. Thanks for the pics and related info. That's today's "thing learned" Smile
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micro_pix



Joined: 11 May 2012
Posts: 141

PostPosted: Sun Feb 04, 2018 2:28 pm    Post subject: Reply with quote

Thanks Rik and Beatsy.

I was amazed that, in some parts of the world, the sand grows in the water! I understand that those Endolithic Cyanobacteria are pretty much unchanged from those found in the oldest fossilised stromatolites dating back 2.7billion years - I suppose that there wasn’t much other than rocks on the menu back then!

Dave
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vasselle



Joined: 05 Jan 2014
Posts: 1497
Location: France

PostPosted: Sun Feb 04, 2018 2:38 pm    Post subject: Reply with quote

very nice
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