Colours in speleothems

[thehungrytroglobite]

Does anyone know why this curtain in FOUL pot is so red/orange? Is it because of peat or iron?

I know blue can be because of copper ions and black because of manganese oxide. Are the blue formations in South Wales because of the former and the black formations in Gingling because of the latter? Or might there be other causes too?

 

[Fulk]

I think that you're basically correct about the colours red/orange blue/green and black, but I suppose you can't really be sure without removing a sample and analysing it. For example, I once read of someone who was convinced that a red stal was coloured by iron, but he did analyse it to find not a trace of iron – the colour was entirely caused by peat staining.

 

[Cantclimbtom]

I think that you're basically correct about the colours red/orange blue/green and black, but I suppose you can't really be sure without removing a sample and analysing it. For example, I once read of someone who was convinced that a red stal was coloured by iron, but he did analyse it to find not a trace of iron – the colour was entirely caused by peat staining.

Removing a sample... why? just lick them and you'll learn right away.
Green is caused by lime, yellow is lemon, red is strawberry, blue is raspberry, black is licorice. White is tricky though, it could be pear or it could be mint. Just don't lick the brown ones!

 

[tim.rose2]

I'm lead to believe the most common cause of yellow / orange / red colours is iron. There's info in Cave Geology by Arthur N. Palmer about formation colours if you want to read more / like the science.

Pretty much all the stal on Portland is coloured. I know of a couple of white / translucent bits well hidden away which I've found recently but it's very rare. A while ago I started this thread: https://ukcaving.com/board/index.php?threads/formation-colours-on-portland.28420/

Didn't spark much conversation at the time so I obtained a copy of Cave Geology and had a read. Other than that I've found little info. I do have some stal samples from Portland which were found in quarries where rifts have been destroyed so if anyone wanted to do some proper testing it's not impossible. In terms of non-scientific testing I've identified lemon, orange, raspberry, strawberry.

 

[cap n chris]

Organic chemistry* (humic and fulvic acids) can give rise to blood-and-guts colouration of speleothems, many examples of which abound on Mendip. *IIRC.

 

[pwhole]

This is is on the shale margin in a mine adit in the Peak District - five metres further along, where the limestone starts, they immediately turn to pure white.

.

 

[Inglesport]

Removing a sample... why? just lick them and you'll learn right away.
Green is caused by lime, yellow is lemon, red is strawberry, blue is raspberry, black is licorice. White is tricky though, it could be pear or it could be mint. Just don't lick the brown ones!

Asked ChatGPT to visualise this.

You're welcome.

 

[tim.rose2]

Perhaps derailing this thread a little as it started up North but...

Chris - I think your onto something there. Certainly on Portland the percolation water will have to travel through 3 layers of ancient soils (palaeosols) including what's known as the great dirt bed. This band is typically 20 to 30 cm thick and rich in organics including (according to Ian West's webpages, who's definitely a Jurassic coast expert) humic substances. Beneath the great dirt bed is a hard cap stone which would definitely slow the pace of percolation / increase pooling and give plenty of opportunity for the water to dissolve and hence 'pick up' humic substances which could later be deposited during stal formation. A bit of an internet rummage shows humic substances can give colours from yellows, through to oranges into browns and reds. I'm definitely preferring this explanation over the standard 'iron in the water' line.

Having cut through stal samples (obviously not the stal in the caves) the colouring goes right through, though does vary in shade suggesting varying concentrations over the time period in which the stal formed. I've seen examples up to 10cm thick so we're not talking a short time period here.

Regarding the wonderful picture of the shale margin in the Peak mine - d'internet tells me shale is rich in ferric oxide which I think we all know is that red-brown colour!

Please pick my posting to pieces if you disagree with anything or have more to add. Stal colours fascinate me and I'd love properly understand what causes it.

 

[mikem]

Coombefield pics show some brown staining:

Isle of Portland Quarries - Geology by Ian West

Geological field guide to the Quarries of the Isle of Portland, Dorset, England

wessexcoastgeology.soton.ac.uk

Another page suggests it is the great dirt bed:

Withies Croft Wall - Isle of Portland - Geology by Ian West

Geology of a Quarry Wall - Withies Croft, Isle of Portland, Dorset, England

wessexcoastgeology.soton.ac.uk

Apparently cement can also cause colouring:

Maintenance - Portland Stone Firms

Learn how to maintain and care for your Portland limestone to ensure it lasts for decades. We list recommended steps for customers to follow.

www.stonefirms.com

Of course peat also colours water brown (although not on Portland, but it is organic)

 

[Andy Farrant]

And mud... clay minerals washed in with the percolation water can impart various brown shades to the stal, more especially larger flowstones formed by larger drips, rather than straws.

 

[caving_fox]

"Of course peat also colours water brown (although not on Portland, but it is organic)"

The colour of peat comes from the organics, and humic acids as mentioned above.

 

[mikem]

Indeed, but there isn't any peat there