[Rockhounds] What is this rock?

[Carolyn Reynard]

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

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