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Post by snowmom on Jul 31, 2014 15:06:08 GMT -5
Since I started collecting rocks from the beaches near here I have from time to time run across rocks that seemed to be either coated with silica or with strings of stuff. The color varies depending on location. One beach seems to have red threads over rocks, and filling any cracks. One beach seems to coat various rocks with black. I have found a few that have greyish or transparent stuff too. Not all of them... it seems only certain rocks of certain types collect this coating. Something in the chemistry of those particular rocks that attracts the stuff that coats them? Some of the coatings can be very thick. I have decided it is silica based because it does not change in acid, and if it was calcite it should soften or fog or dissolve. (am I on the right track here?). I have tried smacking some of the rocks with a hammer, too and it is a very tough coating. A few very small dings, but no breaks. Did you ever try to look something up when you didn't know what it was called? I had a heck of a time finding the two words above... perimorph and epimorph. Found them through the use of the word pseudomorph which I have seen here. The rocks I am finding seem to be unchanged inside as yet, though I do not have a saw to make cuts and tell for sure. most I find seem to be larger than fist size... but I have not come to recognize the coating 'for sure' on many rocks I have/had until recently. I may have some small enough to tumble if I explore a bit. So, here is my question/request. Talk about perimorphs and epimorphs and show me pictures if you have some in your collections? I have been reading all the links I have found on the internet, now that I know what the darn things are called, or at least what they are trying to be! I'll see if I can get a couple of photos taken. Input please?
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jamesp
Cave Dweller
Member since October 2012
Posts: 36,186
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Post by jamesp on Aug 1, 2014 4:45:27 GMT -5
The cavities in coral psuedomorphs often resilicify after they have been damaged by some force snowmom. In these photos the tubes broke and were recoated by clear silica. I see a lot of this recoated phenom inside the cavities. Something disturbed it during the the creation process. I think fortifications in Montana agate do similar when they tilt to another angle while forming.Causing straight bands in one angle, and others at another angle. in the case of the coral, there may have been erosion that caused them to shift. Or tectonic activity. this one from a dealer, recovered damage by druzy crystals www.paleodirect.com/pgset2/cor094.htm
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Post by snowmom on Aug 2, 2014 11:43:32 GMT -5
here are the rocks I ran for the first time in my new vibe... I was surprised at how the camera picked up the yellow on most of the rocks. What is see with my bare eye is smoky brownish.. wonder what sort of mineral does that for a refractive trick? It is in the silica coating on many of these rocks, much more evident on camera than in 'person'. Not all the rocks in the first photo have that coating, but a lot of them do! this one does look very yellow gold, and the silica seems to be laid on in strands. Inside seems to be layered quartz or chalcedony, you can barely see it through all the yellow, easier when you hold it in your hand. I do think the yellow is all on the outside. this one is probably basalt or basanite with porphyry... but covered with yellow/brown silica as well. Probably a quartz/schist pegmatite but covered with yellow and red strands of silica... makes it look like something else... If I cut it will the silica have penetrated and started to work on the original stone to morph it? I don't have a saw! If anybody is interested in cutting these or others I have like this for the sake of satiating curiousity, please let me know, and i'll arrange it! Thanks!
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