Re: Sandstone genesis
From: Carsten Troelsgaard (carsten.troelsgaard_at_mail.dk)
Date: 09/25/04
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Date: Sat, 25 Sep 2004 05:33:44 +0200
"Ookie Wonderslug" <ookie@my-deja.com> skrev i en meddelelse
news:enq8l0p6ql5gpam0r8qp7rkqce4ctvvl0k@4ax.com...
> On Fri, 24 Sep 2004 16:00:23 +0100, Aidan Karley
> <aidan@abuse.demon.co.uk.invalidated> wrote:
>
>
> > Question - are you certain that the aberrant grain was quartz?
> > In small grains (e.g. sand grains), distinguishing dirty quartz
> >from feldspar is surprisingly difficult. Which is one plausible
> >explanation for the disparity in grain shape you mention - if most of
> >the grains are quartz, and the sandstone has been in a chemical
> >environment where silica is capable of being deposited from pore fluids
> >(a process called "quartz overgrowth formation" - Google for it) then
> >the occasional feldspar grains (any non-quartz grains, to be short)
> >would actually show the rounding/ sphericity/ sorting/ grain size of
> >the sediment AS TRANSPORTED. But the sandstone you're looking at is the
> >sum of the erosional history of the hinterland, the transportation
> >history of the sediment, the depositional processes of the sediment,
> >AND the diagenetic history of the sediment as it converts to rock.
> >
>
> I "googled" it and yep, that seems to be it. "Quartz overgrowth
> formation" would explain this particular rock nicely.
I'm still puzzled. Can you link me to the information that lead you to the conclusion?
You describe your observation as crystalish? This would mean that the grains has crystal-faces in
angles characteristic for quartz. To me crystalish doesn't make sense as the faces are there or they
aren't. I sometimes have been taken away by the nice brand-new looking waterclear grains that is
very edgy but has no crystal-faces - and I suspect that's what you refer to. This kind of character
carry a different clue to me: that the grains are juvenile (little transport) ... where the mature
end of the scale is to be understood as the grains you may find at the sea-side being rounded and
loose the virgin shine and clarity in the surf. In general, when you say edgy grains to a geologist,
he instinctively thinks juvenile sediment.
I havn't seen meny sandstones, but where quartz is a cementing agent I have often been left with an
impression of simply a wet glue. I don't recall to have seen thin-sections with quartz-overgrowth as
typical as what leaves a zonation in feldspars and others - that doesn't mean that it doesn't exist.
> >> And is it even remotely possible that an
> >> emerald would be embedded in a sandstone rock in south central West
> >> Virginia?
> >>
> > I don't know enough about the regional geology of the area, but
> >inherently it's pretty unlikely. There are common minerals which are
> >often found in sandstones which do have an intense (and very
> >attractive) green colour. The one that springs to mind is glauconite,
> >an iron mineral that frequently forms little pellets which can be
> >transported as if they were sand grains. They're quite common in
> >sandstones, but more common in mudstones.
>
> And that would explain the "emerald" too. The iron in the rock is
> evident from the dark color in some of the "cement" that holds the
> grains together.
>
> So that pretty much wraps up that one. Thanks for the help.
>
> >
>
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