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Innocentx

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I would like to better understand conchoidal fracturing of chert/flint. I have many pieces where the fracturing is obviously conchoidal, but some others where this isn't obvious. I'll post photos in hopes that knowledgeable folks can point out circular characteristics that I'm not seeing.

 

In this first one I can see small conchoidal divots. It's the larger seemingly straight(long lines) fractures where I don't see conchoidal characteristics.

 

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"Journey through a universe ablaze with changes" Phil Ochs

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And this second one, if someone could point out what's conchoidal, I would appreciate it.

 

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"Journey through a universe ablaze with changes" Phil Ochs

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27 minutes ago, Innocentx said:

I would like to better understand conchoidal fracturing of chert/flint. I have many pieces where the fracturing is obviously conchoidal, but some others where this isn't obvious. I'll post photos in hopes that knowledgeable folks can point out circular characteristics that I'm not seeing.

 

In this first one I can see small conchoidal divots. It's the larger seemingly straight(long lines) fractures where I don't see conchoidal characteristics.

 

5b7a193636be4_IMG_4233resize.jpg.99f90e1c8707cb78fa5eac606ca55fb0.jpgIMG_4234.thumb.jpg.3fc8c944c15df12ff38351c520fd87a9.jpgIMG_4235.thumb.jpg.31723ca68697201ecf18c2dc9f910e3d.jpgIMG_4236.thumb.jpg.c8a034b27f1c992e6de3c8085ac0d89b.jpg

 

 

Ignore the larger fractures, the rock is on its  way to becomming several rocks. Look at the smaller fractures which are as you pointed out conchoidal.

My goal is to leave no stone or fossil unturned.   

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28 minutes ago, Innocentx said:

And this second one, if someone could point out what's conchoidal, I would appreciate it.

 

IMG_4237.jpg

IMG_4238.jpg

IMG_4239.jpg

IMG_4240.jpg

IMG_4241.jpg

This one has conchoidal fracture even though the surface is rougher probably due to more non silica components. Ignore the slightly rough surface and look at the larger surfaces where you will notice that they are gently curving. Conchoidal fractures are curved.

My goal is to leave no stone or fossil unturned.   

See my Arizona Paleontology Guide    link  The best single resource for Arizona paleontology anywhere.       

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So any kind of curved fracture, no matter how incredibly slight, denotes conchoidal. Thanks, @DPS Ammonite.

"Journey through a universe ablaze with changes" Phil Ochs

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Generally but I cannot say always. Granite sometimes cracks in large curved sheets even though the individual minerals do not all fracture conchoidally. (I would not consider the fracturing in granite conchoidal.) Most quartz varieties, if pure enough, will fracture conchoidally. In real grainy varieties it may be less pronounced. 

My goal is to leave no stone or fossil unturned.   

See my Arizona Paleontology Guide    link  The best single resource for Arizona paleontology anywhere.       

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39 minutes ago, DPS Ammonite said:

In real grainy varieties it may be less pronounced. 

I see.

The material here goes from grainy (more like intermediate cortex), to dense and polished where I expect the conchoidal fractures are most pronounced.

I'll get my rocks right one of these days.;)

"Journey through a universe ablaze with changes" Phil Ochs

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The larger cracks in Your rocks are most likely from geologic stress rather than a precision.

This will make the single fracture much larger than what is observed in the small piece. So You are looking at a small area of a very large faulting structure.

I also see "hackle fringe" structures. This feature will distort the conchoidal structure.

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11 hours ago, ynot said:

I also see "hackle fringe" structures. This feature will distort the conchoidal structure.

Thanks. I can see how that would happen. Also good to know the name of these features.

"Journey through a universe ablaze with changes" Phil Ochs

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