Chert help

[EwanCameron]

Hi all

Are the SiO2 parts that make up Chert drawn together by being magnetised ?

Can you keep it simple I'm dumb as a bag of rocks 


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[EwanCameron]

Also why do they also become Flint ?


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[Peter Burgess]

I always imagine something like squidgy silica gel forming deep in the sediments and solidifying over time.

 

[Brains]

Flint is chert from a spedific set of beds within the cretaceous chalk, but as minerals they are identical.
The deposit is a secondary mineral in most instances, formed from remobilised SiO2, which came from silica sediment or silica rich skeletons (eg radiolaria). It is commonly seen to form inconsisitent layers and nodules which cut across bedding, thought to be the redox fronts within the groundwater. The silica in solution will tend to stick together to form a hydrated gel which sets as the chemistry changes to form a crypto crystalline hydrate. magnetism is not invloved. The micro structure of chert is like tiny spheres of amorphous silica hydrate with no macro structure or crystallinity, hence having a glassy fracture and appearence. Relatively commonly the chert will form around an incosistency in the rock, such as a fossil, organic debris, worm tubes, etc

The silica is sticky, it sets as the groundwater changes to form chert, which is also called flint in some places...

 

[EwanCameron]

Thanks all


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[Pitlamp]

I agree with Brains. To which I'd add;

* I doubt it's anything to do with magnetism.

* A special problem (and I mention this because I know you're doing a fair amount of diving) is that the gravel formed from chert nodules has an important difference from normal gravel. Normal gravel encountered in sumps tends to be formed of rounded clasts of hard sandstone. It gets into the mouthpieces of regulators, gets lodged in the 2nd stage exhaust diaphragms and then causes an uncomfortably wet breathe. This is a major inconvenience.

Gravel derived from chert tends to form clasts which have a different shape; they're slender and angular. These can get wedged between the 2nd stage body and various levers which open the 2nd stage valve and jam it closed. This is rather more serious than an "inconvenience" as you then get your air supply shut off. A classic place for this to happen is the restriction at the bottom of Brown's Chamber in Peak Cavern's Far Sump, where the gravel is almost entirely composed of such chert "wedges". The answer deployed by many cave divers is fine nylon mesh, held on the orifice of the 2nd stage housing by the rubber mouthpiece. This in itself can become a hazard if not done optimally and (to cover my own backside) I'm not suggesting folk do this - I merely make the observation that some cave divers do it. (What a sad world we live in when you feel compelled to say things like that . . . )

 

[EwanCameron]

Mega info as always and worth a think about just getting my new cyclones so will have a look

Seen mesh on photos on divers regs in books but will get advice before trying


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[EwanCameron]

Loving that SiO2 is also quarts but formed as a hydrothermal Mineral not a sedimentary rock 

PL - thanks for the info


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[langcliffe]

Loving that SiO2 is also quarts but formed as a hydrothermal Mineral not a sedimentary rock 

SiO2 is the basic chemical formula for ALL quartz.

Most quartz is normally formed from the crystallisation of magma (to form rocks such as granite), although some is precipitated out of hydrothermal fluids. As the result of weathering and erosion it becomes a major constituent of siliciclastic and siliceous sedimentary rocks. As Brains indicated, cherts may then be formed within the sedimentary rocks by diagenetic processes.

 

[EwanCameron]

[emoji106]🏻


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[EwanCameron]

Loving that SiO2 is also quarts but formed as a hydrothermal Mineral not a sedimentary rock 

SiO2 is the basic chemical formula for ALL quartz.

Most quartz is normally formed from the crystallisation of magma (to form rocks such as granite), although some is precipitated out of hydrothermal fluids. As the result of weathering and erosion it becomes a major constituent of siliciclastic and siliceous sedimentary rocks. As Brains indicated, cherts may then be formed within the sedimentary rocks by diagenetic processes.

So quarts formed by hydro heated water is called and hydrothermal mineral

What do you call magma cooling quarts

A igneous mineral ?


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[Brains]

If you need to split them down by formation then yes, but quartz is quartz (except when it isnt*...) Hydrothermal, igneous, diagenetic, metamorphic, primary precipitate - just extra ways of trying to describe the formation path..

*Supplementary for the mineral nerd. SiO2 also forms tridymite, cristobalite and high and low quartz as well as non (crypto) crystalline things like chert, flint, opal and glasses. Other even less common forms maybe mooted. Shock quartz is, wait for it, ordinary quartz that has been shocked (by high impact)!

 

[EwanCameron]

Cool


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[ah147]

A classic place for this to happen is the restriction at the bottom of Brown's Chamber in Peak Cavern's Far Sump, where the gravel is almost entirely composed of such chert "wedges". The answer deployed by many cave divers is fine nylon mesh, held on the orifice of the 2nd stage housing by the rubber mouthpiece. This in itself can become a hazard if not done optimally and (to cover my own backside) I'm not suggesting folk do this - I merely make the observation that some cave divers do it. (What a sad world we live in when you feel compelled to say things like that . . . )

I just popped some tights over the mouthpiece. Stopped getting full of mud (as much) and still works fine down to 80m...

 

[Pitlamp]

Yes, that works too - but is less durable as the mesh gets holed more quickly.
The best deployment is when the mesh is captive beneath the mouthpiece rubber (although this is slightly less convenient to remove if you want to use the regulator with no mesh protection in alternative diving conditions).

Some people find that the mouthpiece feels less secure in the mouth with the mesh over mouthpiece method and there is a possibility that this has a slightly greater effect on breathing resistance as well (compared with when the mesh us below the mouthpiece rubber).

Not that I'm in any way advocating that anyone tries any of this this - I merely make the observation on what a lot of experienced British cave divers do.

 

[EwanCameron]

I'm reading about hydrothermal minerals at the moment

It's interesting stuff


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