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.
Ooids
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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
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