What is Moonmilk?

Peter Burgess

New member
Does anyone have any ideas about what moonmilk is, and more particularly the sort of places it can 'grow'?

In Barons' Cave in Reigate, which is dug in the silver sand beds of the Lower Greensand, and not limestone or chalk, there is a lot of soft white material on the walls. Although I have not checked it, I am told it fizzes in acid, suggesting it is calcium carbonate. Here is a picture. Could it be moonmilk? It has grown in about 200 years as it fills in the carved dates and initials from that time. About 10 years ago, you could also make out the date 1677 in one spot, but I'm blowed if I can see it now. I am sure the white stuff has covered it completely.

barons_ab01_graffiti.jpg
 

Brains

Well-known member
I suspect that you may have Gypsum growing form the eveporation of groundwater and extraction from the air. There will be numerous other salts coming out of the groundwater, including some carbonates. Gypsum is the commonest of the evaporite minerals, but I have usually seen it as needle like growths rather than a milky infill when on the walls of a mine. Without an XRD scan I am afraid it is mostly guesswork. Being in the sandstones rather than lime probably means it is differnt to moonmilk. Friends who used to work in the coal mines tell me that after a weekend shut down, the first one on the man rider had the job of headbutting all the gypsum stalls that had grown over the 48 hours, just had to be careful not to be knocked off...
 

Peter Burgess

New member
That is a very sensible suggestion. Gypsum is quite possible. What is that stuff that oozes out of brick walls sometimes? Might it be that? Next time I am down there, I will try to remember to get a close up photo of it. The stuff is most prolific in the largest passage, which is also the lowest part of the cave. The conditions in this section will probably be the most constant. Water rarely drips here, so the walls seem dry although the air is probably more or less saturated. It is a dead end, unlike the rest of the cave which connects an upper and lower entrance.
 

finster

New member
Apparently - "Moon milk" - A rare powdery mineral formation found covering the cave floor in a white blanket. Occassionally develops into some interesting formations and can be several inches deep. or "moon milk," a rare white bubbly coating made of calcite or even  An investigation of a material known as "moon milk" resulted in conclusions regarding its composition and occurence. It is a calcareous sinter composed of fine grained aggregates of aragonite, with some dolomite, some silica, and traces of iron, manganese, lead, and aluminum. Just a few answers to what is moon milk from the Net, I sifted out the non caving results as some were slightly dubious....
 
D

Dep

Guest
The stuff Peter refers to in Barons' appears to me to be organic - it's soft and fluffy on close inspection.
It could be mineral but I don't think so - we find it in many places, it is very comon in the sand mines.
This particular stuff is a type of mould I think.

Moonmilk is something I have heard off without ever having seen it that I am aware of, I have often wondered what the term means in the context of limestone caves.
 

kay

Well-known member
What's that thick stuff you see in half inch (or more) thick layers for example in Milky Way in Valley Entrance? I'd assumed from the passage name that was moon milk. It looks like thick cream, or very smooth soft cheese, and is soft to the touch. I've seen it several times in the Dales.

What Cap'n'Chris has just posted looks more like the fluffy stuff that you get all over walls and roof - as if you're on the inside of a giant bowl of cauliflower cheese. Is that also moonmilk? Seen lots and lots of that stuff.
 

cap n chris

Well-known member
Moonmilk is a term used to describe aggregates of microcrystalline substances of varying composition (aha! - it's not necessary chemically the same and depends on a variety of factors), usually carbonate minerals of which calcite is the most common in limestone caves. There are a number of ways in which it might originate but those most likely to be relevant in Britain are:

1) Formed as part of the life cycle of micoorganisms; bacteria, algae and fungi have all been isolated from moonmilk deposits. However some moonmilk doesn't contain microorganisms so this is not a hard-and-fast theory.

2) Moonmilk may be a product of the disintegration of bedrock/speleothems (i.e. rotten stone); this is the common theory but does not account for moonmilk flowstone issuing from cracks in walls, or the fact that moonmilk often contains silica and other impurities which did not originate from the surrounding rock.

3) Moonmilk may precipitate directly from groundwater but, for an unknown reason, the crystals deposited do not grow large.

So, there you have it. No-one really knows, is the correct answer.


 

Peter Burgess

New member
Thanks all. I suppose we will have to do some science if we really want to know what this stuff is. It only seems to grow in this site, and not in the neighbouring caves which are geologically identical. I can only think it is a result of something that has soaked into the wall in years past, like limewash or similar, or something that has come down through the ground from above (there was once a castle above it), or something that got into the walls from whatever used to be kept in the cave - it may have been a cellar, possibly for wine, possibly for something else.
 

Peter Burgess

New member
Isn't Kay describing that stuff that is all over Salubrious Passage in Ffynnon Ddu? I was told that was a sort of redeposited decomposed calcite.
 

gus horsley

New member
Peter Burgess said:
Does anyone have any ideas about what moonmilk is, and more particularly the sort of places it can 'grow'?

In Barons' Cave in Reigate, which is dug in the silver sand beds of the Lower Greensand, and not limestone or chalk, there is a lot of soft white material on the walls. Although I have not checked it, I am told it fizzes in acid, suggesting it is calcium carbonate. Here is a picture. Could it be moonmilk? It has grown in about 200 years as it fills in the carved dates and initials from that time. About 10 years ago, you could also make out the date 1677 in one spot, but I'm blowed if I can see it now. I am sure the white stuff has covered it completely.

The stuff you're describing is almost certainly gypsum and it can grow at an extrememly rapid rate.  Does pyrite occur in the rock?  If so then I'm 100% certain it's gypsum (calcium sulphate).

I was always led to believe that moonmilk is mainly precipitated by bacteria (not always) and is a sort of underground version of tufa which is less crystalline than the latter because of the high humidity.

Salubrious passage does contain some moonmilk, as do a few other areas in OFD, but many of the deposits (eg Edward's Shortcut area) are in fact gypsum due to a chemical reaction with pyrite which is in some abundance in the limestones of the area.
 

Peter Burgess

New member
The stuff you're describing is almost certainly gypsum and it can grow at an extrememly rapid rate.  Does pyrite occur in the rock?  If so then I'm 100% certain it's gypsum (calcium sulphate).

The rock is silica sand with virtually no impurities, other than a small amount of iron oxides. When sand pits are dug in the neighbourhood into this deposit, they sometimes strip off the overlying gault clay, and at the boundary some impressive selenite crystals have been found. However, the sand itself is unlikely to contain sulphates or pyrite. The growth being limited to this one place strongly suggests to me that there is something about its history that has encouraged the white deposit to form. Has anyone noticed how places used to store alcohol often have odd growths all over the walls? Is this due to fumes of some sort? Perhaps the buried castle remains above have something interesting in them - like a cess-pit for example? Could this not be the source of sulphates leaching through?

 

gus horsley

New member
Selenite is a variety of gypsum so I'm pretty sure your deposits are the same.  If you can determine the source of the selenite at the boundary layer you may find it's the same process of formation as those in the mines.
 
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