I have recently noticed that the entrance passages of Rod's Pot, once devoid of moonmilk, are now displaying areas of dramatically rapid moonmilk deposition.  This has led me to reflect on this strange formation and consider its origins.  I have found various articles online, some suggesting a mico-biological component but no simple consensus explanation. 

I would like to suggest a very simple explanation for the formation of moonmilk.  This chalky calcium carbonate deposit is clearly not formed by the dripping or trickling water that forms stals or flowstone.  Cave walls are often naturally damp and it seems to me that moonmilk precipitates from this film of moisture. This being the case it raises a question - what mechanism fills the cave air with this fine aerosol of water containing dissolved calcium carbonate?  Picture the classic back-lit photograph of the drip landing on the stalagmite and we may have the answer.  The drip fragments explosively and a small portion of the water, water containing dissolved calcium carbonate, becomes part of the aerosol suspended in the cave atmosphere.  The aerosol is transported by draughts to cave walls where evaporation and/or gassing off of CO2 occurs and calcium carbonate is deposited. There seems to be a 'clumpiness' in the deposition, as if the precipitate deposits preferentially upon itsself.

So why the the new and rapid growth in Rods Pot - what has changed?  One obvious answer is the draught.  Since the Bath Swallet connection was opened nearly 2 years ago the cave seems to consistently draught out from the Rod's entrance.  This seems to be strong evidence to support a correlation between draughts and moonmilk.


Anyway that is ther theory and I welcome other views and comments (even from Graham, (as if I had a choice)).

The draught existed before the opening of the link between Bath and Rods. It became really noticable when we broke through at the top of Shower Pot in 2001.

There are various ways the air passes from one cave to the other, including the narrow continuation of the main passage at the bottom of Big aven.  That passage was shown to link to the top of Echo Aven by the egress of bang fumes, and we had a voice connection from the bottom of Echo Aven into Rods before the link was opened.

I am not sure of the date, but a few years ago someone dumped a car in the entrance of Rods and the petrol smell was almost overpowering in the top part of Bath. The smell was not obvious below the pitches, suggesting that there is a link in the upper levels of the caves.
One draught we never traced comes into Rods Pot just above the Bear Pit in winter where it leaves a large dry patch on the wall.

Andy Sparrow said:

(even from Graham, (as if I had a choice)).

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Love you too, Andy.  :

I would presume the air needs to be moist - not always the case as some draughts tend to dry out entrance passages.

mrodoc said:

I would presume the air needs to be moist - not always the case as some draughts tend to dry out entrance passages.

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That could suggest a better explanation than Andy's aerosol theory.
The dry air enters via Bath and becomes warmed on the way through the caves. When it reaches the area near the Rods entrance the air could be at the correct temperature/humidity to cause the calcium carbonate to be precipitated as moonmilk from saturated moisture on the walls. If there was not such a high airflow then the deposit would be some other form e.g. flowstone.

I suggest a combination of the two. the aerosol theory depositing a fine mist of calcium carbonate laden water on the cave wall and the water then being evaporated quickly by the strong draught (in the manner of the mrs blowing on her freshly polished nails to dry them) this could account for the rapid growth and small crystals that form moonmilk rather than heavier deposits.

The draught existed before the opening of the link between Bath and Rods. It became really noticable when we broke through at the top of Shower Pot in 2001.

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Presumably the original draught wasn't strong enough to dry the wall prior to Shower Pot being opened up. Also isn't most of the moisture on cave walls saturated with calcium carbonate, so a breeze drying it out would result in moonmilk?

Mike

I have been considering Andy?s original question since it was first posted and have been wondering whether to add another element into the debate and finally decided that I should (I expect I will be jumped on) ?..

The delay has been largely caused because I am not certain about the actual facts of ?moonmilk? and I am not certain it is actually properly understood by anyone ?.. (perhaps it is ?.?)

As I understand it, the powdery white(ish) substance found on the walls and ceilings in caves is either Calcium Nitrate or Potassium Nitrate (Saltpetre). It certainly begins as Calcium Nitrate but I don?t know specifically how it transmutes to Potassium Nitrate except that the existence of bat guano is one of the factors that will cause the precipitation of potassium instead of calcium (and, interestingly, appears to account for the theory that ?moonmilk? has a bacterial origin).

Barometric pressure changes are solely (I think solely) responsible for the flow of nitrogen into and out of the rock and it is this flow that (I think) is at the root of the  cause of the formation.

However, there has to be another factor (like the bat guano) for the deposit to crystallise and I do not know what this is although there may be numerous factors.

As for the sudden appearance that Andy mentions ?. Well, I am not sure it has anything to do with water or being damp but possibly more to do with the passage of gases in the air (especially any evidence of methane perhaps from a field outside the cave full of cow pats).

Other than that, I am stumped  :-\

Ian

I don't think its my imagination that rapid moonmilk growth appears to be happening in other Mendip caves.  Next time you climb up the Lavatory Pan in the Wet Way of Swildons look at the walls above you, also in the entrance passage to Goatchurch.  Maybe it's not a change of draughts but climatic conditions more generally that are having an effect.  Please take a look in the areas and tell me if you agree with this observation.

I dont know if the following info sheds any light on the question.

From Speleology--Caves and the Cave Environment (1997), by George W. Moore and Nicholas Sullivan, St. Louis, Cave Books, 176 p

The mineralogy of moonmilk has been intensively studied in Europe and North America during the past 30 years. We now know that the microscopic grains in the moonmilk of limestone caves consist mainly of calcite. But in relatively warm caves where the wall rock contains appreciable quantities of magnesium as a constiuent of the mineral dolomite, the grains may consist of any of the following magnesium minerals: hydromagnesite, magnesite, huntite, nesquehonite, and dolomite.

When the mineral constituents of moonmilk are removed by dissolving it in a weak acid, an abundant organic residue remains, which consists chiefly of such bacteria as Macromonas bipunctata, along with actinomycetes and algae. This microflora may assist in breaking down the minerals of the wall rock, and it aids in their conversion to the solids contained in the moonmilk.

The larger mineral bodies in calcite moonmilk have a distinctive surface sculpture that can be seen under the scanning electron microscope. The bodies consist of rods of calcite with an average size of 1 x 8 micrometers. A diagonal grain is impressed on the surfaces of the rods, and parallel ridges commonly trend along the lengths of the rods superimposed on the diagonal grain. The combined effect produces bodies somewhat resembling ears of corn.

The diagonal grain is aligned with the crystal structure, as can be seen through an optical microscope with polarized light. Because the crystal structure of calcite normally parallels the long dimension of calcite crystals, the grains in calcite moonmilk were once erroneously identified and named as a separate new mineral, "lublinite."

In many samples, the rods are enmeshed in a net of calcite filaments about 0.1 micrometer in width. These filaments are believed to have been associated with microbes that serve as nuclei for growth of the calcite bodies. An inclined crystal face of a seed crystal is nucleated by the microorganism and subsequent growth leads to the unusual crystal orientation and surface sculpture of the moonmilk grains.

Most moonmilk occurs where water may reasonably be inferred to move through the substance to its surface where deposition takes place by loss of carbon dioxide. The life processes of individual microorganisms cause a microvariation of the chemical environment that leads to deposition of discrete mineral grains, rather than to a more solid speleothem such as cave coral.

In a specimen of fresh calcite moonmilk from Caverns of Sonora, Texas, imaged by transmission electron microscopy, several of the thinnest (and shortest) mineral filaments have round organic bodies 0.1 micrometers in diameter at one end. We suggest that this offers an explanation for the small size of moonmilk grains. The organism nucleates the rod from one end only, until it divides. Subsequent thickening and sculpturing of the rod then takes place mainly by inorganic mineral overgrowth.

More research is needed to determine the energy source of the microorganisms in moonmilk. The snow-white rather than brown color of most moonmilk suggests that oxidation of iron is not the source.

In 1994, Rudolph Reinbacher studied freshly forming moonmilk in Mondmilchloch, Switzerland, the type locality for the substance. The moonmilk there is calcite with a color (wet) in the Munsel color system of very pale orange (10YR8/2). A sample stored in a jar in dim light for several months acquired brown, gray, and green spots, presumably from microbial contamination. This growth proves that nutrients remained in the sample and suggests that soluble organic compounds from the soil provide the energy for the microorganisms that control the growth of this strange substance.

If you google "moonmilk" you may end up with this impressive series of photos (not of moonmilk itself):

http://www.tinyvices.com/Ryan_McGinley_Moonmilk_29

http://www.tinyvices.com/Ryan_McGinley_Moonmilk_40

Mike

Martin,

Your submission makes a very interesting read and, I think, widens the debate yet further.

Although it is clear that a lot of analysis was done on ?moonmilk?, sadly, the chemical formula is missing (which is central).

I don?t profess to understand the subject at all but do understand something of the chemistry and I am aware (well, strongly believe) that there has to be an additional factor which is acting as a catalyst. As I said in my post, I don?t know what that is but the presence of micro-organisms may well be it (and almost certainly is in the case of saltpetre).

In any case, the deposit must be derived from calcium carbonate as that is the base compound from where it emanates. There is some consensus though, in that the study provides that there has to be a loss of carbon (although it then refers to dioxide and I suspect the reference is actually to ?carbonate?) and for calcium nitrate to be formed the carbon has to be displaced by nitrogen which would cause the carbon to bond with two parts oxygen (thusly, carbon dioxide is the by-product).

What could cause the nitrogen to displace the carbon? The uneven flow of gases within the rock caused by barometric pressure changes could do so but there would still need to be another catalyst as otherwise the moonmilk would be present everywhere. (This is where I get stumped).

Potassium Nitrate (Saltpetre) is not mentioned in the article at all although it is hugely documented that people used to enter caves to avail themselves of it for use in gunpowder. It is also widely suggested (possibly proved?) that it exists where there is bat guano which would indicate that something in guano is displacing the calcium in favour of the potassium (again, I am stumped).

Another thought as to why it might appear by an entrance (especially a new one) is that the entrance is likely to be a bottleneck for the draught and is likely to harbour a greater concentration of the missing catalyst.

Possibly   

Ian

Saltpetre (potassium nitrate) occurs in warm but not overly humid conditions (excessive dampness would wash it away) where there is a considerable amount of nitrogenous organic matter (e.g. urine, excrement), the saltpetre growing as crystals on the surface of the organic material and adjacent rock. It takes some yeras to grow to a commercial volume, hence particularly rich sources (caves with plentiful bat guano, urine-saturated earth floors of privies, stables, etc) were periodically scraped clean of crystals every few years. Occurrences of saltpetre in UK caves must be very rare on account of the low levels of organic matter and the usually damp conditions. Caves that are (or, rather, were) sources of saltpetre are mostly confined to radically different climates to the UK.

Jackalpup said:

There is some consensus though, in that the study provides that there has to be a loss of carbon (although it then refers to dioxide and I suspect the reference is actually to ?carbonate?) and for calcium nitrate to be formed the carbon has to be displaced by nitrogen which would cause the carbon to bond with two parts oxygen (thusly, carbon dioxide is the by-product).

What could cause the nitrogen to displace the carbon? The uneven flow of gases within the rock caused by barometric pressure changes could do so but there would still need to be another catalyst as otherwise the moonmilk would be present everywhere.

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Simply put, there is no direct interchange of carbon and nitrogen.  What you have are two ionic species, carbonate (CO3) and nitrate (NO3).  The CO3 comes from the compound H2CO3 which in water is mostly in the form of H+ ion and HCO3 - ion (the + and - signs indicate the charge on the ions) and which can further "dissociate" to two H+ and CO3 2-.  The carbonate ion will precipitate out of water in the presence of Calcium ions (Ca 2+) to form calcium carbonate CaCO3 in what ever form it takes (a fluffy white precipitate known to most chemists or if done slowly and very carefully in a crystal form which then grows into a speleotherm).  But calcium bicarbonate Ca(HCO3) is soluble in water, as is calcium nitrate Ca(NO3)2.

The problem with nitrogen in the air is it is very hard to get a reaction to transform that nitrogen into another chemical compound.  It hardly occurs in plants (it is called nitrogen fixation) and only a few plants like peas and beans do it to a limited extent.  Most plants leach it out of the soil, hence the need for nitrate fertiliser.  In the UK, nitrate ions from a geological origin would have been washed away by rain over the eons since virtually every compound of nitrate is soluble in water, thought there is a limit above which any more will precipitate out.  I am afraid I just don't accept that nitrate concentrations could be so high as to exceed this level, except for special cases such as an isolated pool with no outlet of water except by evaporation.  If I recall the posts correctly, we are talking about moon milk on walls as well as pools.  I am unsure about micro organisms ability to generate nitrate ions but doubt it, since as I understand it, humans generate the nitrate they excrete by breaking down more complex organic molecules containing nitrogen which have been taken into the human body by eating plants.  Thus normally the presence of nitrate indicates pollution such as cowsh. 

Perhaps and this is pure speculation, the micro organisms are generating CO2 from air as part of their growth which is then causing the local precipitation of calcium carbonate from calcium bicarbonate.  What the presence of nitrate ions would do is increase the concentration of calcium ions in the water (as calcium nitrate) so increasing the rate at which calcium ions are becoming available to react to form calcium carbonate.  But if this were true then it would be simple to determine this process and should have been proven by now.  Perhaps a project for a final year chemistry or geology undergraduate?

Bob Mehew said:

The problem with nitrogen in the air is it is very hard to get a reaction to transform that nitrogen into another chemical compound. It hardly occurs in plants (it is called nitrogen fixation) and only a few plants like peas and beans do it to a limited extent.

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from http://www.biology.ed.ac.uk/research/groups/jdeacon/microbes/nitrogen.htm

Biological nitrogen fixation can be represented by the following equation, in which two moles of ammonia are produced from one mole of nitrogen gas, at the expense of 16 moles of ATP and a supply of electrons and protons (hydrogen ions):

N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi

This reaction is performed exclusively by prokaryotes (the bacteria and related organisms), using an enzyme complex

I don't think we are in dis-agreement. 

Bob, I know it is diffilcult to get nitrogen to do pretty much anything (I do keep saying I don't understand this very well) and I still believe there must be a catalyst which I think you are suggesting too.

Your reference and Martins link to nitrogen fixation is, of course, perfectly proper but I think the original question is how "moonmilk" would occur in caves. I was stumped (and theorising) as to how calcium nitrate could manifest because I could not get my head around how the nitrate could form and I think you might just have found an answer 

Coooolies 

Ian

I believe that Calcium Nitrate from the Saltpetre Caves in the USA is formed within the cave earth by bacteria. I don't know if this helps.

It does help Les ....... All along I have pondered what the catalyst is and now it seems even more certain that it is a baterial agent ...... 

I don't think that is the end of it though ..... There is "moonmilk" deep inside some of the Yorkshire caves (like a lush cavern in Swinsto). It is possible the bacteria is being carried through in the water though as it is a very active system.



Ian

The likelihood of nitrates being involved in moonmilk are very low. Nitrates are incredibly soluble; many are in fact deliquescent (dissolve in the water they absorb from the air); in caves the impact of bacteria/life is likely to be low (no food!) wheras there is a handy source of carbonate (rock!). Some variant on Andy's theory therefore sounds reasonable; nitrate-based theories less so.

The nitrates (from cowsh, batsh, wherever) if present in the moisture film on the cave walls (analysis, anyone?) might act as a fuel/food source for the microbes who seem to be implicated in the growth or deposition of moonmilk.

Don't know that that helps at all to explain why the stuff should be appearing so quickly in the Mendips....