[Rockhounds] What is this rock?

[Dora Smith]

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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