Remaining vapor and silica gas phase extruded through degassing, as
you wrote it, from the increasingly cooling and crystallizing "mafic"
phase, subject to crystallization of its own once the temperatures
reach solidus (would that be liquidus?) for quartz? A wild, wild
guess... been away from the books for a long time...
On Sun, Jan 31, 2010 at 8:08 AM, Douwe van Hinsbergen
<[log in to unmask]> wrote:
> Hi all,
>
> I have a question, just out of curiosity. Not particularly a tectonic
> question, but maybe some of you have given it a thought.
>
> Many basaltic lavas contain amygdales/geodes of quartz, amethist, etc. I
> have come across them particularly in trap-like basalt fields of Mongolia,
> and Etendeka (great way to waste a field day). They fill vesicles in the
> lavas, and some of them are half-filled, with nice crystals, some of them
> are entirely filled.
>
> How do those geodes form? I assume that the vesicles form due to degassing,
> when the lavas are still flowing at high temperature, so I assume that
> quartz crystallisation occurs after or during cooling. I always kind of
> assumed that these geodes crystallize out of ground water, or hydrothermal
> fluids...but basalt does not really strike me as particularly permeable, and
> I have examples where I cannot see any crack leading to the geode, so how do
> you get the water in those vesicles? And if quartz crystallises out of
> water, how do you fill those vesicles entirely?
>
> I'm sure this is known, but the internet directs you to gemology sites, and
> those sites focus on where they form, and what they are, not really how they
> form.
>
> All the best,
> Douwe
>
> --
> http://www.geologist.nl
> Dr. Douwe J.J. van Hinsbergen
> Physics of Geological Processes (PGP)
> University of Oslo
> Sem Sælands vei 24
> NO-0316 Oslo
> Norway
> Tel: +47 22856254
> [log in to unmask]
>
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