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Posted by: Donald Kasper
Go to any hydrothermal vent site on this planet spewing out silica and hot water, and show me an agate. You cannot. Your hot water and silica model just died. I collect for myself. When I get BS in the literature I can check it in the field as I live just south of one of the largest agate trends in the world. Beware of PhD's writing about agates in playa lakes while living in Pennsylvania in coal country. Really, they should study ferns and leave that agates alone.
The Milford, Utah vent site is world famous. I have been there. I have samples. Opal-CTA and Opal-CT. No agate. No agate or jasper on this planet has opal-A, the only opal formed by weathering and surface hydrothermal deposition. Infrared proves it. You just have to look.