What Is/Was Pipeclay And How Is It Formed?

[wiley coyote]

A guy was telling me that Pipeclay is super heated Quartz that has melted down and dissolved into Pipeclay. I'd never given it much thought? So I'd like to know if he's right?

Wiley.

 

[mbasko]

Pipeclay is fine grained usually white/grey clay. Can be yellow/orange too & Catlinite pipeclay is red.
The make up would depend on location & can include calcite, dolomite, silica/quartz, kaolinite, gypsum etc. etc. Some areas show quartz as only a minor inclusion.
Can't find any reference to super heated quartz? More likely to come from weathering/leaching of fine gravels/sands in quartz areas combined with an absence of iron minerals for lighter colours IMO?
I think the true definition of pipeclay probably gets misused a bit in gold areas - we tend to call any lighter coloured clay layer pipeclay whether it is a true pipeclay or not?
Goldirocks would know.

 

[silver]

I'd recon it'd be malleable and fine enough to make a backy pipe out of it that wouldn't crack when drying and wouldn't crack when lit n used.

 

[Swinging & digging]

Pipeclay is decomposed slate. Its NOT QUARTZ, whoever told you that is simply WRONG.

 

[user 4386]

Pipeclay is decomposed slate. Its NOT QUARTZ, whoever told you that is simply WRONG.

You are quite correct - it is usually what is commonly called kaolin (the mineral kaolinite).

The physical properties of clays and the uses they can be put to varies with their mineralogy. Kaolinite has many uses because it is the main component of good quality pottery clay and can be fired to a fine-grained uncoloured (white) hard finish as in fine quality white china - although you can add ochres to colour it.

For example, during weathering a basalt or shale might become smectite clay (which contains things like sodium, calcium, iron and magnesium in it) - it has its uses, such as in filtering oils. This is an expanding clay and fires to an dirty finish. More weathering and the iron and magnesium leaches out but any potassium remains (or more commonly a clay forms from a potassium-rich shale), to become illite clay. Even more weathering and it loses its potassium and becomes kaolinite, a pure hydrated aluminium silicate. The other clays have their uses - eg we usually don't try to make bricks out of kaolinite but out of a more illite-rich mixture, preferably with some iron oxide mixed with it to give it a nice red colour when fired. You may have noticed that a lot of cream-brick houses were built in the 1950s. This was because in the post war building boom, they could not find enough iron-oxide stained clay.

Horses for courses.

https://geologyscience.com/minerals/clay-minerals/

 

[UnderEmployedGeo]

Add this to the list of confusing historic terms that actually make less sense to the geologists among us (guilty) than to normal, sensible folk with better things to worry about. Whenever I see references to "pipeclay" my mind just shuts down and stops working (if it was working to begin with that is).

Thank you once again to Goldierocks for some actual geology.

One other one is references in older materials to "pitted shale" or pitted whatever - THAT I understand - the pits would be where there used to be little sulphide crystals in the original fresh rock. Slates are pretty resistant to weathering, but sulphides are not. First they oxidise, then they get washed out completely, leaving little "pits". Sulphides are pretty common around gold deposits like you see in Vic (and parts of WA).

A similar thing happens in quartz-carbonite veins (of the sort that often carry gold in gold fields) - if there were enough sulphides in the fresh rock, the preferential weathering of the sulphides can lead to a sort of honeycomb texture, that is better described in geology as "boxwork texture" - lots of little roughly cubic holes that once were full of sulphide crystals, now just full of iron staining. Boxwork textures are a good sign, just as laminated iron-quartz can also be.

Back to clay minerals - in the outer western suburbs of Melbourne, where the Newer Volcanics (basalt) have weathered to clay, there is a particular smectitic clay type known as Montmorillonite - and it is incredible at absorbing water, expanding hugely when wet. People have literally broken their house slabs in half by accidentally leaving a garden sprinkler on...

 

[Jaros]

That's all very interesting re pipeclay and colours. Bit of a worry with the house slab cracking.

 

[20x]

..

 

[Goldenau]

Shale pic above

It's an interesting question. Yes, it's from decomposed slate. Another thought is, slate was metamorphosed from shale. When you find shale in a gold mine, and you see the soft grey talcum like material between layers of shale, kind of greasy to touch, that's called pug. Or Fulcon. Often pug is present in gold mines in Gippsland. Shale is present, not slate. Slate is a much harder and tougher material whereas your shale tend to be flakey and very layered. I've found fossils in shale, between the layers. You kind of get an idea of the age of the material with a little bit of knowledge. Pipe clay as prospectors call it, is very old. Often objects don't move past pipe clay and become imbedded or cover in it. Why nuggets are often found in pipe clay as 'kinder surprises'. And yes, pipe clay comes in a variety of course, sometimes with a marbled look of browns and reds with yellow lines through it, quite pretty. I found a near 2 gram nugget in this type of material a week ago.

 

[UnderEmployedGeo]

If you google "sedimentary rock cycle" and look at the images, you will quickly discover that geologists are really bad at doing diagrams. This one I found, however, is almost useful:

The main thing to note is that clay, shale and slate are all of the same "family", with slate being the metamorphosed version of shale (which is lithified clay). As the grain size of shales and slates is clay-size, the weathered product will also be clay sized - so clay begat shale which begat slate which in turn may decompose again to clay.

 

[UnderEmployedGeo]

Better pics:

 

[Mackka]

Excellent.
Mackka

 

[wiley coyote]

The pipeclay where I detect is a creamy/white colour, feels slippery to touch, like talcum powder. Is it correct that the old timers dug through the wash dirt to the pipe clay, then tunnelled along in it as It was easier to work, took It up to the surface, then dropped the wash dirt down into the tunnel, then took that up to the surface to process. I've heard differing versions of how they worked It, but this seems the most logical way to me. Any thoughts on this? wiley.

 

[Moneybox]

Better pics:
https://www.prospectingaustralia.co...2/1642559128_screenshot_2022-01-19_132504.jpg
https://www.prospectingaustralia.co...2/1642559129_screenshot_2022-01-19_132443.jpg

Excellent illustrations UnderEmployedGeo and I love the colours but where is the gold? layful:

 

[UnderEmployedGeo]

Better pics:
https://www.prospectingaustralia.co...2/1642559128_screenshot_2022-01-19_132504.jpg
https://www.prospectingaustralia.co...2/1642559129_screenshot_2022-01-19_132443.jpg

Excellent illustrations UnderEmployedGeo and I love the colours but where is the gold? layful:

Don't ask me!
There's a saying about geologists - get 6 of them in a room together and you'll get 7 opinions.

 

[user 4386]

Better pics:
https://www.prospectingaustralia.co...2/1642559128_screenshot_2022-01-19_132504.jpg
https://www.prospectingaustralia.co...2/1642559129_screenshot_2022-01-19_132443.jpg

Excellent illustrations UnderEmployedGeo and I love the colours but where is the gold? layful:

Don't ask me!
There's a saying about geologists - get 6 of them in a room together and you'll get 7 opinions.

12 actually - "on the one hand...but on the other...."

But there is general consensus as to what the two of us have described.

 

[user 4386]

Better pics:
https://www.prospectingaustralia.co...2/1642559128_screenshot_2022-01-19_132504.jpg
https://www.prospectingaustralia.co...2/1642559129_screenshot_2022-01-19_132443.jpg

Excellent illustrations UnderEmployedGeo and I love the colours but where is the gold? layful:

Stuck in the clay - which is why puddlers were invented to release the gold (you cant pan some sticky clay).

 

[user 4386]

The pipeclay where I detect is a creamy/white colour, feels slippery to touch, like talcum powder. Is it correct that the old timers dug through the wash dirt to the pipe clay, then tunnelled along in it as It was easier to work, took It up to the surface, then dropped the wash dirt down into the tunnel, then took that up to the surface to process. I've heard differing versions of how they worked It, but this seems the most logical way to me. Any thoughts on this? wiley.

Usually that is because gold fell into cracks in the clay that formed the bedrock of the streams - the clay in that case is usually still carrying recognisable features of the original rock such as bedding and cleavage, and is difficult to break down by panning or sluicing. About a third of gold in alluvial gold leads was often in this, two-thirds in the overlying gravel.

Clay that OVERLIES the gravel is usually a dead loss. That is because gold concentration is a mechanical process - high density gold grains tend to concentrate in gravel or drop into cracks in underlying bedrock.

 

[user 4386]

..
https://www.prospectingaustralia.co...569/1642205485_20220115_110327_compress14.jpg

This photo illustrates the point - you can still see the original fabrics like bedding in the rock. It has altered to clay in situ.

 

[user 4386]

Add this to the list of confusing historic terms that actually make less sense to the geologists among us (guilty) than to normal, sensible folk with better things to worry about. Whenever I see references to "pipeclay" my mind just shuts down and stops working (if it was working to begin with that is).

Thank you once again to Goldierocks for some actual geology.

One other one is references in older materials to "pitted shale" or pitted whatever - THAT I understand - the pits would be where there used to be little sulphide crystals in the original fresh rock. Slates are pretty resistant to weathering, but sulphides are not. First they oxidise, then they get washed out completely, leaving little "pits". Sulphides are pretty common around gold deposits like you see in Vic (and parts of WA).

A similar thing happens in quartz-carbonite veins (of the sort that often carry gold in gold fields) - if there were enough sulphides in the fresh rock, the preferential weathering of the sulphides can lead to a sort of honeycomb texture, that is better described in geology as "boxwork texture" - lots of little roughly cubic holes that once were full of sulphide crystals, now just full of iron staining. Boxwork textures are a good sign, just as laminated iron-quartz can also be.

Back to clay minerals - in the outer western suburbs of Melbourne, where the Newer Volcanics (basalt) have weathered to clay, there is a particular smectitic clay type known as Montmorillonite - and it is incredible at absorbing water, expanding hugely when wet. People have literally broken their house slabs in half by accidentally leaving a garden sprinkler on...

A bit like "indicator" and "reef". The first is any pyritic, quartzose or graphitic seam in slate (eg the "pecilmark" at Ballarat East - not an indicator of gold directly, just an indication of how many metres you are from the main quartz reefs - described by Mark Twain - see #9953 in Australian History). I find that most people dont realise that in the 1800s "reef" meant hard rock (eg sandstone, quartz). It confuses a lot of people in deep lead reports (another annoying term) because it is commonly referring there to a hard bed of sandstone or quartzite that they could not easily dig out wth a pick and shovel. So "struck a reef and followed it" often means different to what modern readers think!