[Rockhounds] What is this rock?

[Mr EMan]

A note on the acid reaction to what appears to be gneiss cobbles from The Adarondack Mtns.

Until 13-12,000 years ago, this region of the North East US was covered by an ice cap.  That scraping shoving glacial process, produces a high concentration solution of CaCO3. This is an excellent cementing agent and will cement smaller cobbles, pebbles and sands together. It also leaves a residue in cracks which may be the source of the fizzing you tested for.  

I have several examples of granite cobbles covered in fluorescing aragonite. In fact any rock from the region can be found somewhere with a coating of aragoinite.  I surmise the reaction you had to hydrocloric acid was limited to this aragonite. Yes-- this example is apparently gneiss and neither sandstone nor granite.  BUT I loved the fact that you set about using the tools, logic, and followed a process to ID the sucker!!!  For that my hat is off to Dora

Elton

--- On Tue, 11/11/08, Carolyn Reynard <sunstone3 at hvc.rr.com> wrote:

> From: Carolyn Reynard <sunstone3 at hvc.rr.com>
> Subject: Re: [Rockhounds] What is this rock?
> To: "Dora Smith" <tiggernut24 at yahoo.com>, "Rockhounds at drizzle.com: A mailing list for rock and gem collectors" <rockhounds at lists.drizzle.com>
> Date: Tuesday, November 11, 2008, 10:43 PM
> Dora, Your rocks certainly look like gneiss. The banding of
> light and dark
> minerals is typical.  The dark mineral(s)
> would be bioitie or hornblende. The light mineral(s) would
> be quartz and/ or
> feldspar.  One would not expect any reaction to HCL. Rinse
> them well, they
> probably won't be a problem in a fish tank.
> 
> I'm not familiar with an Adirondack smell. Your Austin
> smell would seem to
> be like an earthy moldy clay odor.
> 
> Carolyn Reynard
> Poughkeepsie, NY
> 
> 
> ----- Original Message ----- 
> From: "Dora Smith" <tiggernut24 at yahoo.com>
> To: <rockhounds at lists.drizzle.com>
> Sent: Tuesday, November 11, 2008 10:05 PM
> Subject: [Rockhounds] What is this rock?
> 
> 
> I asked my sister to bring me home some Adirondack granite
> from our family
> home for my fish tank, and she brought me these two rocks. 
>  Photos are at
> http://good-times.webshots.com/album/568626982hMlTIK
> 
> They look sedimentary; they are banded.  They are heavy and
> softer than
> granite ought to be; they scratch with a knife.  They react
> weakly with 10%
> hydrochloric acid (toilet bowl cleaner).  Some parts of the
> rock definitely
> fizz, especially if the rock is hot, but not as vigorously
> as limestone.
> Sometimes acid toilet bowl cleaner leaves behind a rough
> residue of large
> sand grains easily scratched off.  They do not fizz with
> vinegar.
> 
> The rock smells distinctly like Adirondack rock and
> distinctly NOT like
> Austin rock.  In Austin nearly all rock is soft limestone,
> and it always
> smells yucky.   There is no other way to describe it.   The
> soil smells the
> same sort of yucky.
> 
> I thought they were limestone until I saw photos of gneiss;
> they look alot
> like gneiss.  but from what I am reading gneiss is not made
> from limestone.
> Usually it is made of solid silicate rock that is already
> hard and durable.
> 
> Can silicate react with hydrochloric acid?
> 
> What is this rock?
> 
> If it is limestone based, is it hard enough not to throw
> off the chemistry
> of my fresh water tropical fish tank?   In Austin the water
> is hard when it
> is taken from the river but it is treated with water
> softeners.
> 
> Yours,
> Dora Smith
> 
> Yours,
> Dora Smith
> Austin, TX
> tiggernut24 at yahoo.com
> 
> --- StripMime Report -- processed MIME parts ---
> multipart/alternative
>   text/plain (text body -- kept)
>   text/html
> ---
> -- 
> _______________________________________________
> Rockhounds at drizzle Mailing List
> Subscription Services:
> http://lists.drizzle.com/mailman/listinfo/rockhounds
> List Home Page, with a link to the List Usage Policy:
> http://www.eclecticlapidary.com/Rockhounds/index.html
> 
> -- 
> _______________________________________________
> Rockhounds at drizzle Mailing List
> Subscription Services:
> http://lists.drizzle.com/mailman/listinfo/rockhounds
> List Home Page, with a link to the List Usage Policy:
> http://www.eclecticlapidary.com/Rockhounds/index.html