Re: Quartz Origins (Uh-Oh, Another Geology Post)

On Jan 6, 6:15 am, Joe <see_real_...@xxxxxxx> wrote:
On Fri, 02 Jan 2009 19:57:13 -0600, Jo Schaper



<jo34schape...@xxxxxxxxxxxxxxx> wrote:
Pictures look fine. Did you try the muriatic acid yet? OR scratching it
with a pocketknife?

Superficially, the two closest things the photos look like are a quartz
(?) version of the chert cryptozooan reef structures we have around here
(Cambrian, early Ordovician in age) or some sort of wormtrack structures
eroded out of limestone (very close in morphology) and then silicified,
although how I'm not sure this could happen. Or my first guess-- weird
slag. Do you have any idea what the age of nearby rocks are?

Definitely looks more like quartz than calcite.
Wish photos 3 and 4 were larger. Photo 5 looks like limestone where the
quarter is, and silica elsewhere,  so I would test both for fizzy.
Trouble is, this mix of chemistry may occur both in an algal reef
structure which is partly silicified or chertified, and you guessed it,
 in slag with part glassy and part hardened flux (limestone.) What is
the reddish stuff in the pockets like? Have you taken a magnet to the
rock, to check for magnetic iron?

Thanks, Jo. I'll get the acid today, and bring the rock back in for
testing. Funny thing, but my wife got tired of seeing it sitting on
the kitchen table. The table itself is terrazzo, so it's not like we
don't already have rock there. (OK, in fairness to her, she's usually
very tolerant to pretty enthusiastic about rocks as well.)

Joe

Oh, while I'm on lemons, I should mention that calcite fizzes
vigorously in cold lemon juice, dolomite with some calcite (they form
a crystalline solution) will fizz according to the calcite content,
and pure dolomite with not fizz in cold lemon juice, but will fizz in
hot.

Also, dolomite will fizz in hydrochloric acid, if it's not made
properly (1 molar solution).

Sorry about that. I glanced at the excellent photos and noticed
rounded grains with a relatively clear, crystalline cement. This is
not suggestive of a metamorphic rock, whose crystals have completely
recrystallized and are homogenous. In metamorphic rocks, carbonate
cement also would react chemically with quartz to create the so-called
cal-silicate minerals (Ca-garnet, for example).

So, it's important to see, if you can, what composes the grains and
what composes the cement. (Assuming the rock actually has these.) Note
that, unless one can identify fossils, it is important to first
identify the minerals, then the rock, then the environment of
formation. The latter usually requires mapping a large area of the
rock, in place.

My original letter was supposed to offer a systematic procedure:

1. What parts scratch glass?
2. What parts fizz in cold vinegar?
3. What parts fizz in vinegar when powdered?
4. If none of the above, visit your local petrologist.

Some other relevant question would be, do you live in the east or west
side of South Carolina? Is the rock fill from a quarry or local? Do
you live in a large valley or on a hill? If the rock hasn't travelled
far, you can likely find outcrops of the rock, with bands filled, in
road cuts.

Best of luck,


Bruce

.

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