Tuesday 25 August 2020

The Colour of the Stones

 "The majority of the stones are grey, but Stones 54, 55, 101, and 156 exhibit an orange hue while Stones 53, 56, and 154 are purple-grey. " *

Out of interest I charted the PXRF data for Fe % for each stone from Nash et al 2020** 

PXF penetrates a little way into the stone so doesn't just measure the crust but it seems there is only a weak correlation between the perceived colours and the Fe concentration of the surface layers. Which is a bit mysterious.

Maybe the colouration is shallower than the PXRF sampling or it isn't an Iron colour that is being seen. Any thoughts?



The purple colour is thought to be from Manganese compounds so the same chart for Mn - no correlation at all!



It has been suggested that the colouration of Sarsens is by "Rock Varnish"  - see Steve Marshall - Geology of Avebury http://www.exploringavebury.com/geology  - And http://www.exploringavebury.com/assets/uploads/files/DornMeekRapidVarnish95.pdf for details and http://www.public.asu.edu/~atrid/NewYorkVarnish.pdf for more.  But if that is the case then elevated Mn would be expected on stones identified as "purple"

*David Field, Hugo Anderson-Whymark, Neil Linford, Martyn Barber, Mark Bowden, Paul Linford, Peter Topping, , Marcus Abbott, Paul Bryan, Deborah Cunliffe, Caroline Hardie, Louise Martin, Andy Payne, Trevor Pearson, Fiona Small, Nicky Smith, Sharon Soutar and Helen Winton (2015). Analytical Surveys of Stonehenge and its Environs, 2009–2013: Part 2 – the Stones. Proceedings of the Prehistoric Society, 81, pp 125-148 doi:10.1017/ppr.2015.2 


**Origins of the sarsen megaliths at Stonehenge BY DAVID J. NASH, T. JAKE R. CIBOROWSKI, J. STEWART ULLYOTT, MIKE PARKER PEARSON, TIMOTHY DARVILL, SUSAN GREANEY, GEORGIOS MANIATIS, KATY A. WHITAKER SCIENCE ADVANCES29 JUL 2020 : EABC0133

8 comments:

  1. Tim
    The orange color is probably from iron. Unfortunately, the criterion used for sampling involved the sarsen surface being accessible, lichen-free and flat with no consideration or measurement of color which makes the lack of correlation unsurprising. The orange hue is very common in sandstones and invariably associated with iron oxide in the interstitial quartz cement. Though iron can also color quartz purple, as in amethyst, this is an unlikely source as strong sunlight leaches the purple from amethyst and, God knows, the sarsen have definitely been exposed to strong sunlight. This means the best candidate for purple is manganese, also in the interstitial cement as an oxide.

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  2. Interesting plots Tim. While they don’t provide any answers regarding the question of colour, what they do demonstrate is how unusual stones 26 and 160 are compared to the others at the monument. We didn’t include either iron or manganese in our statistical analyses (both are too mobile) but these are clearly very different sarsens and likely from different sources.

    David Nash

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  3. Very interesting Tim. An overlay helps to compare, but I haven't time to upload to Flickr right now. We shouldn't forget that the Moon offers additional colour variations, including a true Blue moon, which would be associated with dust in the atmosphere, perhaps long lasting as might occur after a volcanic eruption.

    https://journals.plos.org/plosone/article/file?id=10.1371/journal.pone.0084711&type=printable

    So colour doesn't necessarily relate to stone positions, rather a representative range of colours contributed by various communities, "tribes", in the area towards the Stonehenge "project". And as the references suggest, possibly a longstanding history of design elements from experience at Avebury.

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  4. Just and observation ...
    Has anyone else noticed that the stones with the most iron are the ones laying on the ground?
    Notice also the appreciable difference in the Fe content between -59A and -B -- the very same stone. Is it because -59A isn't all the way flat on the surface?

    I'm no mineralogist, but doesn't there seem to be some correlation?

    Neil

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  5. I'm glad you brought this up Tim, as I've been pondering it too. We are told that 50 of Stonehenge's 52 sarsens have identical chemistry, and that the exceptions are upright 26 and lintel 160. But I am not the first to notice that, as in Avebury, there are actually several different colours of sarsen in Stonehenge. Stones 53 and 54, for example, are both uprights of the same great trilithon, yet they are quite different. Stone 53 is blue-white and stone 54 is orange, presumably because its sand grains contain iron. According to the new survey, stones 53 and 54 are chemically identical but how can this be? As far as I can understand, the laser-scan analysis of the stones purposefully excluded iron, which seems to be what is mainly responsible for the differences.
    Not being a geologist I find all of this rather baffling. I wish I could post some macro pics here but I don't think that's possible is it?

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  6. I should just clarify what Tim wrote about rock varnish. It’s a brownish, sometimes bronze, mineral coating deposited in rocks in splash zones of rivers. It accumulates very quickly – in tens of years, rather than thousands of years as does wind-blown desert varnish. I have noted that rock varnish is found on some sarsens in and around Avebury, notably the Z-Stones and many stones of the West Kennet long barrow. But I’ve never suggested that rock varnish is found on any of the Stonehenge stones!

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    Replies
    1. Thanks Steve - my misphrasing, it was to point people to your expertise on it rather than imply you commented on Stonehenge but I can see now it reads that way.

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    2. Sorry, that should read '...deposited ON rocks' not IN rocks.

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