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Hi... This is Susan in Jamaica again. I received so much great information in my last posts that it has taken a long time to digest it. Much still to learn but I believe I am leaps and bounds from my initial puzzelment over the rocks I am finding.I understand that Ophiolitic exposures provide most of the very most interesting samples. But where in Jamaica might that exposure be?
I am still a bit confused by cherts and their relationship to chalcedony. In the first picture I show just a few examples of the plentiful "banded chert". Can it be related to contact metamorphism? Maybe it means nothing, but the rock on the upper left appears to have a piece of volcanic rock still attached to it.The second group I have guessed is chalcedony, but why does it sometimes appear as softly shaped agates and "opals", and at other times appear as tightly compacted horizontally stacked crystals? None of the samples can be scratched by a knife
Were I capable of rational thought I would answer forthwith - but I am so tired and worn out from my bout with bronchitis I can only come up with a poorly framed insult here and there. So have a snotty day - do nothing.
Thanks Bass...You think JimBob might have more detailed information?
You think JimBob might have more detailed information?
I'm sure he'll weigh in with his opinion whether or not he has more detailed information
I did a lot of internet searching over the weekend and I found what seems like a good explanation in that the chert nodules began with sponge skeletons (which are apparently silica)that were deposited with limestone sediments.
Chert can also come from plankton such as radiolarians and diatoms, who build their tests from silica as well. Im sure there are many other sources, including hydrothermal activity (Bass?), but the main sources that I know of are organic in origin.
Quote from: susanshirleyjamaica on 18/12/2008 00:48:37 You think JimBob might have more detailed information?Classic! You are right to question that.Quote from: Bass on 18/12/2008 01:41:49I'm sure he'll weigh in with his opinion whether or not he has more detailed informationThat made my day.
The more likely source is your organic suggestion- radiolarians and diatoms, especially in or downwind from volcanically active areas. Or the silica could be provided by nearby more silica-rich sediments during diagenesis. Deep ocean chert layers (ie top of ophiolite sequences)are derived from radiolarian and diatom tests as they filter to the ocean bottom (plus lots of time!)
What is your speciality? I have another mystery that I need solved.
He also finds methane (CH4) in caves and crevasses.Of course, the largest methane emitting crack he has to study is planted firmly in his computer chair. My apologies, JimBob, if I was unclear, but I was not intending to say that chert forms mostly from sponges. That was the topic at hand and was explaining HOW they can form from sponges. Dont worry though, I have been properly self-flagellated and am reciting 100 "hail JimBob's" before nightfall.