Post by Deleted on Jan 2, 2018 21:25:58 GMT -5
Dec 15, 2017 4:58:46 GMT -5 @rocks2dust said:
Thundereggs with jasper interiors formed in rhyolite flows. Even the small particle size of clays and caliches would not be sufficiently small to penetrate through the flow into the interior chambers of these gas bubbles. Even were one to postulate that clay could make it through the surrounding rock to fill the voids, one would expect the surrounding rhyolite to be saturated with clay particles filtered out as it was carried through the surrounding rhyolite, and this simply is far from what I've encountered when digging any of these. Rather, it is the silica that moves through the rock and fills the voids (unless pressure causes the shell to crack or collapse). Moreover, there are places where you can dig a jasper-filled egg and a few inches beyond it dig an agate-filled egg, caused (as are varying colors between eggs) by the particular minerals in the gases, solution or rhyolite shell at that point.
Clays can form a part of sedimentary jaspers and agates (particularly with traditional European agate materials). Kasper, on the other hand, denies that chalcedony forms from silica in water solutions traveling through rock or flowing into cavities at all. From what I recall, his conjecture is that all material necessary for formation of the chalcedony (agate or jasper) is already present in the volcanic rock in which it forms. When it comes to what one observes in thunderegg formations, this falls apart. They show evidence of multiple depositional phases (gas, gel and/or waterborne in fairly rapid succession) with intrusion of clays or even claylike minerals not a necessary factor. They are often found in perlite beds, some of which have weathered into near-clays long after the agate formation and not affecting the insides of the nodules at all. He denies that agates and zeolites can form in the same nodule, when they in fact do (zeolites also form along with or encapsulated within macrocrystaline quartz). Because he is so certain that chalcedony does not form from water-borne silica deposition, he has posited that only volcanic origin can explain agates that form in seabed deposits, when there is certainly silica in seawater and held in limestone - corals, seaweeds and other sea life depend on it. Saying that chalcedony (whether agate or jasper) doesn't form from water-borne deposition doesn't explain fossil-containing jaspers or jasperized wood, agate limb casts that preserve permineralized wood features, etc. He makes distinctions between jasper and chert that few accept as well.
Jasper and agate are both traditional names given to chalcedony. There is no agreement among either mineralogists or gemologists as to what makes them different, other than the traditional distinction of opacity. I've mostly heard jasper derisively labeled with mud-related terms by some gemologists who dismiss jaspers as entirely unworthy of their attention - you know the type, their knowledge is focused on diamonds and sapphires with a nod to the one or two other gemstones they've happened to run across. I thus have a problem with the term and any association with jaspers all being formed from mud (fine, very high-silica ash, sometimes, among the several likely ways that it can form). There are materials that form from mud (mudstones and shales) that have nothing to do with jasper.
Oh yeah, the Golden Gate bridge's "kinradite" was a great jasper. I've seen a very few slabs offered over the last few decades - some looks like the old material, some doesn't. Someone on another forum said that they'd found some small pieces along the shore, but I don't recall seeing any photos to confirm. Kinradite does closely resemble the "Oregonite" jasper (not the same as the mineral oregonite) that was dug in Oregon's Josephine county back in the 1930s and 1940s and is just as hard to find these days. Hornitos jasper can have a similar look and is more readily available. I'll second your recommendations for stephoinite and howardite/rattlesnake, too.