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Posted by: Donald Kasper
It is quite well established that geodes have agates cores and that the plastic state of rhyolite is 374 to 575 C. Above that is a melt and below that is cold rock that only brecciates. So this is the temperature they formed at with silica migrating to the core and the exsolution and expulsion of feldspar. Studies that say agates are formed at 35 C are based on O-18/O-16 ratios which can say anything you like as migration of water with salt changes the ratio and the presumed temperature. In fact, minerals formed at volcanic spreading center trenches are mapped at this temperature by this method while the lava erupts at 1250 C. In terms of chlorite and celadonite, they form directly in volcanic systems and not from weathering and are found in agates consistently. More celadonite than chlorite.
There is a few problems with weathering models. First, all amygdules and geodes have an outer lining of silica in every agate of this planet. Groundwater cannot deposit silica films to do this. Groundwater will make laminar silica structures only, wall-to-wall. This so-called sticky physics to line all geodes/amygdules on earth does not exist with water. There is only one physical state where the voids can be lined uniformly with silica and that is called vapor phase deposition which only occurs in supercritical fluids.
Then you have that small problem with interior waterlines and vapor tops with crystal quartz or banded agate. Blow it off as coincidence as you wish, but if you want an actual model of the physics by which this occurs, weathering does not work. However, when the lava with its silica cools below 374 C and goes subcritical, the system must form two phases. One is water and the other is water vapor. Supercritical fluid and silica going subcritical form waterline floors and vapor phase crystal or banded tops is prima facie proof of a supercritical transition occurrence.
When rocks are exposed to ash only in sedimentary strata, those agates form only in subcritical systems. These are totally different, sinuous and concave structural agates and never have waterlines. These form at or around 150 C. There is no such thing as the one method of agate formation. There are the 70 geologic systems of formation with common themes.
Acicular morphologies of minerals are not accidents of nucleation. Acicular morphologies only form in supercritical fluids where there is no surface tension. There are 4 geologically important supercritical fluid types, each with its own transition temperature. Morphologies (shape-types) are treated in the literature as accidents of God, but are formed in very specific geochemical conditions.
Transition times from super- to subcritical matters and can oscillate depending on heat sources in the volcanics. Decompression in transition causes boiling where the fluid starts to cycle up the walls and drip down the center. This forms the silica pendant structures, always and only perpendicular to the horizontal waterlines.
So we can go through every major agate structural morphology and define it in terms of supercritical transit times and types. For example, rapid subcritical transitioning causes an implosion to occur, and sucks in the exterior volcanic rock around the void structure. When the shell celadonite comes in, its water is expelled. Anywhere water contacts these silica fluids, opal is formed. So the celadonite filaments are encased in opal.
If I am correct in identifying beta-moganite in infrared, this mineral forms at the lower supercritical transition start at 354 C. As such, I have only two populations of moganite, no intermediate forms, with the beta only found in the hottest pyroclastic rocks. Both populations are known to be moganite in Raman infrared, as specimens shared with NASA scientists have found. If correct, and I do collaborate with some planetary scientists at JPL Pasadena on this model, then your agates are filled with beta-moganite and your groundwater model drops dead. it occurs in a variety of very specific structures and is different than moganite in banding layers. These two populations of moganite have totally different spectral behaviors. That is not a conclusion based on 4 rocks like say, oh, Moxon might publish on, but based on 35,000 total agate and volcanic rock graphs with a set of 2500 moganite graphs to study.