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Sunday, 15 January 2012

Looking at sand under the microscope

Karekare Beach, North Island, New Zealand (polarising filter, somewhat digitally enhanced)

I have been messing around with sand for a while, and testing to see if you can easily distinguish sand from different sources.  One of my tests was to take same samples of "black sand" from Piha Beach on the west coast of New Zealand's North Island, about an easy hour from Auckland, which is on the east coast (the North Island of New Zealand is quite skinny around those parts!)

Sand from the hind dune at Piha Beach.
New Zealand is mostly volcanic, and the black sands of Piha and Karekare (mainly Karekare) feature in the film 'The Piano', so our visit was partly scenic, partly scientific.

By an odd chance, the mp3 CD in the car's player, as we left Karekare was playing Michael Nyman's main theme from 'The Piano', The Heart Asks Pleasure First.

You can see clips from the film and hear this music on Youtube, with a few shots of the beach (see the scene at 49 seconds, for example, and compare it with my picture above).
Wind-sorted sand from high on Piha
Beach, selected because it appeared
to be light in colour.

Now back to the science.  I noticed that the black sands in the dunes at the back of the beach were sorted by the wind into different colours, and in the wave zone by water, so I took small samples to see if there were any big differences.

Because I was aware of possible quarantine issues, I washed and boiled each sample before drying it for a prolonged period in a microwave. It was then sealed in a zip-lock bag for transport.

Even though the sands looked different at a macroscopic level, there was less to spot at the micro level.  I need to pursue this in a more scientific way.

Water-sorted sand from near the wave
zone, selected from the darkest patches.
I think there is a visible difference here.

Now a note about scale here.  Most of the sand shots are x60, but that means little to the reader, since shots are sized, cropped and resized and then viewed on screens of different size.  The key thing to not here is that an uncropped shot covers an area about 4 mm wide and 3 mm high.  I will go into how you measure scales in my next entry.

That gets a bit tricky in the next shot and the last one, which are both composites, with a x10 base carrying a x60 inset.  The full field in x10 is about 16 mm wide and 12 mm high (and it doesn't take too much higher maths to realise that the microscope makers have been playing a bit fast and loose with their "magnifications"!

Anyhow, now you know what you are looking at, and all of these show the full field of the original shot: there may be resizing, but there has been no cropping.
Weathering products, Triassic Hawkesbury sandstone, Sydney.
This is a sample of sand taken from a bush track running through Triassic Hawkesbury sandstone near Sydney.

I used x10 magnification then clipped a part of a x60 shot of the same sample: you can safely assume that the material in the inset appears at the same size that it would have in a full picture of the size: the inset covers an area about 2.5 mm wide and 2 mm high.  Notice how coarse these grains are.

Squeaking sands, New South Wales South Coast: Lake Tabourie and Rennies Beach.
Here are two portions of two x60 shots of "squeaking sands".

My attempts to explain why some hot, dry, Australian sands make a squeaking sound when you walk on them may have delivered a result, but I leave it to the reader to decide whether or not there is the hint of an answer in these shots, when you compare the squeaking sands with the non-squeaking Piha sands or the Triassic grains which are also non-squeaking.

Scale here: the whole frame covers an area 3 mm high.  One of these days, I will make some scale bars to drop in on these pics!

Shell grit from Coller's Beach near Mollymook, New South Wales
South Coast. A careful look at this may provoke a few new and
different investigations from those I have been undertaking.
If you look carefully, there are two views of the same thing here.
I'm going to keep going with this study, and see where it leads.  It may have a place, only a small place, but a possible place, in book project number 5 as mentioned in my last entry.  See if you can race me to an answer or five!

The key variables are probably the mineral content of the sand, its dryness, the grain size and the roundness of the grains, though I have also been looking at uniformity of grain size.

Then there's the amount of organic matter, ranging from zero, up to 100% in the case of bands of shell-grit, like the sample above.

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This blog covers quite a few different things, so I tag each post. I also blog about history, and I am currently writing a series of books called Not your usual... and the first two have been accepted by Five Mile Press, The offcuts appear here with the tag Not Your Usual... . For a taste of Australian tall tales, try the tags Speewah or Crooked Mick.   For a miscellany of oddities, try the tag temporary obsessions. And language us covered under the tags Descants and Curiosities, while stuff about small life is under Wee beasties.

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