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Re: a simple explaination of agate formation

Posted by: Gregg Little

I find some strange and down right erroneous statements, oh dear where do I start;

1). "Agates form in volcanic rocks or volcanic ash related to volcanism, only." The adjective "volcanic" can only mean from volcanism and its processes.

2). "Silica and rhyolite intermix. Andesites have vein agates. Basalts have amygdules." The void spaces that agate forms in is a function of tectonic stresses that create the void space, not a function of agate formation.

3). "Agates are calcite-clay-silica-hydrate rocks." If you look in any book on mineralogy they all basically say "agate is a banded form of finely-grained, microcrystalline quartz". All other constituents are impurities, inclusions, etc. Agate is quartz.

4). "Groundwater does not dissolve silica and it does not just move around to and fro. It takes alkaline systems to dissolve silica." Since when does groundwater that becomes alkaline cease to be ground water? Sedimentary basins around the world can have and have alkaline waters. In my petroleum geology work have seen chalcedony (aka agate) in sedimentary rocks. I have also seen stylolites (pressure solution features) in quartz arenites (sandstones) which puts silica in solution which then precipitates as quartz overgrowths on quartz grains.

5). "So, agates are rock, they are not varietal quartz" Is Donald Kasper trying to dismiss the whole field of geological science?

6). "They are typically found in deserts.". As Ralph B. stated and I reiterate British Columbia and Nova Scotia are not deserts, but a source of agate in volcanic terrains in a temperate climatic zone.

7). "no popular model of weathering has any scientific basis ". I am not sure how weathering was brought up except by Donald Kasper; weathering is post depositional.

8). "Most models of geologic formation of agates pick and chose what fits their concept of formation.". See my comment #5. Geologists in research do not pick and choose, they use the volumes of past research work and their evidence to support their research thesis.

9). "Quartz does not weather. It must be mechanically broken down.". Weathering is both a mechanical and chemical process, again check the literature. Many, many water analyse show dissolved silica.

And last but probably not finally, 10). "In essence, a galaxy of complexity reduced to stupidity for bland generalizations." Agate formation definitely has "a galaxy of complexity" but the latter part of this statement appears to be what you are advocating.

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