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Post by johnjsgems on Mar 31, 2011 16:39:58 GMT -5
I guess that's what breccia is. Rock that formed, broke up and reformed. A little different than fractured rock.
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Post by deb193redux on Apr 1, 2011 11:39:33 GMT -5
Yes John. I have thought about this. Fractured is bad but a fractured history is generally great. Ironic. Because so much that we value in jaspers comes from fracturing. Picture jaspers would hardly exist without fracturing.
In my take, breccia is generally more broken up bits, mostly in place but with some turning about and shifting, that gets considerable silica inflow, such that the agate seams re-cementing the rock are often as wide as the bits of breccia between seams. For example some stone canyon, or the African breccias, especially tabu tabu.
Picture jasper is fracturing where pigmentation flows in on fracture lines. Often silica flows in later and heals the fractures. Sometime it breaks on the fractures though because the healing is not as complete as the re-cementing in breccia.
A lot of the stuff from the Franciscan fromation, including poppy jaspers, was fractured from faulting and earthquakes, but there was insufficient silica to flow back in and really heal it well. But the poppies are fascinating, and when you get a small healed area, it is a joy to cut.
Then there is conglomerate where the rock broke up, got transported, rounded, sorted, and then re-cemented in another rock. Different from breccia, often has mixed hardness, and can be challenging. The matrix is often a bit drab, but there is some fine material there too.
I think the thing that fascinates me about chappenite is that the fractures are generally curved and not as angular as the fractures in berccias, picture jaspers, or poppy jasper. Being silicified seafloor, essentially silicified mud that was under water - most picture jaspers are silicified mud that was above sea level. Anyway, in seafloor sediments processes of slumping and folding are much more common and active (think of a CA hillside after a good rain.)
The result is curved fractures in the semi-lithified mudstones. Then later (in many cases after the seafloor has been lifted above sea level) the rock is silicified and you have the wonderful beige and pastel swirls and curved but healed fractures. Sometimes a healed seam will be wide and even have a little breccia inside - like an island in a river.
I like the stuff, and have not had any poorly healed material in what little I have cut.
Issues of fracturing are also a matter of scale too. Completely unfractured rock has to be violently mined, sometimes with explosives, but often with jack hammers and/or sledges. This can add fractures, not to mention it is just hard to mine.
When you have rock where the bigger fractures are about 1-2 feet apart and poorly healed, but the little fractures in between are all healed, this is easy to mine, and mining does not cause much new fracturing, and you get all the added beauty introduced by the fractures.
Thinking of the geology, rockhounds, miners, traders and how it all works out in certain ways and intersects our notions of aesthetic beauty is just mind candy for me. Maybe as much fun as holding a nice piece of breccia, or chappenite, or picture jasper, or poppy jasper .... but nah, I guess holding the rock is a little more fun.
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