Author Topic: Formation colours on Portland  (Read 502 times)

Online tim.rose2

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Formation colours on Portland
« on: August 19, 2021, 09:55:08 pm »
I'm hoping there's a suitably knowledgeable person out there who can give me an explanation for the array of colours seen in the formations on Portland?  I have a chemistry background so looking for some science here - i.e. what minerals / elements cause these and where they've come from (presumably the soil above?).  What sort of concentrations / co-crystal structures etc. exist?  What's the mechanism for transport / deposition (presumable dissolution / precipitation from water similar to CaCO3 driven by pH change?)   The islands caves are often described as void of formations, but if you know where to look this is far from the truth.  We've also just found another ton of the stuff so it's perked my interest. 

A few observations which may or may not influence the colours...
1. The majority of the flowstone is found in rifts in the higher limestone beds (freestone).  Very little in the lower cherty series.  There is less flowstone in the phreatics but again where it exists most is in the higher level tunnels in the freestone.
2. The vertical distance from roof of decorated rifts to ground level above is usually only a few metres (3 to 10 m).  What soil there is on Portland is a very sticky clay.  Much of the area above the caves is built on, quarried or peoples gardens.
3. The colours seen are cream, yellow, orange, red & grey (particularly interested in the grey colours as I've not seen this anywhere else).  I've personally never seen a single bit of pure white calcite on Portland, though some of the cream coloured stal is nearly white but this is rare.  Yellow / Orange is by far the most common.
4. Being an island surrounded in sea and since most of the cave entrances are on the cliffs there's no shortage of sea air (i.e. salt).  No idea if this is relevant or not, however as salt premotes oxidation of iron and I've read somewhere iron (presumably the various iron oxides) causes red colours in stal I guess it might be.
5. Obviously Portland stone has been extensively quarried over the years as a building material, but I'm unaware of any mining for minerals and whilst being underground I've never noticed any mineral veins.  I therefore don't think there's any significant mineralisation in the rock - well nothing valuable.
6. The caves are dry - There are some damp bits and some get a bit drippy when it's pi**ing down but there's no flowing water / streamways.  There's only a few places where water forms small pools in the caves.

To give an idea of what I'm talking about here's some photos:

This one is actual bright yellow moon milk:

Well that turned into a bit of a photo splodge!  Hopefully gives enough of an idea to start some discussion.

Offline Pitlamp

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Re: Formation colours on Portland
« Reply #1 on: August 20, 2021, 11:00:38 am »
I suppose the obvious answer is that the common yellowy brown colour is likely to be caused by one or more compounds of iron. The grey may be a low concentration of a hydroxide of manganese. There is some useful information along these lines (relating to far greater concentrations though!) in P Crabtree's article about Black Reef Cave in a BSA Cave Science journal from the 1950s or 60s.

Or, I suspect a grey colour may be caused by accumulations of airborne dust (spores, pollen etc) which coats layers of growing calcite then gets incorporated by overgrowth but may still be visible through subsequent translucent crystal layers.

But there are plenty of forum users who know a lot more than me about such things.

Have you looked in Art Palmer's cave minerals book for help?

Online tim.rose2

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Re: Formation colours on Portland
« Reply #2 on: August 20, 2021, 06:46:35 pm »
Totally unaware that book exists, but looks very interesting.  I will have to see if I can obtain a copy.
Airborne dust is certainly a possibility, though you'd think this would be more concentrated nearer the entrances / sources of draughts but that's not the case.  The caves are pretty draughty places in a gale.  Just trying to think; I believe I've only every seen the grey stal on the West side of the island.


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