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Posted by: Donald Kasper
Agates form in volcanic rocks or volcanic ash related to volcanism, only. So-called sedimentary agates are found under ash beds that was the silica source. Agates form in supercritical fluids. As magma comes to the surface, the feldspar and silica separate from each other. The silica under certain conditions forms agate in conjunction with supercritical fluid. The lower the silica content of the magma, the less agates are found. Silica and rhyolite intermix. Andesites have vein agates. Basalts have amygdules. It is not the net content of silica that is the sole driver, it is the catalyst to force it out of solution that also matters. Agates are calcite-clay-silica-hydrate rocks. They have many species of opal, moganite, quartz, cristobalite. They are commonly included with calcite which plays a key role in their formation by creating alkaline conditions. The inclusions and shell structures are commonly clays. Celadonite and bentonite are the most common. Infrared spectroscopy equipment I have shows for example that the Union Road Missouri agates all have celadonite shells. Even though the host is sedimentary, celadonite cannot form from weathering. It forms only in volcanic rocks and their ash equivalents. Snakeskin agates have shells of bentonite and celadonite glaze, which disproves their magadiite precursor and weathering model. With infrared, no zeolites, no salts, no evaporites are found in agates. Sulfides are rare and only occur with a few types that can form in neutral pH systems. They form in agates when calcite is present. The calcite forms sulfate and neutralizes the acidity. Overall, the inclusions tell us their temperature of formation. Chlorite and celadonite found in them are formed around 425 C and not with weathering. The glass state of rhyolite between hard rock and melt is the range of 374 to 575 C. Above 374 C is only supercritical water. A fluid gas with the solubility of water and diffusion of a gas. No surface tension exists. In this system, the charge on silica is negative, and you will note that only cation positively charged minerals or metals are found in agates. Anion mineral states are never found. Groundwater does not dissolve silica and it does not just move around to and fro. It takes alkaline systems to dissolve silica. So, agates are rock, they are not varietal quartz, and they form very quickly (hundreds of years max time scale), and only in volcanic rocks. In fact, they are almost never found near water, near rivers, water tables, or lakes, or river mouths. They are typically found in deserts. You never go to the tropical rainforests of Brazil to hunt agates, you go to the Mojave Desert of California or Sahara of Morocco. This defeats a weathering water model for rocks not found with water. Where does their water come from? Right out of the melt. It is volcanic water, and not hydrothermal water. Hydrothermal water is rich in calcite, not silica. To get pure silica, you cannot have groundwater. Agates have no humic acid to show they were in contact with ground water. Humic acid is not found in agates cores in infrared, just the shells. That comes from weathering, not agate formation. As you look closely, no popular model of weathering has any scientific basis to be believable as no data fits a weathering model.